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19-1694; Rev 0; 4/00
3V to 12V Current-Limiting Hot Swap Controllers with Autoretry, DualSpeed/BiLevel Fault Protection
General Description
The MAX4271/MAX4272/MAX4273 comprise a complete family of integrated 3V to 12V hot swap controllers. They allow the safe insertion and removal of circuit cards into live backplanes. The discharged filter capacitors of the circuit card provide a low impedance to the live backplane. High inrush currents from the backplane to the circuit card can burn up connectors and components, or momentarily collapse the backplane power supply leading to a system reset. This family of hot swap controllers prevents such problems by regulating the current to a preset limit when the board is plugged in, allowing the system to stabilize safely. After the startup cycle is completed, two on-chip comparators provide DualSpeed/BiLevelTM protection against short circuits, load glitches, and overcurrent conditions. In the event of a fault condition, the load is disconnected. Fault recovery is handled by unlatching (MAX4271), autoretry (MAX4272), or programmed (MAX4273) methods. The MAX4271 family includes many integrated features that reduce component count and design time. An onboard charge pump provides the gate drive for a lowcost, external N-FET. Integrated features like startup current regulation and current glitch protection eliminate external timing resistors and capacitors. Also featured are an open-drain status output to indicate a fault condition, and an adjustable overcurrent response time. The MAX4271 (latched fault protection) and MAX4272 (autoretry fault protection) come in the space-saving 8pin SO package, and the MAX4273 (full function) comes in the space-saving 16-pin QSOP package. All parts are specified across the extended temperature range, and have an absolute maximum rating of 15V to provide extra protection against inductive kickback during board removal.
DualSpeed/BiLevel is a trademark of Maxim Integrated Products.
Features
o Provide Safe Hot Swap for +3V to +12V Power Supplies with Minimum External Components o Unique Current Regulation Architecture Eliminates External Timing Resistors and Capacitors and Minimizes N-FET Linear Mode Duration o Autoretry Feature (MAX4272/MAX4273) Allows Self-Management During Fault o DualSpeed/BiLevel Operation Protects Against Current Glitches and Short Circuits o Power-On RESET (MAX4273) o 15V Absolute Maximum Rating Protects Against Inductive Kickbacks During Board Removal o Internal Charge Pump Generates Gate Drive for External N-MOSFET o Status Output Pin Indicates Fault/Safe Condition o Space-Saving 8-Pin SO, 16-Pin QSOP Packages
MAX4271/MAX4272/MAX4273
________________________Applications
Base Stations RAID Remote-Access Servers Network Routers Network Switches ISDN
Ordering Information appears at end of data sheet. Pin Configurations appear at end of data sheet.
Typical Operating Circuit
BACKPLANE VCC REMOVABLE CARD RSENSE M1 N VOUT CBOARD IN STAT GND ON STAT SENSE GATE
ON
MAX4271 MAX4272
CTIM GND CTIM
________________________________________________________________ Maxim Integrated Products
1
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3V to 12V Current-Limiting Hot Swap Controllers with Autoretry, DualSpeed/BiLevel Fault Protection MAX4271/MAX4272/MAX4273
ABSOLUTE MAXIMUM RATINGS
IN to GND...............................................................................15V STAT, OUTC, LLMON, AUXVCC to GND ...............-0.3V to +14V GATE to GND ..............................................-0.3V to (VIN + 8.5V) GATE to LLMON (Note 1).........................................-1V to +8.5V INC, ON to GND. (Note 2) ........................................-1V to +14V CEXT to GND ...............................................-8.5V to (VIN + 0.3V) CSPD, CTON, REF to GND ...........-0.3V to the lower of (VIN + 0.3V) or +12V VSEN, RTH, CTIM to GND ...........................-0.3V to (VIN + 0.3V) Current into INC, ON (Note 2) ............................................2mA Current into Any Other Pin ...............................................50mA Continuous Power Dissipation (TA = +70C) 8-Pin SO (derate 5.9mW/C above +70C)..................471mW 16-Pin QSOP (derate 8.3mW/C above +70C)...........667mW 16-Pin SO (derate 8.7mW/C above +70C)................696mW Operating Temperature Range ...........................-40C to +85C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10s) .................................+300C
Note 1: GATE can be pulled below LLMON, but current must be limited to 2mA. Note 2: INC and ON can be pulled below ground. Limiting the current to 2mA ensures that these pins are never lower than about -0.8V.
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(VIN =+2.7V to +13.2V, TA = -40C to +85C, unless otherwise noted. Typical values are at VIN = +5V and TA = +25C.) (Note 3)
PARAMETER POWER SUPPLIES Input Voltage Range Supply Current CURRENT CONTROL Slow Comparator Threshold Slow Comparator Response Time VSC,TH tCSPD VIN - VSENSE CSPD = floating CSPD = 100nF to GND 5k on RTH to VIN 75k on RTH to VIN TA = +25C TA = TMIN to TMAX TA = +25C TA = TMIN to TMAX TA = +25C TA = TMIN to TMAX 45 43.5 10 10 45 43.5 675 650 180 200 350 0.2 21 21 31 31 5.5 100 10 41 41 750 20 20 50 50 55 56 40 40 55 56 825 840 220 ns A mV mV s ms VIN IQ ON = VIN 2.7 0.6 13.2 1 V mA SYMBOL CONDITIONS MIN TYP MAX UNITS
Fast Comparator Threshold
VFC,TH
MAX4273 only
MAX4273 RTH = GND, MAX4271/MAX4272 Fast Comparator Response Time SENSE Input Bias Current MOSFET DRIVER Startup Period (Notes 4, 5) Gate Charge Current MAX4271/MAX4272 CTIM = 100nF tSTART IGATE MAX4273 CTON = 100nF No capacitor GATE = IN (Note 6) tFCD IB,SENSE 10mV overdrive, from overload condition to gate discharging VSENSE = VIN
ms s A
2
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3V to 12V Current-Limiting Hot Swap Controllers with Autoretry, DualSpeed/BiLevel Fault Protection
ELECTRICAL CHARACTERISTICS (continued)
(VIN =+2.7V to +13.2V, TA = -40C to +85C, unless otherwise noted. Typical values are at VIN = +5V and TA = +25C.) (Note 3)
PARAMETER SYMBOL CONDITIONS Time from trigger to VGATE < 0.1V (triggered by either the ON input or the slow comparator), CGATE = 1000pF to GND Time from current overload to VGATE < 0.1V (triggered by the fast comparator, during normal operation), CGATE = 1000pF to GND Voltage at which internal zener clamp circuitry is triggered, measured with respect to VIN (MAX4271/MAX4272), measured with respect to VLLMON (MAX4273) Measured with respect to VIN, IGATE = 8.5A VIN 5V VIN 2.7V 5 V 2.7 0.4 25 1 70 2.75 195 A 75 200 550 mA MIN TYP MAX UNITS
MAX4271/MAX4272/MAX4273
Slow Turn-Off Time
tSLOW,OFF
60
s
Fast Turn-Off Time
tFAST,OFF
15
s
Maximum Gate Protection Voltage
6.7
7.5
V
Minimum Gate Drive Voltage
During fast discharge, due to a fast comparator fault in normal operation Gate Discharge Current IGATE,DIS During startup (current regulation provided by fast comparator) During normal discharge, due to a slow comparator fault in normal mode, or by ON going low LLMON Overvoltage Threshold GATE Overvoltage Threshold LLMON Impedance REFERENCE (MAX4273) Output Voltage Line Regulation Load Regulation ON AND RESET COMPARATORS Threshold Voltage Hysteresis Power-Supply Rejection Ratio Propagation Delay VHYST PSRR tD,COMP 2.7V VIN 13.2V 10mV overdrive, ON going positive or negative, INC going negative INC going positive VREF
VREF, LINE
Startup is initiated only after VLLMON is less than this voltage (MAX4273) Startup is initiated only after VGATE is less than this voltage Impedance to GND, after a fault (MAX4273) No load, VIN = 5V 2.7V VIN 13.2V, no load IREF = 0 to 100A, VIN = 5V
0.1 0.1 1 1.176 1.2 4 1 1.224 8 3 0.6
V V k V mV mV
VREF,
LOAD
VIN = 5V, rising threshold at ON or INC
0.575
0.6 3
0.625 1
V mV mV/V s
10 100 150 200
ms
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3
3V to 12V Current-Limiting Hot Swap Controllers with Autoretry, DualSpeed/BiLevel Fault Protection MAX4271/MAX4272/MAX4273
ELECTRICAL CHARACTERISTICS (continued)
(VIN =+2.7V to +13.2V, TA = -40C to +85C, unless otherwise noted. Typical values are at VIN = +5V and TA = +25C.) (Note 3)
PARAMETER Input Voltage Range Input Bias Current ON Pulse Width Low (Note 7) DIGITAL OUTPUTS (STAT, OUTC) Output Leakage Current Output Voltage Low RETRY TIMING Retry Timeout Period tRETRY 100nF capacitor on CTIM (Note 5) Startup is initiated when this threshold is exceeded at IN 0.5 1 2 s UNDERVOLTAGE LOCKOUT (UVLO) Threshold Hysteresis Delay VUVLO VUVLO,HYST tD,UVLO Time the input voltage must exceed undervoltage lockout before startup is initiated 100 2.25 100 150 200 2.67 V mV ms VOL VSTAT 13.2, VOUTC 13.2 ISINK = 1mA 1 0.4 A V IB,COMP tRESTART To unlatch a fault MAX4271, MAX4273 with CTIM = IN 20 SYMBOL CONDITIONS Input can be driven to the absolute maximum limit without false output inversion MIN -0.1 0.001 TYP MAX 13.2 1 UNITS V A s
Note 3: All devices are 100% tested at TA = +25C. All temperature limits are guaranteed by design. Note 4: Startup time is the time during which the slow comparator is ignored and the fast comparator regulates the sense current. It is measured from the time ON is brought high. Note 5: Inferred from test with 1nF. Note 6: The current available at GATE is a function of VGATE (see Typical Operating Characteristics). Note 7: Guaranteed by design.
4
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3V to 12V Current-Limiting Hot Swap Controllers with Autoretry, DualSpeed/BiLevel Fault Protection
Typical Operating Characteristics
(VIN = 5V, RSENSE = 100m, CBOARD = 47F, TA = +25C, unless otherwise noted.)
MAX4271/MAX4272/MAX4273
SUPPLY CURRENT vs. INPUT VOLTAGE
MAX4271/3-01
SUPPLY CURRENT vs. TEMPERATURE
0.9 0.8 SUPPLY CURRENT (mA) 0.7 0.6 0.5 0.4 0.3 0.2 0.1 VIN = 3V 20.0 19.0 18.0 -15 10 35 60 85 TEMPERATURE (C) VIN = 12V VIN = 5V ON = VIN
MAX4271/3-02
SLOW COMPARATOR RESPONSE TIME vs. INPUT VOLTAGE
24.0 23.0 22.0 tCSPD (s) 21.0
MAX4271/3-03
1.0 0.9 0.8 SUPPLY CURRENT (mA) 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 2 4 6 8 10 12 ON = GND ON = VIN IGATE = 10A
1.0
24 23 22 21 20 19 18 tCSPD (ms)
CSPD = 110nF TIME IN ms
CSPD = 0 TIME IN s
14
0 -40
0
2
4
6
8
10
12
14
VIN (V)
VIN (V)
SLOW COMPARATOR THRESHOLD vs. INPUT VOLTAGE
MAX4271/3-04
FAST COMPARATOR RESPONSE TIME vs. OVERDRIVE VOLTAGE
MAX4271/3-05
FAST COMPARATOR RESPONSE TIME vs. TEMPERATURE
MAX4271/3-06
51.0 50.8 50.6 50.4 VSC,TH (mV) 50.2 50.0 49.8 49.6 49.4 49.2 49.0 0 2 4 6 8 10 12 TA = +85C TA = -40C TA = +25C
450
500 450 RESPONSE TIME (ns) 400 350 300 250 200 VIN = 3V VIN = 5V VIN = 12V
400 RESPONSE TIME (ns)
VIN = 12V
350 VIN = 5V 300 VIN = 3V
250
14
200 1 10 100 1000 OVERDRIVE VOLTAGE (mV)
-40
-15
10
35
60
85
VIN (V)
TEMPERATURE (C)
FAST COMPARATOR THRESHOLD vs. INPUT VOLTAGE
208 206 204 VFC,TH (mV) 202 200 198 196 194 192 190 0 2 4 6 8 VIN (V) 10 12 14 0.1 0.01 TA = +85C TA = -40C TA = +25C
MAX4271/3-07
TIME TO FAST DISCHARGE GATE vs. CGATE
MAX4271/3-08
STARTUP TIME vs. INPUT VOLTAGE
350 CTIM = 100nF TIME IN ms
MAX4271/3-09
210
100k TIME TO DISCHARGE GATE (s) 10k 1k 100 10 1 VIN = 3V
35
330
33 tSTART (ms)
tSTART (s)
VIN = 12V VIN = 5V
310 CTIM = 1nF TIME IN s
31
290
29
270
27
0.1
1
10
100
1000
250 0 2 4 6 8 10 12 14 VIN (V)
25
CGATE (nF)
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5
3V to 12V Current-Limiting Hot Swap Controllers with Autoretry, DualSpeed/BiLevel Fault Protection MAX4271/MAX4272/MAX4273
Typical Operating Characteristics (continued)
(VIN = 5V, RSENSE = 100m, CBOARD = 47F, TA = +25C, unless otherwise noted.)
FAST GATE DISCHARGE CURRENT vs. VGATE
MAX4271/3-10
GATE CHARGE CURRENT vs. GATE VOLTAGE
VIN = 12V
MAX4271/3-11
GATE CHARGE CURRENT vs. TEMPERATURE
VGATE = 0 125 100 VIN = 12V
MAX4271/3-12
1.8 FAST GATE DISCHARGE CURRENT (mA) 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 0 2 4 6 8 VIN = 3V VIN = 5V VIN = 12V
120 100 80 IGATE (A) VIN = 3.0V 60 40 20 0
150
VIN = 5.0V
IGATE (A)
75 50
VIN = 5V
VIN = 3V 25 0 0 2 4 6 8 10 12 14 16 18 20 -40 -15 -10 35 60 85 VGATE (V) TEMPERATURE (C)
10 12 14 16 18 20 VIN (V)
GATE VOLTAGE vs. INPUT VOLTAGE
MAX4271/3-13
SLOW GATE DISCHARGE CURRENT vs. GATE VOLTAGE
MAX4271/3-13a
TIME TO CHARGE GATE vs. CGATE
NO EXTERNAL MOSFET TIME TO CHARGE GATE (ms) 100 10 1 0.1 0.01 0.001 0.01 VIN = 3V TO VGATE = 6V VIN = 12V TO VGATE = 17V
MAX4271/3-14
25 IGATE = 10A 20 TA = +85C TA = +25C
400 350 300 IGATE (A) 250 200 150 100 50 TRIGGERED BY A FAULT OR BY ON FALLING 0 2 4 6 8 VIN = 5V VIN = 12V VIN = 3V
1000
VGATE (V)
15
10
TA = -40C
VIN = 5V TO VGATE = 10V
5
0 0 2 4 6 8 10 12 14 VIN (V)
0 10 12 14 16 18 20 VGATE (V)
0.1
1
10
100
1000
CGATE (nF)
FAST GATE DISCHARGE CURRENT vs. VIN
MAX4271/3-15
SLOW GATE DISCHARGE CURRENT vs. TEMPERATURE
MAX4271/3-16
TIME TO SLOW DISCHARGE GATE vs. CGATE
NO EXTERNAL MOSFET DISCHARGE TO VGATE = 0.1V
MAX4271/3-17
4.0 FAST GATE DISCHARGE CURRENT (mA) RISING VIN 3.5 3.0
400 350 VIN = 3V 300 IGATE (A) 250 200 150 100 VIN = 5V, 12V VGATE = VIN TRIGGERED BY A FAULT OR BY ON FALLING -40 -15 10 35 60
1000 TIME TO DISCHARGE GATE (ms) 100 10 1
VIN = 12V
2.5 2.0 1.5 FALLING VIN 1.0 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 VIN (V)
VIN = 5V 0.1 0.01 VIN = 3V
50 0
85
0.001 0.01
0.1
1
10
100
1000
TEMPERATURE (C)
CGATE (nF)
6
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3V to 12V Current-Limiting Hot Swap Controllers with Autoretry, DualSpeed/BiLevel Fault Protection
Typical Operating Characteristics (continued)
(VIN = 5V, RSENSE = 100m, CBOARD = 47F, TA = +25C, unless otherwise noted.)
FAST GATE DISCHARGE CURRENT vs. TEMPERATURE
FAST GATE DISCHARGE CURRENT (mA) VIN = 3V 2.2 2.0 1.8 1.6 1.4 VIN = 5V, 12V 1.2 1.0 -40 -15 10 35 60 85 TEMPERATURE (C) 50s/div CBOARD = 470F, RSENSE = 100m, CGATE = 0 200s/div CBOARD = 470F, RSENSE = 100m, CGATE = 22nF, RS = 0 0V VGATE 2V/div 0V VOUT 2V/div VGATE = VIN
MAX4271/3-18
MAX4271/MAX4272/MAX4273
MAX4271/MAX4272 TURN-OFF TIME (CBOARD = 470F)
MAX4370-18
MAX4271/MAX4272 TURN-OFF TIME (EXTERNAL CGATE = 22nF, CBOARD = 470F)
MAX4370-20
2.4
ON
ON VGATE 2V/div ILOAD 1A/div 0A
ILOAD 1A/div 0A
VOUT 2V/div
ON COMPARATOR THRESHOLD vs. INPUT VOLTAGE
MAX4271/3-21
ON COMPARATOR THRESHOLD vs. TEMPERATURE
MAX4271/3-22
0.605 ON COMPARATOR THRESHOLD (V)
0.6100 ON COMPARATOR THRESHOLD (V) 0.6075 0.6050 0.6025 0.6000 VIN = 3V 0.5975 0.5950 VIN = 12V VIN = 5V
0.603
RISING
0.601 FALLING 0.599
0.597
} }
10 35 60 85 TEMPERATURE (C)
0.595 0 2 4 6 8 10 12 14 VIN (V)
-40
-15
UVLO THRESHOLD VOLTAGE vs. TEMPERATURE
MAX4271/3-26
UVLO DELAY vs. TEMPERATURE
MAX4271/3-27
2.60 2.55 UVLO THRESHOLD (V) RISING 2.50 2.45 FALLING 2.40 2.35 2.30 -40 -15 10 35 60
160
155 UVLO DELAY (ms)
VIN = 5V, 12V VIN = 3V
150
145
140 85 -40 -15 10 35 60 85 TEMPERATURE (C) TEMPERATURE (C)
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FALLING RISING
7
3V to 12V Current-Limiting Hot Swap Controllers with Autoretry, DualSpeed/BiLevel Fault Protection MAX4271/MAX4272/MAX4273
Typical Operating Characteristics (continued)
(VIN = 5V, RSENSE = 100m, CBOARD = 47F, TA = +25C, unless otherwise noted.)
MAX4273 REFERENCE VOLTAGE vs. INPUT VOLTAGE
MAX4271/3-29
INC THRESHOLD vs. INPUT VOLTAGE
MAX4271/3-28
INC THRESHOLD vs. TEMPERATURE
620 618 616 THRESHOLD (mV) 614 612 610 608 606 FALLING RISING 1.2040 1.2038 REFERENCE VOLTAGE (V) 1.2036 1.2034 1.2032 1.2030 1.2028 1.2026 1.2024 1.2022 1.2020 -40 -15 10 35 60 85 3 5
RISING 0.615 THRESHOLD (V)
0.605
FALLING
0.595
0.585
604 602
0.575 4 6 8 10 12 14 16 INPUT VOLTAGE (V)
600 TEMPERATURE (C)
7
9
11
13
15
INPUT VOLTAGE (V)
REFERENCE VOLTAGE vs. TEMPERATURE
MAX4271/3-31
RETRY TIMEOUT vs. INPUT VOLTAGE
CTIM = 100nF 945 RETRY TIMEOUT (ms) 940 935 930 925 920
MAX4271/3-32
1.2040 1.2035 REFERENCE VOLTAGE (V) 1.2030 1.2025 1.2020 1.2015 1.2010 1.2005 1.2000 -40 -15 10 35 60
950
85
3
5
7
9
11
13
15
TEMPERATURE (C)
INPUT VOLTAGE (V)
RETRY TIMEOUT PERIOD vs. TEMPERATURE
MAX4271/3-33
STARTUP TIME (CBOARD = 470F)
MAX4271/3-34
995 990 RETRY PERIOD (ms) 985 980 975 970 965 -40 -15 10 35 60 3V 5V
ON VGATE 2V/div
12V
ILOAD 1A/div
VOUT 2V/div
85
TEMPERATURE (C)
500s/div CBOARD = 470F, RSENSE = 100m, CTIM = 10nF, CGATE = 0
8
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MAX4271/3-30
0.625
3V to 12V Current-Limiting Hot Swap Controllers with Autoretry, DualSpeed/BiLevel Fault Protection
Typical Operating Characteristics (continued)
(VIN = 5V, RSENSE = 100m, CBOARD = 47F, TA = +25C, unless otherwise noted.)
TURN-OFF TIME (CBOARD = 0)
MAX4271/3-37
MAX4271/MAX4272/MAX4273
STARTUP TIME (CBOARD = 0)
MAX4271/3-35
ON VGATE 2V/div VGATE 2V/div
ON ILOAD 1A/div 0A
VOUT 2V/div
VOUT 2V/div
0V
100s/div CBOARD = 0, RSENSE = 100m, CTIM = 10nF, CGATE = 0
50s/div CBOARD = 0, RSENSE = 100m, CGATE = 0, RS = 0
Pin Description
PIN MAX4271 MAX4272 MAX4273 NAME FUNCTION Current-Sense Threshold Setting Input. Connect a resistor from RTH to VIN to set the fast comparator threshold. Bypass to VIN with 0.1F. Connect RTH to VIN to disable both the fast comparator and current regulation at startup. Short RTH to GND for a 200mV threshold. See Fast Comparator Threshold section. Auxiliary Supply Input. Supply input for short-circuit switchover. To use this, connect a 1F capacitor from AUXVCC to GND; otherwise, leave floating. See Auxiliary VCC section. Input Voltage. Connect to +2.7V to +13.2V supply; 15V absolute maximum voltage rating. Current-Sense Resistor Voltage Input. RSENSE is connected from IN to SENSE. Gate Drive Output. Connect to gate of external N-channel pass transistor. External Gate Capacitance Connection. Connect a capacitor from CEXT to GATE to increase the gate charging time. This pin goes high impedance during a fast comparator fault for fast discharge. Load Line Monitor. Connect to the source of the external N-channel MOSFET. The external FET is turned on only when the load voltage is less than 100mV. Ground Startup Timer Input. Leave floating or connect the timing capacitor from CTON to GND. See Startup and Retry Timer section. Slow Comparator Speed Setting. Leave floating or connect the timing capacitor from CSPD to GND. See Slow Comparator Timer Setting section.
--
1
RTH
--
2
AUXVCC
1 2 3 --
3 4 5 6
IN SENSE GATE CEXT
-- 4 -- 5
7 8 9 10
LLMON GND CTON CSPD
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9
3V to 12V Current-Limiting Hot Swap Controllers with Autoretry, DualSpeed/BiLevel Fault Protection MAX4271/MAX4272/MAX4273
Pin Description (continued)
PIN MAX4271 MAX4272 MAX4273 NAME FUNCTION Startup and Retry Timers Input. Controls the startup time and the autoretry time (32x startup time) in the MAX4272, only the autoretry time in the MAX4273, and only the startup time in the MAX4271. Leave floating or connect the timing capacitor from CTIM to GND. Connect to IN for latched mode (this prevents autoretry in MAX4273). See Startup and Retry Timer section. Status Output. High indicates startup completed with no fault (Table 1). STAT is an open-drain output. ON Comparator Input. Connect high for normal operation; connect low to force the MOSFET off. Comparator threshold VTH,ON = 0.6V allows for precise control over shutdown feature. Pulse ON low for 20s min to unlatch after a fault (MAX4273 in latched mode, or MAX4271). Negative pulses are ignored during autoretry (MAX4273 in autoretry mode, or MAX4272). See ON and Delayed Comparator section. 1.2V Reference Output. Do not bypass with a capacitor to GND. Uncommitted Comparator Input. Controls OUTC. Uncommitted Comparator Output. OUTC goes high 150ms after INC goes high. OUTC goes low immediately after INC goes low. OUTC is an open-drain output.
6
11
CTIM
7
12
STAT
8
13
ON
-- -- --
14 15 16
REF INC OUTC
Detailed Description
The MAX4271/MAX4272/MAX4273 are circuit breaker ICs designed for hot swap applications where a line card is inserted into a live backplane. Normally, when a line card is plugged into a live backplane, the card's discharged filter capacitors provide a low impedance that can momentarily cause the main power supply to collapse. The MAX4271/MAX4272/MAX4273 are designed to reside either in the backplane or in the removable card to provide inrush current limiting and short-circuit protection. This is achieved by using an external N-channel MOSFET, an external current-sense resistor, and two on-chip comparators. Figures 1 and 2 show the functional diagrams. The timing and voltage levels for several critical parameters can be adjusted with external resistors, external capacitors, or by pin strapping. The timing components are optional; without them, the part is set to its nominal values, as shown in the Electrical Characteristics. The parameters that can be adjusted are: * Current-limit threshold * Slow comparator response time * Startup timer * Fast comparator threshold
10
* Autoretry timeout period (time the part is shut down after an overcurrent event) * Shutdown configuration (latched/autoretry) * Current overload threshold
Startup Mode
CTIM (MAX4271/MAX4272) or CTON (MAX4273) sets the startup period (see Startup and Retry Timers). The startup period begins after three conditions are met: 1) 150ms after VIN exceeds the UVLO threshold (see Undervoltage Lockout) AND 2) 10s after VON >0.6V AND 3) The device is no longer in retry mode. During startup, the slow comparator is disabled and the inrush current can be limited in two ways: 1) Slow ramping of the current to the load, by controlling the external MOSFET gate voltage OR 2) Limiting the current to the load by regulating the voltage across the external current-sense resistor
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3V to 12V Current-Limiting Hot Swap Controllers with Autoretry, DualSpeed/BiLevel Fault Protection MAX4271/MAX4272/MAX4273
CSPD VIN CSPD IN 6A 2.45V
MAX4271 MAX4272
SLOW COMPARATOR
6A
VSC,TH 50mV RSENSE
INPUT UVLO 150ms DELAY ON VIN RISING STAT
VSC,TH 200mV FAST COMPARATOR
SENSE GATE DRIVE CHARGE PUMP 0.1V 4A GATE OVLO ON ON COMPARATOR 0.6V VOUT GND GATE
DISCHARGE LOGIC CONTROL VIN ENABLE
M1 N
CTIM CTIM
Figure 1. MAX4271/MAX4272 Functional Diagram
Unlike other circuit breaker ICs, the MAX4271/ MAX4272/MAX4273 regulate the current to a preset level instead of completely turning off the external N-FET if an overcurrent condition occurs during startup. In startup mode, the gate drive current is limited to 100A and decreases with the increase of the gate voltage (see Gate Charge Current vs. Gate Voltage in the Typical Operating Characteristics). This allows the controller to slowly enhance the MOSFET. If the fast comparator detects an overcurrent, the gate voltage is momentarily discharged with a fixed 70A current until the load current through the sense resistor (RSENSE) decreases below its threshold point. This effectively regulates the inrush current during startup. Figure 3 shows the startup waveforms. STAT goes high at the end of the startup period if no fault condition is present.
Normal Operation (DualSpeed/BiLevel)
In normal operation (after the startup timer has expired), protection is provided by turning off the external MOSFET when a fault condition is encountered. DualSpeed/BiLevel fault protection incorporates two comparators with different thresholds and response times to monitor the current: 1) Slow comparator. This comparator has an externally set response time (from 20s to seconds) and a fixed 50mV threshold voltage. The slow comparator ignores low-amplitude momentary current glitches. After an extended overcurrent condition, a fault is generated and the MOSFET gate is slowly discharged. 2) Fast comparator. This comparator has a fixed 350ns response time and a 200mV threshold voltage (adjustable from 50mV to 750mV in MAX4273). The fast comparator turns off the MOSFET immediately after it detects a large amplitude event such as a short circuit.
11
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3V to 12V Current-Limiting Hot Swap Controllers with Autoretry, DualSpeed/BiLevel Fault Protection MAX4271/MAX4272/MAX4273
CSPD VIN AUXVCC VIN 2.45V 6A VFC,TH = 50mV IN
MAX4273
6A REF REF 1.2V INPUT UVLO 150ms DELAY ON VIN RISING SLOW COMPARATOR VFC,TH = 200mV RTH RSENSE
RTH FAST COMPARATOR VFC,TH = 50mV TO 750mV 10A SENSE CEXT CEXT AUXVCC LOGIC CONTROL DISCHARGE ENABLE CHARGE PUMP GATE DRIVE GATE Q1 N
AUXVCC
STAT
0.1V
GATE OVLO
VIN 0.1V 4A CTIM CTIM VIN 4A CTON CTON ON COMPARATOR 0.6V RESET COMPARATOR 150ms DELAY ON INC RISING ON OUTPUT OVLO LLMON
OUTC
INC
0.6V VOUT
GND
Figure 2. MAX4273 Functional Diagram 12 ______________________________________________________________________________________
3V to 12V Current-Limiting Hot Swap Controllers with Autoretry, DualSpeed/BiLevel Fault Protection
In each case, when a fault is encountered, the status pin (STAT) goes low, and for the MAX4273, the device discharges the output voltage through a 1k resistor from LLMON to GND. After a fault, the MAX4271 stays latched off and the MAX4272 enters retry mode, while the MAX4273 has selectable latched or retry mode. Figure 4 shows the waveforms of a fault condition. the trip point of 1.2V. Therefore, a pulsing current with a duty cycle of 50% or greater (i.e., the current is above the threshold level > 50% of the time) is considered a fault condition even if it is never higher than the threshold for a longer period than the slow comparator's set response time. The discharge rate depends on the N-FET gate capacitance and the external capacitance at GATE. In the MAX4273, CEXT remains connected and capacitance to this point has to be discharged by the same current. This increases the discharge time. Once the fault condition is detected, the STAT pin goes low and the device goes into retry or latched mode. Fast Comparator The fast comparator behaves differently according to the operating mode. During startup, the fast comparator is part of a simple current regulator. When the sensed current is above the fast comparator threshold, the gate is discharged with a 70A current source. When the sensed current drops below the threshold, the charge pump turns on again. The sensed current will rise and fall near the threshold due to the fast comparator and charge-pump propagation delay. The gate voltage will be roughly saw-tooth shaped, and the load current will present a 20% ripple. The ripple can be reduced by adding a capacitor from GATE to GND.
MAX4271/MAX4272/MAX4273
BiLevel Fault Protection
Slow Comparator The slow comparator is disabled at startup while the external MOSFET is turning on. This allows the part to ignore the higher-than-normal inrush current charging the board capacitors when a card is first plugged in. If the slow comparator detects an overload condition while in normal operation (after startup is completed), it turns off the external MOSFET by discharging the gate capacitance with a 200A current. The slow comparator threshold is set at 50mV and has a default delay of 20s (CSPD floating), allowing it to ignore power-supply glitches and noise. The response time can be lengthened with an external capacitor at CSPD (Figure 10). If the overcurrent condition is not continuous, then the duration above the threshold minus the duration below must be greater than 20s (or the external programmed value) for the device to trip. When the current is above the threshold, CSPD is charged with a 6A current source. A fault is detected when CSPD is charged to
STAT ON STAT tSTART VGATE 2.7V TO 6V VOUT VGATE VOUT IFAST, SET VTH VGATE 2.7V TO 6V VOUT
CBOARD = LARGE CBOARD = 0 ILIM ILOAD tON tOFF
ILOAD
Figure 3. Startup Waveforms
Figure 4. Response to a Fault Condition 13
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3V to 12V Current-Limiting Hot Swap Controllers with Autoretry, DualSpeed/BiLevel Fault Protection MAX4271/MAX4272/MAX4273
If the sensed current is still high after the startup timer expires, the MOSFET gate is discharged completely as described below. In normal operation (after startup), the fast comparator is used as an emergency off switch. If the load current reaches the fast comparator threshold, the device quickly forces the MOSFET off completely. This could happen in the event of a serious current overload or a dead short. The fast comparator has a 350ns response time and discharges GATE with a 1mA current. Given a 1000pF gate capacitance and 12V gate voltage, the MOSFET will be off in about 12s. Any additional capacitance connected between GATE and GND to slow down the startup time also increases the turn-off time. In the MAX4273, CEXT goes high impedance during the fast discharge. This reduces the effective capacitance on GATE if a capacitor is used between GATE and CEXT, and allows the MOSFET to quickly turn off. In turn, this allows adjustment of the MOSFET charging time without affecting the fast discharge rate, although it does affect the slow discharge rate. The MAX4271/MAX4272 fast comparator threshold is set to four times the slow comparator threshold (i.e., 200mV). The MAX4273 fast comparator threshold is set to 200mV by connecting RTH to GND, is disabled by connecting RTH to IN, or is adjustable by an external resistor connected to IN (see Fast Comparator Threshold (RTH)). Pulse ON low for 20s (min) to restart after a fault (MAX4271/MAX4273 in latched mode). Negative pulses are ignored during autoretry (MAX4273 in autoretry mode, or MAX4272). The capacitor on CTIM affects the MAX4272's retry timeout period (time the part is shut down after an overcurrent event) and the startup time (see the Electrical Characteristics). The retry timeout period is fixed at 32 times the startup time in order to minimize power dissipation in the external MOSFET in case of a short-circuit condition (see MOSFET Thermal Considerations). This is not an issue for parts latched off during a fault condition since they stay off until commanded on. The MAX4273 configured in retry mode has a separate startup timer capacitor (CTON) and retry timeout capacitor (CTIM). This allows the user to change the ratio between startup time and retry timeout period.
Status Output
The status output is an open-drain output that goes low under the following conditions: * During the UVLO delay period * In startup * Forced off (ON <0.6V) * In an overcurrent condition * In the retry timeout period (or latched off, for the latched parts) STAT is high only if the part is in normal mode and no faults are present (Table 1). Figure 5 shows the status (STAT) output timing diagram.
Latched/Autoretry
The MAX4271 MOSFET driver stays latched off after a fault condition until it is reset by a negative-going pulse on the ON pin. The MAX4272 is periodically turned on after a fault condition with a timeout duration set by an external timing capacitor on CTIM. The MAX4273 has a selectable latched mode or retry mode. Connect CTIM to IN to set the device in latched mode, or use an external capacitor at CTIM to set the retry timeout.
Over/Undervoltage Lockouts
The UVLO prevents the MAX4271/MAX4272/MAX4273 from turning on the external MOSFET until VIN exceeds the lockout threshold (2.25V min) for 150ms. The UVLO protects the external MOSFET from insufficient gate drive voltage. The 150ms timeout ensures that the board is fully plugged into the backplane and that VIN
Table 1. Status Output Truth Table
IN UVLO DELAY PERIOD Yes X X X X No PART IN STARTUP X Yes X X X No ON PIN X X Low X X High OVERCURRENT CONDITION X X X Yes X No PART IN RETRY-TIMEOUT PERIOD (OR LATCHED OFF DUE TO OVERCURRENT CONDITION) X X X X Yes No STAT PIN (STATUS) Low Low Low Low Low High
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3V to 12V Current-Limiting Hot Swap Controllers with Autoretry, DualSpeed/BiLevel Fault Protection
is stable. Any input voltage transient at IN below the UVLO threshold will reset the device and initiate a startup sequence. These devices also have an overvoltage lockout (OVLO) feature that prevents the device from restarting after a fault condition if the discharge has not been completed. VGATE has to be discharged to below 0.1V. Additionally, the MAX4273 LLMON pin discharges the load line with a 1k pulldown and prevents startup until the load voltage is below 0.1V. Since the MAX4271/MAX4272 do not monitor the output voltage, a startup sequence can be initiated while the board capacitance is still charged. A large board capacitance or a short startup period may prevent the MAX4272 from charging completely in one startup period. The MAX4272 responds to these conditions by charging the capacitor with bursts defined by a tON duty cycle and a period of tON + tRETRY. The charging will be complete after several retries unless the resistive load or current load excessively discharges the board capacitance during the retry timeout. This feature applies to the MAX4273 if LLMON is left floating or is connected to GND. To prevent multiple charging bursts, ensure that the tON timer exceeds the minimum time required to complete the charge of the board capacitance (see Component Selection).
__________Applications Information
Component Selection
N-Channel MOSFET Select the external N-channel MOSFET according to the application's current level. Table 2 lists some recommended components. The MOSFET's OnResistance (RDS(ON)) should be chosen low enough to have a minimum voltage drop at full load to limit the MOSFET power dissipation. High RDS(ON) can cause output ripple if the board has pulsing loads or triggers an external undervoltage reset monitor at full load. Determine the device power-rating requirement to accommodate a short circuit on the board at startup
IN
MAX4271/MAX4272/MAX4273
RSENSE MAX4271 MAX4272 SENSE
VGD GATE DRIVE CHARGE PUMP GATE M1 N VGS VOUT D1 CBOARD
Gate Overvoltage Protection
New-generation MOSFETs have an absolute maximum rating of 8V for the gate-to-source voltage (VGS). To protect these MOSFETs, the MAX4271/MAX4272 limit the gate-to-drain voltage (the MAX4273 limits the gate-tosource voltage) to +7.5V with an internal zener diode. No protection is provided for negative V GS (MAX4271/ MAX4272). If GATE can be discharged to ground faster than the output voltage, an external small-signal protection diode (D1) can be used, as shown in Figure 6. The MAX4273 has the protection diode internal.
ON VIN OV tSTART 1.2V CTIM (CTON)* OV FAULT CONDITION, OR ON FALLING EDGE VIN STAT OV NO FAULT CONDITIONS PRESENT *MAX4273 ONLY
Figure 6. External Gate-Source Protection
Table 2. Recommended N-Channel MOSFETs
PART NUMBER IRF7413 IRF7401 IRL3502S MMSF3300 MMSF5N02H MTB60N05H FDS6670A NDS8426A FDB8030L Fairchild Motorola International Rectifier MANUFACTURER DESCRIPTION 11m, 8 SO, 30V 22m, 8 SO, 20V 6m, D2PAK, 20V 20m, 8 SO, 30V 30m, 8 SO, 20V 14m, D2PAK, 50V 10m, 8 SO, 30V 13.5m, 8 SO, 20V 4.5m, D2PAK, 30V
Figure 5. Status (STAT) Output Timing Diagram ______________________________________________________________________________________ 15
3V to 12V Current-Limiting Hot Swap Controllers with Autoretry, DualSpeed/BiLevel Fault Protection MAX4271/MAX4272/MAX4273
with the device configured in automatic retry mode (see Thermal Considerations). Using the MAX4271/MAX4273 in latched mode allows the consideration of MOSFETs with higher RDS(ON) and lower power ratings. A MOSFET can typically withstand single-shot pulses with higher dissipation than the specified package rating. Low MOSFET gate capacitance is not necessary since the inrush current limiting is achieved by limiting the gate dv/dt. Table 3 lists some recommended manufacturers and components. Sense Resistor The slow comparator threshold voltage is set at 50mV. Select a sense resistor that causes a 50mV voltage drop at a current level above the maximum normal operating current; typically, set the overload current at 1.2 to 1.5 times the nominal load current. The fast comparator threshold is typically set at 200mV. This will set the fault current limit at four times the overload current limit. The MAX4273 fast comparator threshold can be set between 50mV and 750mV; see Table 4 for a detailed listing. Choose the sense resistor power rating according to the device configuration. If no retry mode is selected, PRSENSE = (IOVERLOAD)2 x RSENSE; if retry is selected, then PRSENSE = (IFAULT)2 x RSENSE x (tON/tRETRY). Fast Comparator Threshold (RTH) (MAX4273) The fast comparator threshold is determined by the external resistor connected at RTH. To select threshold voltages between 50mV and 750mV, use resistor values between 5k and 75k according to Figure 7. Resistor values between 200 and 5k are not recommended. Setting the threshold voltage of the fast comparator below 50mV will effectively override the slow comparator operation. The MAX4273 fast comparator can be disabled by shorting the RTH pin to VIN (VIN 25mV or less). Ground RTH to set the threshold to 200mV internally. Startup and Retry Timers (CTIM, CTON) The startup (tSTART) and retry (tRETRY) timers are determined by the capacitors connected at CTIM and CTON. The capacitor connected to CTIM has two functions for the three devices as follows:
DEVICE MAX4271 MAX4272 MAX4273 CTIM FUNCTION Startup time Startup time and sets retry timer Sets retry timer
Table 3. Component Manufacturers
COMPONENT Sense Resistors IRC Fairchild MOSFETS International Rectifier Motorola MANUFACTURER Dale-Vishay PHONE 402-564-3131 704-264-8861 888-522-5372 310-322-3331 602-244-3576 WEB www.vishay.com www.irctt.com www.fairchildsemi.com www.irf.com www.mot-sps.com/ppd/
Table 4. Current Levels vs. RSENSE
RSENSE (m) 10 50 100 PART NUMBER MAX4271/MAX4272 MAX4273 MAX4271/MAX4272 MAX4273 MAX4271/MAX4272 MAX4273 OVERLOAD THRESHOLD SET BY SLOW COMPARATOR (A) 5 5 1 1 0.5 0.5 FAULT CURRENT THRESHOLD SET BY FAST COMPARATOR (A) 20 5 to 75 4 1 to 15 2 0.5 to 7.5
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3V to 12V Current-Limiting Hot Swap Controllers with Autoretry, DualSpeed/BiLevel Fault Protection
CTON determines the maximum time allowed to complete turn-on for the MAX4273. The default values for turn-on time (tON) and tRETRY are chosen by leaving these pins floating; they are 10s and 320s, respectively. These are also the minimum values (not controlled and dependent on stray capacitance). Longer timings are determined by the size of the capacitor according to Figure 8, and can be determined in Table 5, which lists the startup and retry timing parameters Set the tON timer long enough to allow for the MOSFET to be enhanced and the load capacitor to be charged completely . There are two ways of completing the startup sequence. Case A describes a startup sequence that does not use the current-limiting feature and slowly turns on the MOSFET by limiting the gate dv/dt. Case B uses the currentlimiting feature and turns on the MOSFET as fast as possible while still preventing a high in-rush current. Case A. Startup Without Current Regulation There are three ways to turn on the MOSFET without reaching the fast comparator current limit: 1) If the board capacitance is small, the inrush current is low. 2) If the gate capacitance is high, the MOSFET turns on slowly. 3) The fast comparator can be disabled (MAX4273 only). In all three cases, tON is determined only by the charge required to enhance the MOSFET. Effectively, the small gate-charging current limits the output voltage dv/dt. This time can be extended by connecting an external capacitor between GATE and GND (MAX4271/ MAX4272) (Figure 9) or between GATE and CEXT (MAX4273). The turn-on time is dominated by the external gate capacitance if this value is considerably higher than the MOSFET gate capacitance. Table 6 shows the timing required to enhance the recommended MOSFET with or without the external capacitor; Figure 3 shows the related waveforms and timing diagrams. (See Time to Charge Gate vs. C GATE and Startup Time with CBOARD = 0 in the Typical Operating Characteristics.) Remember that a high gate capacitance also increases the turn-off time (t OFF ), except in the case of a MAX4273 fast fault. If an external gate capacitor is not used, RS is not necessary. RS prevents MOSFET source oscillations that can occur when CGATE is high while CBOARD is low. Electrical characteristics as specified by the manufacturer's data sheet are: FDS6670A: CISS = 3200pF, QT(MAX) = 50nC, RDS(ON) = 8.2m
70 80
MAX4271/MAX4272/MAX4273
FAST COMPARATOR THRESHOLD VOLTAGE vs. RTH
800 700 600 VTH, FC (mV) 500 400 300 200 100 0 0 10 20 30 40 RTH (k) 50 60 DISABLED FOR RTH < 200 INDETERMINATE, NOT RECOMMENDED FOR RTH < 5k RTH (k) = VTH,FC (mV)/10
IRF7401: CISS = 1600pF, QT(MAX) = 48nC, RDS(ON) = 22m MMSF5N03HD: C ISS = 1200pF, Q T (MAX) = 21nC, RDS(ON) = 40m
Figure 7. Fast Comparator Threshold vs. RTH
Table 5. Startup and Retry Timing Parameters
PART MAX4271 MAX4272 MAX4273* tON DEFAULT ( s) 10 10 10 EXTERNALLY SET tON (ms) = 0.31 x CTIM (nF) tON (ms) = 0.31 x CTIM (nF) tON (ms) = 0.31 x CTON (nF) 320 320 DEFAULT ( s) tRETRY EXTERNALLY SET No retry available tRETRY (ms) = 32 x t ON = 10 x CTIM (nF) tRETRY (ms) =10 x CTIM (nF)
*MAX4273 retry feature disabled by connecting CTIM to VIN.
______________________________________________________________________________________
17
3V to 12V Current-Limiting Hot Swap Controllers with Autoretry, DualSpeed/BiLevel Fault Protection MAX4271/MAX4272/MAX4273
Table 6. "No-Overcurrent" Turn-On Timing (Startup Without Current Limit)
DEVICE Fairchild FDS6670A International Rectifier IRF7401 Motorola MMSF5N03HD CGATE (nF) 0 22 0 22 0 22 MOSFET tON (ms) VIN = 3V 0.22 2.3 0.175 1.9 0.101 2.0 VIN = 5V 0.16 2 0.130 1.8 0.074 1.8 VIN = 12V 0.19 3.2 0.16 3.5 0.073 3.2 VIN = 3V 0.07 0.54 0.075 0.54 0.033 0.470 MOSFET tOFF (ms) VIN = 5V 0.13 1.1 0.13 1.1 0.067 1.0 VIN = 12V 0.145 1.95 0.16 2.0 0.085 1.95
RSENSE
tON AND tRETRY vs. CTON AND CTIM
10,000 tRETRY vs. CTIM (MAX4272/MAX4273) 1000 tON AND tRETRY (ms) 100 10 1 0.01 0.1 0.01 0.1 1 10 100 1000 CAPACITANCE (nF) tON vs. CTIM (MAX4272) tON vs. CTON (MAX4273)
VIN
M1
VOUT
CBOARD RS* RPULL-UP IN STAT ON SENSE GATE CGATE
MAX4271 MAX4272 MAX4273 CTIM GND CTIM
CSPD CSPD
*OPTIONAL (SEE TEXT)
Figure 8. Startup and Retry Timeout
Figure 9. Operation with an External Capacitor
Case B. Startup with Current Regulation In applications where the board capacitance (CBOARD) at VOUT is high, the inrush current causes a voltage drop across RSENSE that exceeds the fast comparator threshold (VFC,TH). In this case, the current charging CBOARD can be considered constant and the turn-on time is determined by: tON = CBOARD VIN / IFAST,SET where the maximum load current IFAST,SET = VFC,TH / RSENSE. Figure 3 shows the waveforms and timing diagrams for a startup transient with current regulation. (See Startup Time with C BOARD = 470F in the Typical Operating Characteristics.) When operating under this condition, an external gate capacitor is not required. Adding an external gate capacitor at GATE to GND
reduces the regulated current ripple but increases tOFF by increasing the gate delay (td) (Figure 4). The actual startup time is determined by the longer of the two timings of Case A and Case B. Set the startup timer tSTART at 2 tON to guarantee enough time for the output voltage to settle; also take into consideration device parameter variation. Retry The retry timer defines the dead time before the IC tries to restart a startup sequence following a fault detection. This feature is available on the MAX4272/MAX4273. Before selecting the retry timer value, determine how long a temporary high-current fault condition may be present. In the event of a permanent fault, the automatic retry will effectively force current pulses through the
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3V to 12V Current-Limiting Hot Swap Controllers with Autoretry, DualSpeed/BiLevel Fault Protection
MOSFET with a duty cycle equal to tON/tRETRY and with a current equal to IFAST,SET. Therefore, particular care has to be taken when choosing between immediate retry and board space needed to manage the power dissipation capabilities of the MOSFET (see Thermal Considerations). The duty cycle is fixed to 1/32 for the MAX4272, but can be varied in the MAX4273 by choosing CTIM and CSTART independently. Additional External Gate Capacitance (CEXT) An external gate capacitance can be connected at GATE. This increases the time required to enhance the MOSFET and further limits the output rise time. In the MAX4271/MAX4272, connect the external capacitor between GATE and GND. In the MAX4273, the external capacitor can be connected between GATE and CEXT or GND. If the capacitor is connected to CEXT, it is discharged to ground during a slow comparator fault but it is left floating during a fast comparator fault; this allows the device to turn off the external MOSFET faster during critical faults. (CEXT is biased at VIN; therefore, use a nonpolarized capacitor). Capacitance connected from GATE to CEXT does little to decrease the regulated current ripple. Add a small capacitor (5nF) from GATE to GND. See the charging and discharging time vs. CGATE graphs in the Typical Operating Characteristics. Slow-Comparator Response Time (CSPD) The slow comparator threshold is set at 50mV, and its response time is determined by the external capacitor connected to CSPD (Figure 10). A minimum response time of 20s (typ) is achieved by leaving this pin floating. This time is determined internally and is not affected by stray capacitance at CSPD (up to 100pF).
SLOW COMPARATOR RESPONSE TIME vs. CSPD
1000
Set the slow comparator response time to be longer than the normal operation load transients, (see Slow Comparator).
MAX4271/MAX4272/MAX4273
ON and Reset Comparators
The ON comparator controls the ON/OFF function of these devices. The ON comparator is a precision voltage comparator that can be used for temperature monitoring or as an additional UVLO (Figure 11). The MAX4273 also features an uncommitted delayed comparator. This comparator can be used for voltage monitoring, power sequencing, or for generating a power-on reset signal for on-card microprocessors (Figure 12). Both comparator threshold voltages are set at VREF/2 = 0.6V with a 3mV (typ) hysteresis. The uncommitted comparator OUTC output is an opendrain output, and it is asserted low when its input voltage (INC) is below the threshold voltage. It goes into a high-impedance state 150ms after the voltage has risen above the threshold. The delay for negative-going edges is 10s. Figure 13 shows the MAX4273 used to monitor precisely the temperature of an external device such as the MOSFET. This configuration uses the uncommitted comparator to set the UVLO at a higher level by running its output into the ON comparator's input. The ON comparator initiates startup when its input voltage (VON) rises above the threshold voltage and turns off the MOSFET when the voltage falls below the threshold. The propagation delay is 10s going high or low. The ON comparator is also used to reset the MAX4271/MAX4273 (when CTIM = VIN) after a fault condition (see Latched/Autoretry).
VREF MAX4271 MAX4272 MAX4273 ON LOGIC CONTROL NTC R2 0.6V
R1
100 RESPONSE TIME (ms) tCSPD (ms) = 0.2 x CSPD (nF) 10
1
0.1 0.01 0.01
0.1
1
10
100
1000
CSPD (nF)
R1 = R2 (VREF / O.6 - 1) R2 = VALUE OF THE NTC RESISTOR AT THE LIMIT TEMPERATURE VREF = ANY REFERENCE VOLTAGE AVAILABLE OR VIN
Figure 10. Slow Comparator Response Time vs. CSPD
Figure 11. Temperature Monitoring and Protection 19
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3V to 12V Current-Limiting Hot Swap Controllers with Autoretry, DualSpeed/BiLevel Fault Protection MAX4271/MAX4272/MAX4273
VIN
BACKPLANE VCC
IN SENSE GATE LLMON INC P
REMOVABLE CARD
10k
IN
SENSE GATE MAX4271 MAX4272 MAX4273
MAX4273
OUTC RESET
ON
Figure 12. Power-On Reset
VIN
VOUT
RESET
1M
NTC IN INC SENSE GATE
Figure 14. Fail-Safe Connector
REF
MAX4273
ON OUTC
GND
LLMON
Connect a 1F capacitor from the AUXVCC pin to GND. This capacitor will deliver the necessary energy to the gate drive until the MOSFET is turned off and the main supply recovers. The 1F capacitor is charged from VIN through an internal switch during normal operation.
Maximum Load Capacitance
Figure 13. Power-On Reset and Temperature
The ON and INC comparator inputs and the STAT and OUTC can be pulled to voltages up to 14V independently of VIN, thus allowing parts to be daisy-chained and not be turned on through the internal protection diodes. In some applications, it is useful to use connectors with staggered leads. In Figure 14, the ON pin forces the removable board to be powered up only when all connections are made.
Auxiliary VCC The auxiliary VCC is available on the MAX4273 and is used to sustain the input voltage required for the device to operate during a short-circuit condition on the board. When a short occurs, the main system power supply could collapse and the MAX4273 will not have enough voltage to keep the gate drive operational and turn off the external MOSFET. If the fault is not removed, the system could remain in a sustained short-circuit state.
The MAX4271/MAX4272/MAX4273 can be used on the backplane to regulate current upon insertion of a removable card (Figure 16). This allows multiple cards with different input capacitance to be inserted into the same slot even if the card doesn't have on-board Hot Swap protection. The MAX4271/MAX4272/MAX4273 current-limiting feature is active during the startup period set by CTIM. The startup period can be triggered if VIN is connected to ON through a trace on the card. Once tSTART has expired (timed out), the load capacitance has to be charged or a fault condition is detected. To ensure startup with a fixed CTIM, tSTART has to be longer than the time required to charge the board capacitance. The maximum load capacitance is calculated as follows: CBOARD < tSTART IFAST,SET / VIN
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3V to 12V Current-Limiting Hot Swap Controllers with Autoretry, DualSpeed/BiLevel Fault Protection MAX4271/MAX4272/MAX4273
VIN
100k
IN * * * * * * ON
SENSE
GATE LLMON INC P
MAX4273
OUTC RESET
Figure 15. Adjustable Undervoltage Lockout and Output Voltage Reset Generator
Input Transients
The voltage at VIN must be above the UVLO during inrush and fault conditions. When a short condition occurs on the board, the fault current can be higher than the fast comparator current limit. The gate voltage is discharged immediately, but note that the MOSFET is not completely off until VGS < VTH. If the main system power supply collapses below UVLO, the MAX4271/ MAX4272/MAX4273 will force the device to restart in startup mode with a 150ms delay once the supply has recovered. The main system power supply must be able to deliver this fault current without excessive voltage drop. The MOSFET is turned off in a very short time; therefore, the resulting dv/dt can be considerable. The backplane delivering the power to the external card must have a fairly low inductance to limit the voltage transients caused by the removal of a fault. Bypassing the input with a small capacitor alleviates false UVLO trips due to these transients.
VIN
BACKPLANE
REMOVABLE CARD WITH NO HOT-INSERTION PROTECTION
VOUT
IN
SENSE MAX4271 MAX4272 MAX4273
GATE
CBOARD
CTIM
ON
MOSFET Thermal Considerations
During normal operation, the MOSFET dissipates little power, it is fully turned on, and its RDS(ON) is minimal. The power dissipated in normal operation is P D = ILOAD2 x RDS(ON). A considerable amount of power is dissipated during the startup and turn-off transients. The design must take into consideration the worst-case scenario of a continuous short-circuit fault present on the board. Two cases need to be considered: 1) The single turn-on with the device latched after a fault (when using MAX4271 or MAX4273 in latched mode)
Figure 16. Using the MAX4271/MAX4272/MAX4273 on a Backplane
2) The continuous automatic retry (when using the MAX4272 or MAX4273 in retry mode) MOSFET manufacturers typically include the packagenormalized transient thermal resistance r JA(t ) or rJC(t) that is determined by the startup time and the retry duty cycle (d = tSTART / tRETRY). The following equation is used to calculate the required transient thermal resistance: RJA (t) = (TJMAX - TA) / PDMAX (t)
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3V to 12V Current-Limiting Hot Swap Controllers with Autoretry, DualSpeed/BiLevel Fault Protection MAX4271/MAX4272/MAX4273
where PDMAX (t) = VIN x IFAULT and the resulting RJA = R JA (t) / r JA (t). R JA is the thermal resistance determined with a continuous load and by the layout or heatsink.
HIGH-CURRENT PATH
Layout Considerations
To take full advantage of the switch response time to an output fault condition, it is important to keep all traces as short as possible and to maximize the high-current trace dimensions to reduce the effect of undesirable parasitic inductance. Place the MAX4271/MAX4272/MAX4273 close to the card's connector. Use a ground plane to minimize impedance and inductance. Minimize the currentsense resistor trace length (<10mm), and ensure accurate current sensing with Kelvin connections (Figure 17). When the output is short circuited, the voltage drop across the external MOSFET becomes large. Hence, the power dissipation across the switch increases, as does the die temperature. An efficient way to achieve good power dissipation on a surface-mount package is to lay out two copper pads directly under the package on both sides of the board. Connect the two pads to the ground plane through vias, and use enlarged copper mounting pads on the top side of the board.
SENSE RESISTOR
MAX4271 MAX4272
Figure 17. Kelvin Connections for the Current-Sense Resistors
Ordering Information
PART MAX4271ESA MAX4272ESA MAX4273EEE* MAX4273ESE* TEMP. RANGE -40C to +85C -40C to +85C -40C to +85C -40C to +85C PIN-PACKAGE 8 SO 8 SO 16 QSOP 16 SO
*Future products--contact factory for availability.
Pin Configurations
TOP VIEW
RTH 1 AUXVCC 2 IN 1 SENSE 2 GATE 3 8 7 ON STAT CTIM CSPD LLMON 7 10 CSPD 9 CTON GND 8 IN 3 SENSE 4 GATE 5 CEXT 6 GND 4 5 16 OUTC 15 INC 14 REF
MAX4273
13 ON 12 STAT 11 CTIM
MAX4271 MAX4272
6
SO
QSOP/SO
22
______________________________________________________________________________________
3V to 12V Current-Limiting Hot Swap Controllers with Autoretry, DualSpeed/BiLevel Fault Protection
Package Information
SOICN.EPS
MAX4271/MAX4272/MAX4273
______________________________________________________________________________________
23
3V to 12V Current-Limiting Hot Swap Controllers with Autoretry, DualSpeed/BiLevel Fault Protection MAX4271/MAX4272/MAX4273
Package Information (continued)
QSOP.EPS
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
24 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2000 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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