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19-2497; Rev 0; 7/02
Triple USB Switch with Autoreset and Fault Blanking
General Description
The MAX1940 triple current-limited switch with autoreset supplies a guaranteed 500mA load per channel in accordance with USB specifications. The MAX1940 operates from a 4V to 5.5V input supply and consumes only 60A of quiescent current when operating and only 3A in standby. Selectable active-high/active-low control logic and independent shutdown controls for each channel provide additional flexibility. An autoreset feature latches the switch off in the event of a short circuit, saving system power. The switch reactivates upon removal of the shorted condition. The MAX1940 provides several safety features to protect the USB port. Built-in thermal-overload protection turns off the switch when the die temperature exceeds +160C. Accurate internal current-limiting circuitry protects the input supply against both overload and shortcircuit conditions. Independent open-drain fault signals (FAULTA, FAULTB, and FAULTC) notify the microprocessor when a thermal-overload, current-limit, undervoltage lockout (UVLO), or short-circuit fault occurs. A 20ms fault-blanking feature enables the circuit to ignore momentary faults, such as those caused when hot-swapping a capacitive load, preventing false alarms to the host system. The fault-blanking feature also prevents fault signals from being issued when the device powers up the load. The MAX1940 is available in a space-saving 16-pin QSOP package and operates over the extended (-40C to +85C) temperature range.
Features
o Triple USB Switch in Tiny 16-Pin QSOP Package o Autoreset Feature Saves System Power o Guaranteed 500mA Load Current per Channel o Built-In 20ms Fault-Blanking Circuitry o Active-High/Active-Low Control Logic o Fully Compliant to USB Specifications o 4V to 5.5V Input Voltage Range o Independent Shutdown Control o Independent Fault Indicator Outputs o Thermal-Overload Protection o 3A Standby Current o UL Certification Pending
MAX1940
Ordering Information
PART MAX1940EEE TEMP RANGE -40C to +85C PIN-PACKAGE 16 QSOP
Applications
USB Ports USB Hubs Notebook Computers Desktop Computers PDAs and Palmtop Computers Docking Stations
Typical Operating Circuit
MAX1940
IN1 FAULTA FAULTB FAULTC
Pin Configuration
TOP VIEW
ONA 1 IN1 2 IN2 3 IN3 4 ONB 5 IN4 6 IN5 7 ONC 8 16 FAULTA 15 OUTA 14 GND
INPUT 4V TO 5.5V
IN2 IN3 IN4 IN5 ONA
OUTA
USB PORT A
MAX1940
13 OUTB 12 FAULTB 11 SEL 10 OUTC 9 FAULTC
SEL GND OUTC ACTIVE-HIGH/ACTIVE-LOW SELECTABLE ONB ONC USB PORT C OUTB USB PORT B
QSOP
________________________________________________________________ Maxim Integrated Products
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Triple USB Switch with Autoreset and Fault Blanking MAX1940
ABSOLUTE MAXIMUM RATINGS
IN_, ON_, OUT_, SEL, FAULT_, to GND...................-0.3V to +6V IN1, IN2 to OUTA......................................................-0.3V to +6V IN2, IN3 to OUTB......................................................-0.3V to +6V IN4, IN5 to OUTC .....................................................-0.3V to +6V OUT_ Continuous Switch Current (per channel, internally limited).........................................1.4A FAULT_ DC Current ............................................................20mA Continuous Power Dissipation (TA = +70C) 16-Pin QSOP (derate 8.3mW/C above +70C)...........667mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10s) .................................+300C
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_ = 5V, CIN_ = 0.1F, COUT_ = 1F, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER Supply Voltage Range Switch On-Resistance Standby Supply Current Quiescent Supply Current IIN_ SYMBOL VIN_ RON TA = 0C to +85C, each switch All switches disabled One switch enabled, IOUT_ = 0 Two switches enabled, IOUT_ = 0 All switches enabled, IOUT_ = 0 OUT_ Off-Leakage Current Undervoltage Lockout Threshold Continuous Load Current Continuous Current Limit Short-Circuit Current Limit Short-Circuit Detect Threshold Continuous Current-Limit Blanking Timeout Period Short-Circuit Blanking Timeout Period Turn-On Delay Output Rise Time Turn-Off Delay Output Fall Time Thermal-Shutdown Threshold ON_, SEL Input High Level ON_, SEL Input Low Level ON_, SEL Input Leakage Current VIH VIL tON tRISE tOFF tFALL ILIM ISC VIN_ - VOUT_ = 0.5V VOUT_ = 0 (IOUT_ pulsing) (Note 1) From continuous current-limit condition to FAULT_ asserted From short-circuit current-limit condition to FAULT_ asserted ROUT_ = 10, does not include rise time (from ON_ to 10% of VOUT_) ROUT_ = 10, from 10% to 90% of VOUT_ ROUT_ = 10, does not include fall time (from ON_ to 90% of VOUT_) ROUT_ = 10, from 90% to 10% of VOUT_ 15C hysteresis VIN_ = 4V to 5.5V VIN_ = 4V to 5.5V VON_ = 0 or VIN_ -1 2 0.8 +1 10 7.5 0.5 ILKG VULVO All switches disabled, VOUT_ = 0, TA = 0C to +85C Rising edge, 3% hysteresis 3.0 500 0.7 0.9 0.9 1.2 0.35 1 20 18 1.2 2.5 0.8 2.5 160 3 35 35.0 4.0 1.2 1.6 CONDITIONS MIN 4.0 85 3 47 55 63 0.02 3.4 TYP MAX 5.5 135 10 75 90 100 10 3.8 A V mA A APK ARMS V ms ms ms ms ms ms C V V A A UNITS V m A
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Triple USB Switch with Autoreset and Fault Blanking
ELECTRICAL CHARACTERISTICS (continued)
(VIN_ = 5V, CIN_ = 0.1F, COUT_ = 1F, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER FAULT_ Output Low Voltage FAULT_ Output High Leakage Current OUT_ Autoreset Current OUT_ Autoreset Threshold OUT_ Autoreset Blanking Time SYMBOL VOL CONDITIONS ISINK = 1mA, VIN_ = 4V VIN_ = V FAULT_ = 5.5V In latched off state, VOUT_ = 0 In latched off state, OUT_ rising In latched off state, VOUT_ > 0.5V 10 0.4 10 25 0.5 20 MIN TYP MAX 0.4 1 45 0.6 35 UNITS V A mA V ms
MAX1940
ELECTRICAL CHARACTERISTICS
(VIN_ = 5V, CIN_ = 0.1F, COUT_ = 1F, TA = -40C to +85C, unless otherwise noted.) (Note 2)
PARAMETER Supply Voltage Range Switch On-Resistance Standby Supply Current Quiescent Supply Current OUT_ Off-Leakage Current Undervoltage Lockout Threshold Continuous Load Current Continuous Current Limit Short-Circuit Current Limit Continuous Current-Limit Blanking Timeout Period Short-Circuit Blanking Timeout Period Turn-On Delay Turn-Off Delay ON_, SEL Input High Level ON_, SEL Input Low Level ON_, SEL Input Leakage Current FAULT_ Output Low Voltage FAULT_ Output High Leakage Current OUT_ Autoreset Current OUT_ Autoreset Threshold OUT_ Autoreset Blanking Time VOL tON tOFF VIH VIL ILIM ISC VIN_ - VOUT_ = 0.5V VOUT_ = 0 (IOUT_ pulsing) From continuous current-limit condition to FAULT_ asserted From short-circuit current-limit condition to FAULT_ asserted ROUT_ = 10, does not include rise time (from ON_ to 10% of VOUT_) ROUT_ = 10, does not include fall time (from ON_ to 90% of VOUT_) VIN_ = 4V to 5.5V VIN_ = 4V to 5.5V VON_ = 0 or VIN_ ISINK = 1mA, VIN_ = 4V VIN_ = V FAULT_ = 5.5V In latched off state, VOUT_ = 0 In latched off state, OUT_ rising In latched off state, VOUT_ > 0.5V 10 0.4 10 -1 2 0.8 +1 0.4 1 50 0.6 35 IIN_ ILKG VULVO SYMBOL VIN_ RON Each switch All switches disabled One switch enabled, IOUT_ = 0 Two switches enabled, IOUT_ = 0 All switches enabled, IOUT_ = 0 All switches disabled, VOUT_ = 0 Rising edge, 3% hysteresis 3.0 500 0.7 0.9 10 7.5 0.5 1.2 1.6 35 35.0 4.0 3 CONDITIONS MIN 4.0 MAX 5.5 135 10 75 90 100 10 3.8 A V mA A APK ms ms ms ms V V A V A mA V ms A UNITS V m A
Note 1: Short-circuit detect threshold is the output voltage at which the device transitions from short-circuit current limit to continuous current limit. Note 2: Specifications to -40C are guaranteed by design, not production tested. _______________________________________________________________________________________ 3
Triple USB Switch with Autoreset and Fault Blanking MAX1940
Typical Operating Characteristics
(Circuit of Figure 2, VIN_ = 5V, CIN_ = 0.1F, COUT_ = 1F, ON_ = SEL, TA = +25C, unless otherwise noted.)
QUIESCENT SUPPLY CURRENT vs. INPUT VOLTAGE
MAX1940 toc01
QUIESCENT SUPPLY CURRENT vs. TEMPERATURE
SHUTDOWN SUPPLY CURRENT (A) 69 68 67 66 65 64 63 62 61 60 2.5 -40 -15 10 35 60 85 -40 VIN_ = 4.5V VIN_ = 5V ALL SWITCHES ACTIVE VIN_ = 5.5V
MAX1940 toc02
SHUTDOWN SUPPLY CURRENT vs. TEMPERATURE
ON_ = GND, VSEL = 5V 2.9
MAX1940 toc03
70 ALL SWITCHES ACTIVE QUIESCENT SUPPLY CURRENT (A) 60 50 40 30 20 10 0
70 QUIESCENT SUPPLY CURRENT (A)
3.0
2.8
2.7
2.6
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 INPUT VOLTAGE (V)
-15
10
35
60
85
TEMPERATURE (C)
TEMPERATURE (C)
SWITCH OFF-LEAKAGE (ONE SWITCH) vs. TEMPERATURE
MAX1940 toc04
NORMALIZED ON-RESISTANCE vs. TEMPERATURE
MAX1940 toc05
CONTINUOUS CURRENT-LIMIT THRESHOLD vs. TEMPERATURE
CONTINUOUS CURRENT-LIMIT THRESHOLD (mA)
MAX1940 toc06
1000 ON_ = OUT_, VSEL = 5V SWITCH OFF-LEAKAGE (nA) 100
1.3 1.2 1.1 1.0 0.9 0.8 0.7
920
NORMALIZED ON-RESISTANCE
916 VIN_ = 5.0V
VIN_ = 5.5V
10
912
1
908
0.1
904
VIN_ = 4.5V
0.01 -40 -15 10 35 60 85 TEMPERATURE (C)
900 -40 -15 10 35 60 85 TEMPERATURE (C)
-40
-15
10
35
60
85
TEMPERATURE (C)
TURN-ON TIME (tON + tRISE) vs. TEMPERATURE
MAX1940 toc07
TURN-OFF TIME (tOFF + tFALL) vs. TEMPERATURE
MAX1940 toc08
FAULT-BLANKING TIME vs. TEMPERATURE
MAX1940 toc09
3.5 ROUT_ = 10 3.4 TURN-ON TIME (ms) VIN_ = 5V
VIN_ = 4.5V
3.3 ROUT_ = 10 3.2 TURN-OFF TIME (ms) 3.1 3.0 2.9 2.8 2.7 2.6 VIN_ = 4.5V -40 -15 10 35 60 VIN_ = 5V VIN_ = 5.5V
22.0 21.5 21.0 20.5 20.0 19.5 19.0 VIN_ = 5.5V VIN_ = 4.5V
3.3
3.2
FAULT-BLANKING TIME (ms)
VIN_ = 5V
3.1
VIN_ = 5.5V
3.0 -40 -15 10 35 60 85 TEMPERATURE (C)
85
-40
-15
10
35
60
85
TEMPERATURE (C)
TEMPERATURE (C)
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Triple USB Switch with Autoreset and Fault Blanking MAX1940
Typical Operating Characteristics (continued)
(Circuit of Figure 2, VIN_ = 5V, CIN_ = 0.1F, COUT_ = 1F, ON_ = SEL, TA = +25C, unless otherwise noted.)
FAULT OUTPUT LOW VOLTAGE vs. TEMPERATURE
250 240 230 220 210 200 190 180 170 160 150 140 130 120 110 100
MAX1940 toc10
AUTORESET CURRENT vs. TEMPERATURE
MAX1940 toc11
AUTORESET CURRENT vs. INPUT VOLTAGE
35 AUTORESET CURRENT (mA) 30 25 20 15 TA = +85C 10 5 0 TA = -40C
MAX1940 toc12
MAX1940 toc15
RPU = 4.7k VIN_ = 5V VIN_ = 5.5V
38 36 AUTORESET CURRENT (mA) 34 32 30 28 26 24 22 20 18 VIN_ = 4.5V -40 -15 10 35 60 VIN_ = 5.5V VIN_ = 5V
40 TA = +25C
FAULT OUTPUT LOW VOLTAGE (mV)
VIN_ = 4.5V
-40
-15
10
35
60
85
85
3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5 INPUT VOLTAGE (V)
TEMPERATURE (C)
TEMPERATURE (C)
OVERLOAD RESPONSE INTO 2.5
MAX1940 toc13
OVERLOAD RESPONSE INTO 2.5 (EXPANDED TIME SCALE)
MAX1940 toc14
SHORT-CIRCUIT RESPONSE INTO 0
5V A 5V
A
5V
A
5V B 0 B 0 C C 0 D 0 A: VIN_ 5V/div B: VOUT_ 5V/div 10ms/div C: VFAULT_ 5V/div D: IOUT_ 1A/div A: VIN_ 5V/div B: VOUT_ 5V/div 400s/div C: VFAULT_ 5V/div D: IOUT_ 1A/div D 0 A: VIN_ 5V/div B: VOUT_ 5V/div 10ms/div C: VFAULT_ 5V/div D: IOUT_ 1A/div D 0 5V B C 5V
SHORT-CIRCUIT RESPONSE INTO 0 (EXPANDED TIME SCALE)
MAX1940 toc16
SWITCH TURN-ON TIME (tON + tRISE)
5V A VSEL = 5V ROUT_ = 10 COUT_ = 1F
MAX1940 toc17
SWITCH TURN-ON TIME (tON + tFALL)
VSEL = 5V ROUT_ = 10 COUT_ = 1F 5V A
MAX1940 toc18
A
5V
B C
0 5V 5V B B 0
D 0 A: VIN_ 5V/div B: VOUT_ 5V/div 400s/div C: VFAULT_ 5V/div D: IIN_ 2A/div 1ms/div A: VON_ 5V/div B: VOUT_ 2V/div A: VON_ 5V/div B: VOUT_ 2V/div 1ms/div
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Triple USB Switch with Autoreset and Fault Blanking MAX1940
Typical Operating Characteristics (continued)
(Circuit of Figure 2, VIN_ = 5V, CIN_ = 0.1F, COUT_ = 1F, ON_ = SEL, TA = +25C, unless otherwise noted.)
STARTUP TIME COUT = 1F
MAX1940 toc19
STARTUP TIME COUT = 330F
5V A
MAX1940 toc20
VSEL = 5V, ROUT_ = 10, COUT_ = 1F A
VSEL = 5V, ROUT_ = 10, COUT_ = 330F 5V
B
5V 5V
B
5V
C
C 0
D 400s/div C: VOUT_ 5V/div A: VON_ 5V/div B: VFAULT_ 5V/div D: IOUT_ 500mA/div
500mA D 1ms/div C: VOUT_ 5V/div A: VON_ 5V/div B: VFAULT_ 5V/div D: IOUT_ 500mA/div
500mA
STARTUP TIME COUT = 100F
MAX1940 toc21
UNDERVOLTAGE LOCKOUT RESPONSE
MAX1940 toc22
VSEL = 5V, ROUT_ = 10, COUT_ = 100F A 5V A
ROUT_ = 10, COUT_ = 1F 0
B
5V 5V
B 0
C C 0 D 1ms/div C: VOUT_ 2V/div A: VON_ 5V/div B: VFAULT_ 5V/div D: IOUT_ 500mA/div 500mA
A: VIN_ 5V/div B: VOUT_ 5V/div
10ms/div C: VFAULT_ 5V/div
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Triple USB Switch with Autoreset and Fault Blanking
Pin Description
PIN 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 NAME ONA IN1 IN2 IN3 ONB IN4 IN5 ONC FAULTC OUTC SEL FAULTB OUTB GND OUTA FAULTA FUNCTION Control Input for Switch A. The active polarity of ONA is set by SEL (see Table 1). Power Input. Connect all IN_ inputs together and bypass with a 0.1F ceramic capacitor to GND. Load conditions might require additional bulk capacitance to prevent pulling IN_ down. Power Input. Connect all IN_ inputs together and bypass with a 0.1F ceramic capacitor to GND. Load conditions might require additional bulk capacitance to prevent pulling IN_ down. Power Input. Connect all IN_ inputs together and bypass with a 0.1F ceramic capacitor to GND. Load conditions might require additional bulk capacitance to prevent pulling IN_ down. Control Input for Switch B. The active polarity of ONB is set by SEL (see Table 1). Power Input. Connect all IN_ inputs together and bypass with a 0.1F ceramic capacitor to GND. Load conditions might require additional bulk capacitance to prevent pulling IN_ down. Power Input. Connect all IN_ inputs together and bypass with a 0.1F ceramic capacitor to GND. Load conditions might require additional bulk capacitance to prevent pulling IN_ down. Control Input for Switch C. The active polarity of ONC is set by SEL (see Table 1). Fault Indicator Output for Switch C. Open-drain output asserts low when switch C enters thermal shutdown, undervoltage lockout, or a sustained (>20ms) current-limit or short-circuit condition. Power Output for Switch C. Bypass OUTC to GND with a 1F ceramic capacitor. Load conditions might require additional bulk capacitance. When disabled, OUTC goes into a high-impedance state. Logic Input Polarity Select. SEL sets the active polarity of the ON_ inputs. Connect SEL high to set activehigh inputs. Connect SEL to GND to set active-low inputs. Fault Indicator Output for Switch B. Open-drain output asserts low when switch B enters thermal shutdown, undervoltage lockout, or enters a sustained (>20ms) current-limit or short-circuit condition. Power Output for Switch B. Bypass OUTB to GND with a 1F ceramic capacitor. Load conditions might require additional bulk capacitance. When disabled, OUTB goes into a high-impedance state. Ground Power Output for Switch A. Bypass OUTA to GND with a 1F ceramic capacitor. Load conditions might require additional bulk capacitance. When disabled, OUTA goes into a high-impedance state. Fault Indicator Output for Switch A. Open-drain output asserts low when switch A enters thermal shutdown, undervoltage lockout, or a sustained (>20ms) current-limit or short-circuit condition.
MAX1940
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Triple USB Switch with Autoreset and Fault Blanking MAX1940
IN_ ONA CHARGE PUMP IN1 FAULTA SEL IN2 SWITCH A (EXPANDED)
ILIM OUTA THERMAL SHUTDOWN
25mA
MAX1940
FAULT LOGIC 20ms TIMERS OUTB SWITCH B (SEE SWITCH A) UVLO OSC 25kHz OUTC BIAS ONB SWITCH C (SEE SWITCH A) FAULTC FAULTB
ONC
REF
GND
IN2 IN3
IN4
IN5
Figure 1. Functional Diagram
Detailed Description
The MAX1940 triple current-limited USB power switch provides three independent switches, each with its own enable-control input and fault indicator (see Figure 1). A logic input sets the active polarity of the enable control inputs. The fault indicators notify the system when the current-limit, short-circuit, undervoltage lockout, or thermal-shutdown threshold is exceeded. The MAX1940 operates from a 4V to 5.5V input supply and supplies a minimum output current of 700mA per channel. A built-in current limit of 0.9A (typ) limits the output current in the event of an overload condition. Built-in short-circuit detection pulses the output current if
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the output voltage falls below 1V, resulting in lower RMS output current and reduced power dissipation in the device. Independent thermal shutdown allows normal operation to continue if one channel experiences a prolonged overload or short-circuit condition. Low-RON NMOS switches enable the MAX1940 to provide three switches in the space-saving 16-pin QSOP package. An internal micropower charge pump generates the high-side supply needed for driving the gates of these high-side switches. Separate current-limiting and thermal-shutdown circuitry permits each switch to operate independently, improving system robustness.
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Triple USB Switch with Autoreset and Fault Blanking
On/Off Control and Undervoltage Lockout
SEL sets the active polarity of the logic inputs of the MAX1940. Connect ON_ to the same voltage as SEL to enable the respective OUT_ switch. Connect ON_ to the opposite voltage as SEL to disable the respective output (see Table 1). The output of a disabled switch enters a high-impedance state. The MAX1940 includes a UVLO circuit to prevent erroneous switch operation when the input voltage goes low during startup and brownout conditions. Input voltages of less than 3.4V inhibit operation of the device. FAULT_ asserts low during an undervoltage lockout condition.
MAX1940
Table 1. On/Off Control
SEL GND VIN ON_ GND VIN GND VIN OUT_ BEHAVIOR ON OFF OFF ON
Fault Indicators
The MAX1940 provides an independent open-drain fault output (FAULT_) for each switch. Connect FAULT_ to IN_ through a 100k pullup resistor for most applications. FAULT_ asserts low when any of the following conditions occur: * The input voltage is below the UVLO threshold. * The switch junction temperature exceeds the +160C thermal-shutdown temperature limit. * The switch is in current-limit or short-circuit currentlimit mode after the fault-blanking period (20ms) expires. The FAULT_ output deasserts after a 20ms delay once the fault condition is removed. Ensure that the MAX1940 input bypass capacitance prevents glitches from triggering the FAULT_ outputs. Limit the input voltage slew rate to 0.2V/s to prevent erroneous FAULT_ indications. To differentiate large capacitive loads from short circuits or sustained overloads, the MAX1940 has an independent fault-blanking circuit for each switch. When a load transient causes the device to enter current limit, an internal counter monitors the duration of the fault. For load faults exceeding the 20ms faultblanking time, the switch turns off, FAULT_ asserts low, and the device enters autoreset mode (see the Output Fault Protection and Autoreset Mode section). Only current-limit and short-circuit faults are blanked. Thermal overload faults and input voltage drops below the UVLO threshold immediately turn the switch off and assert FAULT_ low. Fault blanking allows the MAX1940 to handle USB loads that might not be fully compliant with USB specifications. The MAX1940 successfully powers USB loads with additional bypass capacitance and/or large startup currents while protecting the upstream power source. No fault is reported if the switch brings up the load within the 20ms blanking period. See Table 2 for a summary of current-limit and fault behavior.
Output Fault Protection and Autoreset
The MAX1940 senses the switch output voltage and selects continuous current limiting for VOUT_ greater than 1V, or short-circuit current limiting for VOUT_ less than 1V. When VOUT_ is greater than 1V, the device operates in a continuous current-limit mode that limits output current to 0.9A. When VOUT_ is less than 1V, the device operates in short-circuit current-limit mode, sourcing 1.2A pulses to the load. When either fault condition persists for 20ms, the output turns off and its fault flag is asserted. The output automatically restarts 20ms after the short or overload is removed. The MAX1940 detects short-circuit removal by sourcing 25mA from the output and monitoring the output voltage. When the voltage at the output exceeds 0.5V for 20ms, the fault flag resets, the output turns back on, and the 25mA current source turns off. Active loads are not expected to have measurable current when supplied with less than 0.5V.
Thermal Shutdown
Independent thermal shutdown for each channel permits normal operation of two switches to continue while a third experiences a thermal fault. The switch turns off and the FAULT_ output asserts low immediately when the junction temperature exceeds +160C. Thermal shutdown does not utilize the 20ms fault-blanking timeout period. The switch turns on again and FAULT_ returns high when the junction temperature cools by +15C. The switch cycles on and off if the overload condition persists, resulting in a pulsed output that reduces system power.
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Triple USB Switch with Autoreset and Fault Blanking MAX1940
Table 2. Current-Limiting and Fault Behavior
CONDITION MAX1940 BEHAVIOR If a short is detected at the output, the channel turns off, and the blanking timer begins. FAULT_ remains high during the blanking timeout period. If the short persists during the fault-blanking period, the output pulses at 0.35ARMS. If the short is removed before the 18ms short-circuit blanking timeout period, the next ramped current pulse soft-starts the output. FAULT_ remains high. If the short circuit persists after the fault-blanking period, FAULT_ goes low, autoreset mode begins, and the output sources 25mA. If the output voltage rises above 0.5V for 20ms, the output turns on and FAULT_ goes high (see ShortCircuit Response in the Typical Operating Characteristics.)
Output Short Circuit (VOUT_ < 1V)
Output current regulates at ILIM and the blanking timer turns on. FAULT_ remains high during the blanking timeout period. Continuous current at ILIM persists until either the 20ms blanking period expires or a thermal fault occurs. Output Overload Current If overcurrent persists after 20ms, FAULT_ goes low, autoreset mode is enabled, and the output sources (VOUT_ > 1V) 25mA. If the output voltage rises above 0.5V for 20ms, the output turns on and FAULT_ goes high (see Overload Response into 2.5 in the Typical Operating Characteristics.) Thermal Fault (TJ > +160C) A junction temperature of +160C immediately asserts FAULT_ low (the blanking timeout period does not apply for thermal faults) and turns off the switch. When the junction cools by 15C, the thermal fault is cleared and FAULT_ goes high. Note that if other fault conditions are present when a thermal fault clears, those fault states take effect.
Applications Information
Input Power Supply and Capacitance
Connect all IN_ inputs together externally. IN_ powers the internal control circuitry and charge pump for each switch. Bypass IN_ to GND with a 0.1F ceramic capacitor. When driving inductive loads or operating from inductive sources, which may occur when the MAX1940 is powered by long leads or PC traces, larger input bypass capacitance is required to prevent voltage spikes from exceeding the MAX1940's absolute maximum ratings during short-circuit events.
cables, which might add an inductive component to the load. This inductance causes the output voltage at the USB port to oscillate during a load step. The MAX1940 drives inductive loads, but avoid exceeding the device's absolute maximum ratings. Usually, the load inductance is relatively small, and the MAX1940's input includes a substantial bulk capacitance from an upstream regulator as well as local bypass capacitors, limiting overshoot. If severe ringing occurs because of large load inductance, clamp the MAX1940 outputs below +6V and above -0.3V.
Turn-On and Turn-Off Behavior
The MAX1940's slow turn-on and turn-off minimizes load transients on the upstream power source. Under fault conditions, the outputs of the MAX1940 turn off rapidly to provide maximum safety for the upstream power source and downstream devices. Internal blocks shut down to minimize supply current when all three channels are off.
Output Capacitor
Bypass OUT_ to GND with a 1F ceramic capacitor for local decoupling. Additional bulk capacitance (up to 470F) reduces output-voltage transients under dynamic load conditions. Using output capacitors greater than 470F might assert FAULT_ if the current limit cannot charge the output capacitor within the 20ms fault-blanking period. In addition to bulk capacitance, small-value (0.1F or greater) ceramic capacitors improve the output's resilience to electrostatic discharge (ESD).
Layout and Thermal Dissipation
Keep all traces as short as possible to reduce the effect of undesirable parasitic inductance and optimize the switch response time to output short-circuit conditions. Place input and output capacitors no more than 5mm from device leads. Connect IN_ and OUT_ to the
Driving Inductive Loads
A wide variety of devices (mice, keyboards, cameras, and printers) typically connect to the USB port with
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Triple USB Switch with Autoreset and Fault Blanking
An active switch dissipates little power with minimal change in package temperature. Calculate the power dissipation for this condition as follows:
100k
MAX1940
MAX1940
IN1 IN2 IN3 O.1F IN4 IN5 ONA OFF ONB ON ONC SEL GND OUTC OUTB OUTA FAULTA FAULTB FAULTC
100k 100k
INPUT 4V TO 5.5V
USB PORT A 1F*
USB PORT B 1F*
P = (IOUT_)2 x RON At the normal operating current (IOUT_ = 0.5A) and the maximum on-resistance of the switch (135m), the power dissipation is: P = (0.5A)2 x 0.135 = 34mW per switch. The worst-case power dissipation occurs when the output current is just below the current-limit threshold (1.2A max) with an output voltage greater than 1V. In this case, the power dissipated in each switch is the voltage drop across the switch multiplied by the current limit: P = ILIM x (VIN - VOUT) For a 5V input and 1V output, the maximum power dissipation per switch is: P = 1.2A x (5V - 1V) = 4.8W Because the package power dissipation is 667mW, the MAX1940 die temperature exceeds the +160C thermal shutdown threshold, and the switch output shuts down until the junction temperature cools by +15C. The duty cycle and period are strong functions of the ambient temperature and the PC board layout (see the Thermal Shutdown section). If the output current exceeds the current-limit threshold, or the output voltage is pulled below the short-circuit detect threshold, the MAX1940 enters a fault state after 20ms, at which point autoreset mode is enabled and 25mA is sourced by the output. For a 5V input, OUT_ short-circuited to GND, and autoreset mode active, the power dissipation is as follows: P = 0.025A x 5V = 0.125W
USB PORT C 1F*
*USB APPLICATIONS MAY REQUIRE ADDITIONAL BULK CAPACITANCE
Figure 2. Typical Application Circuit
power bus with short traces. Wide power bus planes at IN_ and OUT provide superior heat dissipation as well.
Chip Information
TRANSISTOR COUNT: 4259 PROCESS: BiCMOS
______________________________________________________________________________________
11
Triple USB Switch with Autoreset and Fault Blanking MAX1940
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information, go to www.maxim-ic.com/packages.)
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.
12 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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