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LT1500/LT1501 Adaptive-Frequency Current Mode Switching Regulators
FEATURES
s
DESCRIPTION
The LT(R)1500 is an adaptive-frequency current mode stepup switching regulator with an internal power switch that is rated up to 700mA. In contrast to pulse skipping switching regulators, the LT1500 uses a current mode topology that provides lower noise operation and improved efficiency. Only at very light loads is Burst ModeTM activated to give high efficiency and micropower operation. High switching frequency (up to 500kHz) allows very small inductors to be used, along with ceramic capacitors if desired. The LT1500 operates with input voltages from 1.8V to 15V and has only 200A operating current dropping to 8A in shutdown. A low-battery comparator is included which stays alive in shutdown. A second output feedback pin with negative polarity allows negative output voltages to be regulated when the switcher is connected up as a Cuk or a flyback converter. Two package types are available. The LT1500 comes in a 14-pin SO package, with two options available for fixed output (3.3V or 5V) or adjustable operation. A reduced feature part, the LT1501, comes in the smaller 8-pin SO package with internal frequency compensation. It is also available in adjustable and fixed output voltage versions.
, LTC and LT are registered trademarks of Linear Technology Corporation. Burst Mode is a trademark of Linear Technology Corporation.
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Low Noise Adaptive-Frequency Current Mode Operation Avoids Low Frequency Noise at Most Load Currents Can Be Externally Synchronized (LT1500) Micropower Quiescent Current: 200A Shutdown Current: 8A Typ Internal Loop Compensation Low-Battery Comparator Active in Shutdown Minimum Input Voltage: 1.8V Typ Additional Negative Voltage Feedback Pin (LT1500) Up to 500kHz Switching Frequency Uses Low Profile, Low Cost Surface Mount Inductors
APPLICATIONS
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Portable Instrumentation Battery Operated Systems PDA's Standby Power
TYPICAL APPLICATION
2-Cell to 5V Converter
22H D1 MBR0520L 5V, 200mA 301k 1% 2 EACH NiCd OR ALKALINE CELLS VIN SHDN 33F* 6V 301k 1% 1nF LT1501-5 LBI LBO OUT GND ISENSE SW
+
+ 220F**
10V TANT * AVX, TPSC107M006R0150 ** AVX, TPSD107M010 R0100 SUMIDA CD73-220, CD54-220 OR CD43-220. SELECT ACCORDING TO MAXIMUM LOAD CURRENT
LT1500/01 * TA01
LOW-BATTERY FLAG (USE EXTERNAL PULL-UP)
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LT1500/LT1501
ABSOLUTE MAXIMUM RATINGS
Supply Voltage ........................................................ 20V Switch Voltage (SW)................................................ 30V Shutdown Voltage (SHDN) ...................................... 20V ISENSE Voltage .......................................................... 20V FB Voltage ................................................................. 5V LBI Voltage ................................................................ 5V LBO Voltage ............................................................. 15V Operating Ambient Temperature Range Commercial ............................................. 0C to 70C Industrial ............................................ - 40C to 85C Operating Junction Temperature Range Commercial ........................................... 0C to 100C Industrial .......................................... - 40C to 100C Storage Temperature Range ................. - 65C to 150C Lead Temperature (Soldering, 10 sec).................. 300C
PACKAGE/ORDER INFORMATION
TOP VIEW SHDN 1 VC 2 VIN 3 ISENSE 4 NC 5 GND 6 PGND 7 14 FB 13 NFB 12 SS 11 LBI 10 LBO 9 8 SYNC SW SHDN 1 VC 2 VIN 3 ISENSE 4 NC 5 GND 6 PGND 7 TOP VIEW 14 VOUT (3.3V/5V) 13 SELECT 12 SS 11 LBI 10 LBO 9 8 SYNC SW
S PACKAGE 14-LEAD PLASTIC SO TJMAX = 100C, JA = 100C/ W
S PACKAGE 14-LEAD PLASTIC SO TJMAX = 100C, JA = 100C/ W
ORDER PART NUMBER LT1500CS LT1500IS
ORDER PART NUMBER LT1500CS-3/5 LT1500IS-3/5
Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS
PARAMETER Feedback/Output Pin Reference Voltage CONDITIONS
TJ = 25C, VIN = 2.3V unless otherwise noted.
MIN
q q q q q
LT1500/LT1501, TJ = 25C All Conditions (Note 6) LT1500-3/5, Select Pin Open All Conditions (Note 6) LT1500-3/5, Select Pin Grounded All Conditions (Note 6)
Reference Voltage Line Regulation Feedback Pin Bias Current
VIN = 2.3V to 15V
2
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TOP VIEW SHDN 1 VIN 2 ISENSE 3 GND 4 8 7 6 5 FB/OUT LBI LBO SW
S8 PACKAGE 8-LEAD PLASTIC SO
TJMAX = 100C, JA = 120C/ W
ORDER PART NUMBER LT1501CS8 LT1501CS8-3.3 LT1501CS8-5 LT1501IS8 LT1501IS8-3.3 LT1501IS8-5
TYP 1.265 3.300 5.000 0.02 30
MAX 1.290 1.295 3.370 3.400 5.100 5.15 0.06 100
UNITS V V V V V V %/V nA
1.240 1.235 3.230 3.200 4.900 4.85
LT1500/LT1501
ELECTRICAL CHARACTERISTICS
PARAMETER Internal Divider Current Operating Quiescent Current Supply Current in Shutdown CONDITIONS LT1500-3.3/LT1501-3.3 LT1500-5/LT1501-5 VIN 5V, VSHDN = 2.3V (Note 1) VIN = 15V VSHDN 0.2V, Fixed Voltages (Note 7) TJ 0C TJ < 0C
q q q q q q
TJ = 25C, VIN = 2.3V unless otherwise noted.
MIN TYP 22 33 200 MAX 30 45 280 320 15 20 1.1 3 2.0 1.8 2.0 10 2.1 2.2 2.1 UNITS A A A A A A V A V V V V A V 5 10 A A ns ns 0.42
8 0.4
Shutdown Pin Threshold Shutdown Pin Input Current Input Start-Up Voltage VSHDN = 2.3V VSHDN = VIN TJ 0C TJ < 0C Light Load Full Load
q q
Undervoltage Lockout Power Switch Switch On Resistance Peak Switch Current (Note 3) Switch Breakdown Voltage Switch Leakage Current Switch Turn-On Delay (Note 5) Switch Turn-Off Delay (Note 5) Current Sense Resistor Low-Battery Comparator Low-Battery Threshold Threshold Hysteresis LBI Input Bias Current LBO Output Low State LBO Leakage Current LT1500 Functions SYNC Pin Bias Current SYNC Pin Threshold Error Amplifier Transconductance VC Pin Source Current VC Pin High Clamp Voltage NFB Reference Voltage NFB Pin Bias Current NFB to FB Transconductance Soft Start Bias Current
ISW = 0.7A (Note 2)
q q
0.50 0.7 30 0.85 45 0.2 0.3 800 400
0.72 1.3
ISW = 100A VSW = 5V VSW = 20V
q q q
q
0.28
Falling Edge
q
1.20
1.24 20
1.28
V mV
q
20 0.1 0.3
50 0.25 0.5 2
nA V V A A V mho A
VLBI = 1.2V, ISINK = 100A ISINK = 2mA VLBI = 1.3V, VLBO 15V
q q q
VSYNC = 3.3V
q q
15 0.4 600 20 1.20 1.26 1.265 12 10,000
35 1.3
1.32 1.300 20
V V A mho A
FB Pin Open
q q
1.230
Note 4 Current Flows Out of Pin
q
2
4
7
3
LT1500/LT1501
ELECTRICAL CHARACTERISTICS
The q denotes specifications which apply over the full operating temperature range. Note 1: Feedback pin or output is held sightly above the regulated value to force the VC node low and switching to stop. Note 2: See Typical Performance Characteristics for graph of Guaranteed Switch Voltage vs Saturation Voltage. Note 3: Peak switch current is the guaranteed minimum value of switch current available in normal operation. Highest calculated switch current at full load should not exceed the minimum value shown. Note 4: Loading on FB pin will affect NFB reference voltage. VNFB = IFB/gm. Do not exceed 10A loading on FB when NFB is being used. Note 5: This is the delay between sense pin current reaching its upper or lower threshold and switch transition. Switch delay times cause peak-topeak inductor current to increase and therefore switching frequency to be low. This effect will be significant for frequencies above 100kHz. See Application Information and Typical Performance Characteristics. Note 6: Reference voltage under all conditions includes VIN = 2.1V to 15V, all loads and full temperature range. Note 7: As with all boost regulators the output voltage of the LT1500 cannot fall to less than input voltage because of the path through the catch diode. This means that the output voltage divider on adjustable parts will still be generating feedback voltage at the FB pin (fixed voltage parts have an internal switch to disconnect the divider in shutdown). If the voltage on FB is greater than 0.6V in shutdown, the internal error amplifier will draw current that adds to shutdown current. See graph of Shutdown Current vs FB voltage in Typical Performance Characteristics.
TYPICAL PERFORMANCE CHARACTERISTICS
Switching Frequency (3.3V Output)
1000 VIN = 2.3V 10H 1000
FREQUENCY (kHz)
FREQUENCY (kHz)
FREQUENCY (kHz)
20H 100 50H 100H BURST REGION 10 0 50 100 150 200 LOAD CURRENT (mA) 250 300
LTC1500/01 * TPC20
Efficiency (3.3V Output)
100 90 80 L = 100H 100 90 80
EFFICIENCY (%)
EFFICIENCY (%)
70 60 50 40 30 1
L = 10H
L = 33H
70 60 50
L = 10H
L = 33H
EFFICIENCY (%)
TJ = 25C VIN = 2.3V LOW LOSS INDUCTOR 10 100 LOAD CURRENT (mA) 1000
LTC1500/01 * TPC17
4
UW
Switching Frequency (5V Output)
VIN = 3V 10H
Switching Frequency (12V Output)
1000 10H 20H 50H 100H 100 VIN = 5V
20H 100 50H 100H
BURST REGION 10 0 50 100 150 200 LOAD CURRENT (mA) 250 300
BURST REGION
10 0 25 50 75 100 125 150 175 LOAD CURRENT (mA) 200
LTC1500/01 * TPC21
LTC1500/01 * TPC22
Efficiency (5V Output)
100 L = 100H 90 80 70 60 50 VIN = 3V LOW LOSS INDUCTOR 1 10 100 LOAD CURRENT (mA) 1000
LTC1500/01 * TPC18
Efficiency (12V Output)
L = 100H
L = 33H L = 10H
40 30
40 30 1
VIN = 5V LOW LOSS INDUCTOR 10 100 LOAD CURRENT (mA) 1000
LTC1500/01 * TPC19
LT1500/LT1501 TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency (3.3V Output)
100 90
EFFICIENCY (%)
ILOAD = 100mA
EFFICIENCY (%)
EFFICIENCY (%)
80 70 60 50 TJ = 25C L = 33H LOW LOSS INDUCTOR 2.00
ILOAD = 10mA
40 1.75
2.25 2.50 2.75 INPUT VOLTAGE (V)
Inductor Copper Loss (3.3V Output)
10 R = 1 R = 0.5 10
EFFICIENCY LOSS (%)
R = 0.2
EFFICIENCY LOSS (%)
R = 0.2 R = 0.1
EFFICIENCY LOSS (%)
1
R = 0.1
VIN = 2.3V 0.1 0 50 100 150 200 LOAD CURRENT (mA) 250 300 0.1 0 50 100 150 200 LOAD CURRENT (mA) 250 300
LT1500/01 * TPC14
Maximum Load Current (3.3V Output)
600 500
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
400 L 33H 300 200 100 0 1.50 L = 10H
1.75
2.00 2.25 2.50 INPUT VOLTAGE (V)
UW
3.00
LT1500/01 * TPC11
Efficiency (5V Output)
100 90 80 70 60 50 40 2.0 2.5 3.0 3.5 4.0 INPUT VOLTAGE (V) 4.5 5.0 TJ = 25C L = 33H LOW LOSS INDUCTOR ILOAD = 10mA ILOAD = 100mA 100 90 80 70 60 50 40
Efficiency (12V Output)
ILOAD = 50mA ILOAD = 10mA
TJ = 25C L = 33H LOW LOSS INDUCTOR 0 2 4 6 8 INPUT VOLTAGE (V) 10 12
3.25
LT1500/01 * TPC12
LT1500/01 * TPC13
Inductor Copper Loss (5V Output)
10
VIN = 3V R = 1 R = 0.5
Inductor Copper Loss (12V Output)
VIN = 5V R = 1
R = 0.5 1
1
R = 0.2
0.1 0 25 50 75 100 125 150 175 LOAD CURRENT (mA) 200
LT1500/01 * TPC15
LTC1500/01 * TPC16
Maximum Load Current (5V Output)
600 500 400 L 33H 300 L = 10H 200 100 0 2.75 3.00 2.0 2.5 3.0 3.5 4.0 INPUT VOLTAGE (V) 4.5 5.0 600 500 400 300 200 100 0
Maximum Load Current (12V Output)
L = 33H
L = 100H L = 10H
0
2
4 6 8 INPUT VOLTAGE (V)
10
12
LT1500/01 * TPC08
LT1500/01 * TPC09
LT1500/01 * TPC10
5
LT1500/LT1501 TYPICAL PERFORMANCE CHARACTERISTICS
Burst Mode Threshold
120 100 TJ = 25C LOAD CURRENT IS REDUCED UNTIL Burst Mode OPERATION STARTS VOUT = 3.3V 60 VOUT = 5V 40 VOUT = 12V 20 0 0 2 4 6 8 INPUT VOLTAGE (V) 10 12
PEAK-TO-PEAK INDUCTOR CURRENT
SWITCH VOLTAGE (V)
LOAD CURRENT (mA)
80
Quiescent Input Supply Current
400 350 TJ = 25C VFB OR VOUT HELD 5% HIGH, SO THAT Burst Mode OPERATION IS ACTIVATED. DOES NOT INCLUDE OUTPUT DIVIDER CURRENT
0.6 0.5 0.4 0.3 0.2 0.1 0
SUPPLY CURRENT (A)
300 250 200 150 100 50 0 0
VOLTAGE (V)
5
15 INPUT VOLTAGE (V)
10
Input Current in Shutdown
20 TJ = 25C VSHDN = 0V 16
CURRENT (A)
12
CURRENT (A)
8
4
0 0 5 15 10 INPUT VOLTAGE (V) 20 25
6
UW
LT1500/01 * TPC01
Switch Saturation Voltage
1.0 TJ = 25C 0.8
220 200 180 160 140 120 100 80
Peak-to-Peak Inductor Ripple Current
VIN = 3.3V VOUT = 5V L = 50H NOTE THAT RIPPLE CURRENT INCREASES WITH SMALLER INDUCTORS DUE TO PROPAGATION DELAY IN THE CURRENT COMPARATOR
0.6
0.4
0.2
0 0 0.2 0.6 0.8 0.4 SWITCH CURRENT (A) 1.0
0
0.1
0.6 0.5 0.2 0.3 0.4 AVERAGE SWITCH CURRENT
0.7
LT1500/01 * TPC06
LT1500/01 * TPC07
Low-Battery Output Saturation Voltage
TJ = 25C VLBI 1.2V
20
25
0
1
2 3 4 SINK CURRENT (mA)
5
6
LT1500/01 * TPC04
LT1500/01 * TPC02
Shutdown Input Current vs Feedback Pin Voltage
140 120 100 80 60 40 20 0 0 0.2 1.2 1.0 0.4 0.6 0.8 FEEDBACK PIN VOLTAGE (V) 1.4 TJ = 25C VIN = 5V ADJUSTABLE PARTS ONLY. FIXED VOLTAGE PARTS DO NOT SHOW SHUTDOWN CURRENT INCREASE WITH FEEDBACK VOLTAGE
LT1500/01 * TPC03
LT1500/01 * TPC05
LT1500/LT1501
PIN FUNCTIONS
SHDN: Logic Level Shutdown Pin. This pin must be held high (> 1.1V) for the regulator to run. SHDN can be tied directly to VIN, even with VIN = 18V. The low-battery detector remains active in shutdown, but all other circuitry is turned off. VIN: This pin supplies power to the regulator and is connected to one side of the inductor sense resistor. It should be bypassed close to the chip with a low ESR capacitor. ISENSE: This is one end of the internal inductor-current sense resistor. With most applications, only the external inductor is tied to this pin. GND: This pin carries only low level current in the LT1500, but it carries full switch current in the LT1501. The negative end of the input bypass capacitor should be connected close to this pin and the pin should go directly to the ground plane with the LT1501. PGND (LT1500 Only): This pin is the emitter of the internal NPN power switch. Connect it directly to the ground plane. SW: This is the collector of the internal NPN power switch. To avoid EMI and overvoltage spikes, keep connections to this pin very short. LBI: This is the input to the low-battery detector with a threshold of 1.24V. Maximum pin voltage is 5V. Bypass LBI with a small filter capacitor when used. If unused, tie LBI to ground. The low-battery detector remains active in shutdown. LBO : This is the open collector output of the low-battery detector. It will sink up to 2mA. Leave open if not used. FB/VOUT: FB is the inverting input to the error amplifier with a regulating point of 1.265V and a typical bias current of 30nA. Bias current is reduced with a canceling circuit, so bias current could flow in either direction. FB is replaced with VOUT on fixed voltage parts. VOUT is the top of an internal divider that is connected to the internal FB node. A switch disconnects the divider in shutdown so that the divider current does not load VIN through the inductor and catch diode. NFB/SELECT (LT1500 Only): NFB is a second feedback node used to regulate a negative output voltage. Negative output voltages can be generated by using a transformer flyback circuit, a Cuk converter or a capacitor charge pump added to a boost converter. The regulating point for NFB is 1.265V and the internal resistance to ground is 100k. External divider current should be 300A or greater to avoid negative output voltage variations due to production variations in the internal resistor value. FB should be left open when using NFB. On fixed voltage parts, NFB is replaced with Select. The Select pin is used to set output voltage at either 3.3V or 5V. VC (LT1500 Only): This is the output of the error amplifier and the input to the current comparator. The VC pin voltage is about 700mV at very light loads and about 1.2V at full load. An internal comparator detects when the VC voltage drops below about 750mV and shuts down the current comparator and the power switch biasing to reduce quiescent current. This forces the regulator to operate in Burst Mode operation. SYNC (LT1500 Only): This is a logic level input used to synchronize switching frequency to an external clock. The sync signal overrides the internal current comparator and turns the switch on. Minimum sync pulse width should be 50ns and maximum width should be 300ns. A continuous high sync signal will force the power switch to stay on indefinitely and current will increase without limit. Don't do this! SS (LT1500 Only): This is the soft start function using the base of a PNP transistor whose emitter is tied to the VC pin. Grounding SS will turn off switching by pulling VC low. A capacitor tied from SS to ground will force VC to ramp up slowly during start-up at a rate set by the capacitor value and the internal 4A pull-up current. An external resistor must be used to reset the capacitor voltage completely to 0V at power down.
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LT1500/LT1501
BLOCK DIAGRAM
LBI
+ -
1.24V
SHDN
BIAS BURST COMPARATOR SW FIXED HYSTERESIS
1.265V REFERENCE
+
ERROR AMP VARIABLE HYSTERESIS I2
100k
-
100k
NEGATIVE ERROR AMP
NFB
FB
APPLICATIONS INFORMATION
OPERATION (SEE BLOCK DIAGRAM) The LT1500 uses a current mode architecture without the need for an internal oscillator. Switching frequency is determined by the value of the external inductor used. This technique allows the selection of an operating frequency best suited to each application and considerably simplifies the internal circuitry needed. It also eliminates a subharmonic oscillation problem common to all fixed frequency (clocked) current mode switchers. In addition, it allows for high efficiency micropower operation while maintaining higher operating frequencies. Because the power switch (Q1) is grounded, the basic topology used will normally be a boost converter with output voltage always higher than the input voltage. Special topologies such as the SEPIC, flyback and Cuk converter can also be used when the output voltage may not always be higher than the input or when full shutdown of the output voltage is needed. Operation as a boost converter is as follows. Assume that inductor current is continuous, meaning that it never drops to zero. When the switch is on, inductor current will increase with voltage across the inductor equal to VIN. When the switch is off inductor current will decrease with inductor voltage equal to VOUT - VIN. Switching frequency will be determined by the inductor
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+
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LBO SYNC IN ISENSE RSENSE 0.28 OUTPUT 18mV Rh
+
-
0.75V CURRENT COMPARATOR
+
R1
I1
Q1
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VC
-
150pF S1 R2 GND PGND
LTC1500/01 * BD
LT1500/LT1501
APPLICATIONS INFORMATION
value, the peak-to-peak inductor current (set internally) and the values for VIN and VOUT. The LT1500 controls output voltage in continuous mode by adjusting the average value of inductor current while maintaining the peakto-peak value of the current relatively constant, hence, the name "current mode architecture." The LT1500 sets the peak-to-peak value of switch current internally to establish operating frequency. This peak-topeak value is scaled down somewhat at light load currents to avoid as long as possible the characteristic of other micropower converters wherein their switching frequency drops very low (into the audio range) at less than full load currents. At extremely light loads, even the LT1500 can no longer maintain higher frequency operation, and utilizes a Burst Mode operation to control output voltage. Details of Continuous Mode Operation At the start of a switch cycle, inductor current has decreased to the point where the voltage across RSENSE is less than the internally generated voltage across Rh. This causes the current comparator output to go high and turn on the switch. At the same time, extra current is added to Rh via S1 to create hysteresis in the trip point of the comparator. This extra current is composed of a fixed amount (I1), and an amount proportional to average inductor current (I2). The presence of a variable I2 increases switching frequency at lighter loads to extend the load current range where high frequency operation is maintained and no Burst Mode operation exists. With the switch turned on, inductor current will increase until the voltage drop across RSENSE is equal to the higher voltage across Rh. Then the comparator output will go low, the switch will turn off and the current through Rh will be switched back to its lower value. Inductor current will decrease until the original condition is reached, completing one switch cycle. Control of output voltage is maintained by adjusting the continuous current flowing through Rh. This affects both upper and lower inductor current trip levels at the same time. Continuous Rh current is controlled by the error amplifier which is comparing the voltage on the Feedback pin to the internal 1.265V reference. An internal frequency compensation capacitor filters out most the ripple voltage at the amplifier output. Operation at Light Loads At light load currents the lower trip level (switch turn-on) for inductor current drops below zero. At first glance, this would seem to initiate a permanent switch off-state because the inductor current cannot reverse in a boost topology. In fact, what happens is that output voltage drops slightly between switch cycles, causing the error amplifier output to increase and bring the current trip level back up to zero. The switch then turns back on and inductor current increases to a value set by I1 (I2 is near zero at this point). The switch then turns off, and the inductor energy is delivered to the output, causing it to rise back up slightly. One or more switch cycles may be needed to raise the output voltage high enough that the amplifier output drops enough to force a sustained switch off period. The output voltage then slowly drops back low enough to cause the amplifier output to rise high enough to initiate a switch turn-on. Switching operation now consists of a series of bursts where the switch runs at normal frequency for one or more cycles, then turns off for a number of cycles. This Burst Mode operation is what allows the LT1500 to have micropower operation and high efficiency at very light loads. Saving Current in Burst Mode Operation Internal current drain for the LT1500 control circuitry is about 400A when everything is operating. To achieve higher efficiency at extremely light loads, a special operating mode is initiated when the error amplifier output is toward the low end of its range. The adaptive bias circuit comparator detects that the error amplifier output is below a predetermined level and turns off the current comparator and switch driver biasing. This reduces current drain to about 200A, and forces a switch off state. Hysteresis in the comparator forces the device to remain in this micropower mode until the error amplifier output rises up beyond the original trip point. The regulated output voltage will fall slightly over a relatively long period of time (remember that load current is very low) until the error amplifier output rises enough to turn off the adaptive bias
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LT1500/LT1501
APPLICATIONS INFORMATION
mode. Normal operation resumes for one or more switch cycles and the output voltage increases until the error amplifier output falls below threshold, initiating a new adaptive bias shutdown. DESIGN GUIDE Selecting Inductor Value Inductor value is chosen as a compromise between size, switching frequency, efficiency and maximum output current. Larger inductor values become physically larger but provide higher output current and give better efficiency (because of the lower switching frequency). Low inductance minimizes size but may limit output current and the higher switching frequency reduces efficiency. The simplest way to handle these trade-offs is to study the graphs in the Typical Performance Characteristics section. A few minutes with these graphs will clearly show the trade-offs and a value can be quickly chosen that meets the requirements of frequency, efficiency and output current. This leaves only physical size as the final consideration. The concern here is that for a given inductor value, smaller size usually means higher series resistance. The graphs showing efficiency loss vs inductor series resistance will allow a quick estimate of the additional losses associated with very small inductors. One final consideration is inductor construction. Many small inductors are "open frame ferrites" such as rods or barrels. These geometries do not have a closed magnetic path, so they radiate significant B fields in the vicinity of the inductor. This can affect surrounding circuitry that is sensitive to magnetic fields. Closed geometries such as toroids or E-cores have very low stray B fields, but they are larger and more expensive (naturally). Catch Diode The catch diode in a boost converter has an average current equal to output current, but the peak current can be significantly higher. Maximum reverse voltage is equal to output voltage. A 0.5A Schottky diode like MBR0520L works well in nearly all applications. Input Capacitor Input capacitors for boost regulators are less critical than the output capacitor because the input capacitor ripple current is a simple triwave without the higher frequency harmonics found in the output capacitor current. Peak-topeak current is less than 200mA and worst-case RMS ripple current in the input capacitor is less than 70mA. Input capacitor series resistance (ESR) should be low enough to keep input ripple voltage to less than 100mVP-P. This assumes that the capacitor is an aluminum or tantalum type where the capacitor reactance at the switching frequency is small compared to the ESR.
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C
2 ( f)(ESR)
A typical input capacitor is a 33F, 6V surface mount solid tantalum type TPS from AVX. It is a "C" case size, with 0.15 maximum ESR. Some caution must be used with solid tantalum input capacitors because they can be damaged with turn-on surge currents that occur when a low impedance power source is hot-switched to the input of the regulator. This problem is mitigated by using a capacitor with a voltage rating at least twice the highest expected input voltage. Consult with the manufacturer for additional guidelines. If a ceramic input capacitor is used, different design criteria are used because these capacitors have extremely low ESR and are chosen for a minimum number of microfarads. C (Ceramic) = 1 4f
f = switching frequency A typical unit is an AVX or Tokin 3.3F or 4.7F. Output Capacitor Output ripple voltage is determined by the impedance of the output capacitor at the switching frequency. Solid tantalum capacitors rated for switching applications are recommended. These capacitors are essentially resistive at frequencies above 50kHz, so ESR is the important factor in determining ripple voltage. A typical unit is a 220F, 10V
LT1500/LT1501
APPLICATIONS INFORMATION
type TPS from AVX, or type 595D from Sprague. These have an ESR of 0.06 in a "E" case size. At lower output current levels, a 100F unit in a "D" case size may be sufficient. Output ripple voltage can be calculated from:
1.2(IOUT )(VOUT ) VRIPPLE = ESR 0.1 + VIN
R2 = 1M(1.265) = 118k 12 - 1.265
ERROR AMPLIFIER R2 118K 1% 1.265V
LTC1500/01 * F01
Precautions regarding solid tantalum capacitors for input bypassing do not apply to the output capacitor because turn-on surges are limited by the inductor and discharge surges do not harm the capacitors. Setting Output Voltage Preset 3.3V and 5V parts are available. For other voltage applications the adjustable part uses an external resistor divider to set output voltage. Bias current for the feedback (FB) pin is typically 30nA (it is internally compensated). Thevenin divider resistance should be 100k or less to keep bias current errors to a minimum. This leads to a value for R1 and R2 (see Figure 1) of:
Figure 1. External Voltage Divider
Selectable Output (Fixed Voltage Parts) The Select pin (available only on LT1500-3/5) allows the user to select either a 3.3V or 5V output. Floating the pin sets output voltage at 3.3V and grounding the pin sets output voltage at 5V. The equivalent circuit of the Select pin function is shown in Figure 2.
VOUT
R1 =
Example: VOUT = xxV
GND
100k(12) R1 = = 949k (use 1M) 1.265
Figure 2. Schematic of Select Pin Function
Note that there is a switch in series with the VOUT pin. This switch is turned off in shutdown to eliminate shutdown current drawn by the voltage divider. For adjustable parts
+
R1(1.265) R2 = VOUT - 1.265
ERROR AMPLIFIER
-
100k(VOUT ) 1.265V
1.265V
-
+
Loop frequency stability is affected by the characteristics of the output capacitor. The ESR of the capacitor should be very low, and the capacitance must be large (> 200F) to ensure good loop stability under worst-case conditions of low input voltage, higher output voltages, and high load currents. The 14-pin LT1500 can use external frequency compensation on the VC pin to give good loop stability with smaller output capacitors. See Loop Stability section for details.
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Note that there is an internal switch that disconnects the internal divider for fixed 3.3V and 5V parts in shutdown. This prevents the divider from adding to shutdown current. Without this switch, shutdown current increases because of the divider current directly, but even more so if the FB pin is held above 0.6V by the divider. See graphs in Typical Performance Characteristics.
VOUT = 12V R1 1M 1%
FB
204k
69k SELECT 58k
LTC1500/01 * F02
11
LT1500/LT1501
APPLICATIONS INFORMATION
with an external divider no switch exists and the divider current remains. There may be additional current drawn by the adjustable LT1500 in shutdown if the divider voltage at the feedback node exceeds 0.6V. See Typical Performance Characteristics. Loop Stability The LT1501 is internally compensated since the device has no spare pin for a compensation point. The LT1500 brings out the VC pin to which an external series RC network is connected. This provides roll-off for the error amplifier, ensuring overall loop stability. Typical values when using tantalum output capacitors are 1000pF and 100k. Transient response of Figure 3's circuit with a 30mA to 100mA load step is detailed in Figure 4. The maximum output disturbance is approximately 20mV. The "splitting" of the VOUT trace when load current increases to 100mA is due to ESR of COUT. COUT can be replaced by a ceramic unit, which has lower ESR, size and cost. Figure 5 shows transient response to the same 30mA to 100mA load step, with COUT = 15F ceramic, CC = 2200pF and RC = 10k. The maximum output disturbance in this case is 100mV. Low-Battery Detector The low-battery detector is a combined reference and comparator. It has a threshold of 1.24V with a typical input bias current of 20nA. In a typical application a resistor divider is connected across the battery input voltage with the center tap tied to Low Battery Input (LBI), see Figure 6. The suggested parallel resistance of the divider is 150k
VCOMP 500mV/DIV VOUT 20mV/DIV AC COUPLED I LOAD 100mA 30mA IL 500mA/DIV 500s/DIV
VIN 2V VIN SHDN LT1500
33H MBR0520L CTX33-1
ISENSE SW FB VC
1M
100pF
332k RC 100k CC 1000pF
GND
PGND
*TANTALUM = AVX TPS SERIES CERAMIC = TOKIN 1E156ZY5U
Figure 3. LT1500 2V to 5V Converter
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Figure 4. Transient Response of LT1500 with RC = 100k, CC = 1000pF and COUT = 220F. VOUT Disturbance is 20mV
VCOMP 500mV/DIV
VOUT 50mV/DIV AC COUPLED
VOUT 5V
I LOAD
100mA
30mA IL 500mA/DIV 200s/DIV
+ COUT*
220F
Figure 5. Transient Response of LT1500 with RC = 10k, CC = 2200pF and COUT =15F Ceramic. VOUT Disturbance is 100mV
VCC VIN 470k LBI R4 274k 1% R5 10M
LT1500/01 * F06
LT1500/01 * F03
R3 301k 1%
LT1500 LBO LT1501
GND
PULL-UP RESISTOR SHOULD BE AT LEAST FIVE TIMES SMALLER THAN R5 TO ENSURE LBO HIGH STATE
Figure 6. Low Battery Detection
LT1500/LT1501
APPLICATIONS INFORMATION
and it should be no more than 300k to keep bias current errors under 1%, giving:
R (V ) R3 = DIV BAT 1.24V R4 = R3(1.24) VBAT - 1.24
R4 =
VBAT = low battery voltage RDIV = Thevenin divider resistance = R3 in parallel with R4 There is about 20mV of hysteresis at the LBI pin. Hysteresis can be increased by adding a resistor (R5) from the output (LBO) back to LBI. This resistor can be calculated from the following equation, but note that the equation for R4 will have to be changed when R5 is added. R5 = R3(VCC ) (VHYST ) - 17mV(VBAT )
VCC = supply voltage for LBO pull-up resistor VHYST = desired hysteresis at the battery R4 (When R5 is Used) = R3(R5)(1.24) R5(VBAT - 1.24) + R3(VCC - 1.24) The LBO pin is open collector. The external pull-up resistor value is determined by user needs. Generally the resistor is 100k to 1M to keep current drain low, but the LBO pin can sink several milliamperes if needed. Example: low battery voltage = 2.5V, desired hysteresis = 200mV, VCC = 5V. Use RDIV = 150k
R3 =
150 k(2.5) = 302k (use 301k, 1%) 1.24 301k(5V ) = 9.56M (Use 10M) (0.2) - 0.017(2.5)
R5 =
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(301k)(10M)(1.24) 10M(2.5 - 1.24) + 301k(5 - 1.24)
= 272k (Use 274k 1%)
The total divider resistance will be 274k + 301k = 575k, and this will draw about 7A from a fully charged battery. Synchronizing The SYNC pin on the LT1500 can be used to synchronize switching frequency to an external clock. The pin should be driven with a 50ns to 300ns pulse which will trigger the switch to an on state. There is a fairly restricted range over which synchronizing will work, because the period between sync pulses must be greater than the natural on-time of the regulator when it is running unsynchronized, and the sync frequency must be greater than the unsynchronized switching frequency. This puts the following restrictions on synchronized operation:
fSYNC > fNATURAL f (V ) fSYNC < NATURAL OUT (Use Minimum VIN ) VOUT - VIN
fNATURAL is the natural unsynchronized switching frequency of the regulator. It is a function of load current, so a careful check must be done to ensure that the above conditions are met under all load and input voltage conditions. Soft Start (SS) The LT1500 can be soft started by connecting a capacitor to the SS pin. This pin is the base of a PNP transistor whose emitter is tied to the VC pin. Soft start action will occur over the range of 0V to 0.8V on the SS pin and the pin is clamped at 1.2V with an internal clamp. An internal 4A pull-up current and the external capacitor value determine soft start time. In a typical application a 0.22F capacitor is sufficient to limit input surges and prevent output overshoot, even with overcompensation on the VC pin. Output voltages greater than 6V with very large output
13
LT1500/LT1501
APPLICATIONS INFORMATION
capacitors may require the capacitor to be larger. To ensure proper reset of the soft start capacitor, an external resistor must be connected in parallel with the capacitor. The resistor value should be 470k or more. Calculating Temperature Rise For most applications, temperature rise in the IC will be fairly low and will not be a problem. However, if load currents are near the maximum allowed and ambient temperatures are also high, a calculation should be done to ensure that the maximum junction temperature of 100C is not exceeded. The calculations must account for power dissipation in the switch, the drive circuitry and the sense resistor. PTOTAL = total device power dissipation RSW = switch resistance (0.72 max) RSENSE = sense resistance (0.42 max) With VIN = -2.2V, VOUT = 5V, IOUT = 150mA, an 8-pin SO package and maximum ambient temperature of 85C (industrial range),
(IOUT )2 (RSW )(VOUT )(VOUT - VIN ) PTOTAL = (VIN )2 2 IOUT (VOUT - VIN ) RSENSE (IOUT * VOUT ) + +
30 (VIN ) 2
PACKAGE DESCRIPTION
Dimensions in inches (millimeters) unless otherwise noted.
S8 Package 8-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
0.189 - 0.197* (4.801 - 5.004) 8 7 6 5
0.010 - 0.020 x 45 (0.254 - 0.508) 0.008 - 0.010 (0.203 - 0.254) 0- 8 TYP
0.016 - 0.050 0.406 - 1.270 *DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE **DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
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(0.15)2 (0.72)(5)(5 - 2.2) + 0.15(5 - 2.2) PTOTAL = 30 (2.2)2 2 0.42(0.15 * 5) + (2.2)2
= 0.47 + 0.014 + 0.049 = 0.11W
The SO package has a thermal resistance of 120C/W, so maximum device temperature will be: TJMAX = 85C + 0.11W(120C/W) = 98C
0.228 - 0.244 (5.791 - 6.197)
0.150 - 0.157** (3.810 - 3.988)
1
2
3
4
0.053 - 0.069 (1.346 - 1.752)
0.004 - 0.010 (0.101 - 0.254)
0.014 - 0.019 (0.355 - 0.483)
0.050 (1.270) BSC
SO8 0695
LT1500/LT1501
PACKAGE DESCRIPTION
0.010 - 0.020 x 45 (0.254 - 0.508) 0.008 - 0.010 (0.203 - 0.254) 0 - 8 TYP
0.016 - 0.050 0.406 - 1.270
*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE **DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
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Dimensions in inches (millimeters) unless otherwise noted.
S Package 14-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
0.337 - 0.344* (8.560 - 8.738) 14 13 12 11 10 9 8
0.228 - 0.244 (5.791 - 6.197)
0.150 - 0.157** (3.810 - 3.988)
1
2
3
4
5
6
7
0.053 - 0.069 (1.346 - 1.752)
0.004 - 0.010 (0.101 - 0.254)
0.014 - 0.019 (0.355 - 0.483)
0.050 (1.270) TYP
S14 0695
15
LT1500/LT1501
TYPICAL APPLICATION
Typical LT1500 (14-Pin) Application, 2-Cell to 5V Converter
33H MBR0520L 5V 249k ON(HI) OFF (LO) 2 EACH NiCd OR ALKALINE CELLS IN SHDN LBI LT1500-3.3/LT1501-5 LBO TO SYSTEM SYNC SS SELECT GND PGND COMP 100k 1M 0.22F 1000pF
LT1500/01 * TA02
+
RELATED PARTS
PART NUMBER LTC(R)1163 LTC1174 LT1302 LT1304 LTC1440/1/2 LTC1516 LT1521 DESCRIPTION Triple High Side Driver for 2-Cell Inputs Micropower Step-Down DC/DC Converter High Output Current Micropower DC/DC Converter 2-Cell Micropower DC/DC Converter Ultralow Power Single/Dual Comparator with Reference 2-Cell to 5V Regulated Charge Pump Micropower Low Dropout Linear Regulator COMMENTS 1.8V Minimum Input, Drives N-Channel MOSFETs 94% Efficiency, 130A IQ, 9V to 5V at 300mA 5V/600mA from 2V, 2A Internal Switch, 200A IQ Low-Battery Detector Active in Shutdown 2.8A IQ, Adjustable Hysteresis 12A IQ, No Inductors, 5V at 50mA from 3V Input 500mV Dropout, 300mA Current, 12A IQ
16
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 q FAX: (408) 434-0507 q TELEX: 499-3977
U
ISENSE SW OUT
+
220F 10V
5V 470k
402k
1nF
LT/GP 0896 7K * PRINTED IN USA
(c) LINEAR TECHNOLOGY CORPORATION 1996

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