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LT1573 Low Dropout Regulator Driver
FEATURES
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DESCRIPTION
The LT (R)1573 is a regulator driver IC designed to provide a low cost solution to applications requiring high current, low dropout and fast transient response. When combined with an external PNP power transistor, this device provides load current up to 5A with dropout voltages as low as 0.35V. The LT1573 circuitry is designed for extremely fast transient response. This greatly reduces bulk storage capacitance when the regulator is used in applications with fast, high current load transients. To keep cost and complexity low, the LT1573 uses a new time-delayed latching current protection technique that requires no external current sense resistor. Base drive is limited for instantaneous protection, and a time-delayed latch protects the regulator from continuous short circuits. The LT1573 is available as an adjustable regulator with an output range of 1.27V to 6.8V and with fixed output voltages of 2.5V, 2.8V and 3.3V. Output accuracy is better than 1% to meet the critical regulation requirement of fast microprocessors. A special 8-pin, fused-lead surface mount package is used to minimize regulator footprint and provide adequate heat sinking.
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Low Cost Solution for High Current, Low Dropout Regulators Fast Transient Response Needs Much Less Bulk Capacitance Latching Overload Protection Minimizes Heat Sink Size Single Supply Operation: VIN = 10V to 2.8V Precision Output Voltage (1%) Small Surface Mount Package Capable of Very Low Dropout Voltage (<0.2V) Fixed or Adjustable Outputs Shutdown
APPLICATIONS
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3.3V to 2.5V Regulator Microprocessor Power Source Post Regulator for Switching Supplies High Efficiency Linear Regulators Ultralow Dropout Regulators Low Voltage Linear Regulators
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATION
CC 100pF FB LATCH COMP VOUT VIN DRIVE RC 1k
+
CTIME GND
LT1573 SHDN RD 24 RB 50
50mV/DIV
QOUT MOTOROLA D45H11 R1 1.6k LOAD R2 1k GND
1573 F01
VOUT
CIN 100F TANT
+
VIN 5V
+
COUT1 1F + CER x 24
2.5A/DIV 10s/DIV
COUT2 220F TANT
VOUT = 1.265V (1 + R1/R2) FOR T < 45C, COUT1 = 24 x 1F Y5V CERAMIC SURFACE MOUNT CAPACITORS. FOR T > 45C, COUT1 = 24 x 1F X7R CERAMIC SURFACE MOUNT CAPACITORS. PLACE COUT1 IN THE MICROPROCESSOR SOCKET CAVITY
Figure 1. 3.3V, 5A Microprocessor Supply
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Transient Response for 0.2A to 5A Output Load Step
1573 F01a
1
LT1573
ABSOLUTE MAXIMUM RATINGS
Input Pin Voltage (VIN to GND) ................................ 10V Drive Pin Voltage (VDRIVE to GND)........................... 10V Output Pin Voltage (VOUT to GND) ........................... 10V Shutdown Pin Voltage (VSHDN to GND) .................. 10V Operating Junction Temperature Range .... 0C to 125C Storage Temperature Range ................. - 65C to 150C Lead Temperature (Soldering, 10 sec.)................. 300C
PACKAGE/ORDER INFORMATION
TOP VIEW FB 1 LATCH 2 SHDN 3 GND 4 8 7 6 5 COMP VOUT VIN DRIVE
ORDER PART NUMBER LT1573CS8 LT1573CS8-2.5 LT1573CS8-2.8 LT1573CS8-3.3 S8 PART MARKING 1573 157325 157328 157333
S8 PACKAGE 8-LEAD PLASTIC SO TJMAX = 125C, JA = 85C/ W
Consult factory for Industrial and Military grade parts.
ELECTRICAL CHARACTERISTICS
PARAMETER DC Characteristics LT1573 Reference Voltage (Adjustable)(Note 1)
VIN = 5V, VDRIVE = 3V, TA = 25C, unless otherwise noted.
MIN 1.252
q
CONDITIONS IDRIVE = 20mA, TJ = 25C 5mA < IDRIVE < 250mA, 3V < VIN < 7V, 1.5V < VDRIVE < 7V
TYP 1.265 1.265 3.3 3.3 2.8 2.8 2.5 2.5
MAX 1.278 1.290 3.333 3.366 2.828 2.856 2.525 2.550
UNITS V V V V V V V V
1.240 3.267
LT1573-3.3 Output Voltage (Note 1)
IDRIVE = 20mA. TJ = 25C 5mA < IDRIVE < 250mA, 3.5V < VIN < 7V, 1.5V < VDRIVE < 7V
q
3.234 2.772
LT1573-2.8 Output Voltage (Note 1)
IDRIVE = 20mA, TJ = 25C 5mA < IDRIVE < 250mA, 3V < VIN < 7V, 1.5V < VDRIVE < 7V
q
2.744 2.475
LT1573-2.5 Output Voltage (Note 1)
IDRIVE = 20mA, TJ = 25C 5mA < IDRIVE < 250mA, 3V < VIN < 7V, 1.5V < VDRIVE < 7V
q
2.450
Line Regulation LT1573 (VFB) LT1573-3.3 (VOUT) LT1573-2.8 (VOUT) LT1573-2.5 (VOUT) Load Regulation LT1573 (VFB) LT1573-3.3 (VOUT) LT1573-2.8 (VOUT) LT1573-2.5 (VOUT) FB Pin Bias Current (Adjustable Only) DRIVE Pin Current DRIVE Pin Saturation Voltage SHDN Pin Threshold Voltage SHDN Pin Current
IDRIVE = 20mA, 3V < VIN < 7V IDRIVE = 20mA, 3.5V < VIN < 7V IDRIVE = 20mA, 3V < VIN < 7V IDRIVE = 20mA, 3V < VIN < 7V IDRIVE = 20mA to 250mA IDRIVE = 20mA to 250mA IDRIVE = 20mA to 250mA IDRIVE = 20mA to 250mA VFB = 1.265V VFB = 1.35V, VDRIVE = 7V VFB = 1.15V, VDRIVE = 1.5V IDRIVE = 20mA, VFB = 1.15V IDRIVE = 250mA, VFB = 1.15V VSHDN = 5V
q q q q q q q q q q q q q q
0.17 0.34 0.34 0.25 7 18 15 13 0.8 250 440 0.12 0.73 1.0 1.33 200
2 5 4 4 15 40 34 30 5 2 0.3 1.4 1.6
2
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mV mV mV mV mV mV mV mV A mA mA V V V A
LT1573
ELECTRICAL CHARACTERISTICS
PARAMETER LATCH Pin Latch-Off Threshold Voltage LATCH Pin Charging Current LATCH Pin Latching Current VIN - VOUT Differential Threshold for Latch Disable Input Quiescent Current Minimum Input Voltage for Bias Operation VIN = 7V
VIN = 5V, VDRIVE = 3V, TA = 25C, unless otherwise noted.
MIN
q
CONDITIONS
TYP 1.4 7 0.65
MAX 2.2
UNITS V A mA
0.8
q q q
0.4 2.8
0.7 1.7
1.0 3.5
V mA V
The q denotes specifications that apply over the full operating temperature range. Note 1: Operating conditions are limited by maximum junction temperature. The regulated feedback or output voltage specification will
not apply for all possible combinations of input voltage, drive voltage and drive current. When operating at maximum drive current, the drive voltage range must be limited. When operating at maximum input and drive voltage, the drive current must be limited.
TYPICAL PERFORMANCE CHARACTERISTICS
LT1573 Feedback Pin Voltage vs Temperature
1.290 1.285 3.40 3.38 3.36 OUTPUT VOLTAGE (V) 3.34 3.32 3.30 3.28 3.26 3.24 3.22 0 25 50 75 100 125 150 TEMPERATURE (C)
1573 G01
FEEDBACK PIN VOLTAGE (V)
1.280 1.275 1.270 1.265 1.260 1.255 1.250 1.245 1.240 -50 -25
OUTPUT VOLTAGE (V)
LT1573-2.5V Output Voltage vs Temperature
2.60 2.58 2.5
FEEDBACK PIN CURRENT (A)
2.54 2.52 2.50 2.48 2.46 2.44 2.42 2.40 -50 -25 50 25 0 75 TEMPERATURE (C) 100 125
QUIESCENT CURRENT (mA)
2.56 OUTPUT VOLTAGE (V)
UW
1573 G04
LT1573-3.3V Output Voltage vs Temperature
2.90 2.88 2.86 2.84 2.82 2.80 2.78 2.76 2.74 2.72
-25 50 25 0 75 TEMPERATURE (C) 100 125
LT1573-2.8V Output Voltage vs Temperature
3.20 -50
2.70 -50
-25
50 25 0 75 TEMPERATURE (C)
100
125
1573 G02
1573 G03
Feedback Pin Bias Current vs Temperature
3.0 2.5 2.0 1.5 1.0 0.5
Quiescent Current vs Temperature
2.0
1.5
1.0
0.5
0 -50 -25
0
25 50 75 100 125 150 TEMPERATURE (C)
1573 G05
0 -50 -25
0
25 50 75 100 125 150 TEMPERATURE (C)
1573 G06
3
LT1573 TYPICAL PERFORMANCE CHARACTERISTICS
Drive Pin Current vs Feedback Pin Voltage
450 400
DRIVE PIN CURRENT (mA) 1.0
TJ = 130C TJ = 25C TJ = -45C
DRIVE PIN VOLTAGE (V)
350 300 250 200 150 100 50 0 0
0.6 0.5 0.4 0.3 0.2 0.1 0 TJ = 25C TJ = -45C
VIN - VOUT (V)
0.2
0.4 0.6 0.8 1.0 1.2 FEEDBACK PIN VOLTAGE (V)
Latch Pin Latch-Off Threshold vs Input Voltage
3.0 16
LATCH CHARGING CURRENT (A)
LATCH PIN LATCH-OFF THRESHOLD (V)
2.5 2.0 1.5 TJ = 125C 1.0 0.5 0 TJ = -45C TJ = 25C
LATCHING CURRENT (mA)
2
3
4 5 6 INPUT VOLTAGE (V)
Shutdown Voltage Threshold vs Temperature
1.5
SHUTDOWN THRESHOLD (V)
1.4
SHUTDOWN PIN CURRENT (A)
1.3
1.2
1.1
1.0 -50 -25
4
UW
1573 G07
Drive Pin Saturation Voltage vs Drive Pin Current
0.85 0.80 0.75
Latch-Disable Threshold (VIN - VOUT) vs Temperature
0.9 0.8 0.7 TJ = 130C
0.70 0.65 0.60 0.55 0.50 0.45 VIN = 5V LATCH DISABLED FOR (VIN - VOUT) < LATCH DISABLE THRESHOLD
1.4
0
50
150 200 250 100 DRIVE PIN CURRENT (mA)
300
0.40 -50 -25
0
25 50 75 100 125 150 TEMPERATURE (C)
1573 G09
1573 G08
Latch Charging Current vs Input Voltage
1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 2 3 5 4 6 INPUT VOLTAGE (V) 7 8
1573 G11
Latching Current vs Input Voltage
TJ = 25C TJ = -45C TJ = 125C
14 12 10 8 6 4 2 0 TJ = 125C TJ = 25C TJ = -45C
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1573 G10
0
2
3
4 5 6 INPUT VOLTAGE (V)
7
8
1573 G12
Shutdown Pin Current vs Shutdown Pin Voltage
300 250 200 150 TJ = 125C 100 50 0 TJ = -45C TJ = 25C
0
25 50 75 100 125 150 TEMPERATURE (C)
1573 G13
0
1
2 3 4 5 6 SHUTDOWN PIN VOLTAGE (V)
7
1573 G14
LT1573
PIN FUNCTIONS
FB (Pin 1): The feedback pin is the inverting input of the error amplifier. The noninverting input of the error amplifier is internally connected to a 1.265V reference. The error amplifier will servo the drive to the output transistor, QOUT in Figure 1, to force the voltage at the feedback pin to be 1.265V. Output voltage is set by a resistor divider as shown in Figure 1. For adjustable devices an external resistor divider is used to set the output voltage. For fixed voltage devices the resistor divider is internal and the top of the resistor divider is connected to the VOUT pin. LATCH (Pin 2): The LT1573 provides overcurrent protection with a timed latch-off circuit. The latch-off time out is triggered when the DRIVE pin is pulled below the saturation voltage of the drive transistor. The saturation voltage is a function of the drive current and is equal to approximately 130mV at 20mA rising to 780mV at 250mA (see typical performance curves). The time out is set by the latch charging current and the value of a capacitor connected between the LATCH pin and ground. If the overcurrent condition persists at the end of the timing cycle the regulator will latch off until either the latch is reset or power is cycled off and back on. The latch can be reset by either pulling the SHDN pin high, pulling current out of the LATCH pin greater than latching current or grounding the LATCH pin. Exceeding the thermal limit temperature will trigger the latch with no timing delay. Under normal condition, the DC voltage at the LATCH pin is zero. When the system is latched off, the DC voltage at theLATCH pin is two VBE above ground. SHDN (Pin 3): The SHDN pin has two functions. It can be used to turn off the output voltage by disabling the drive to the output transistor. It can also be used to reset the current limit latch. The shutdown/reset functions are activated by applying a voltage > 1.3V to the SHDN pin. The output voltage will restart as soon as the SHDN pin is pulled below the shutdown threshold. If the shutdown/ reset function is not used, the pin should be grounded. The voltage applied to the SHDN pin can be higher than the input voltage. When the SHDN pin voltage is higher than 2V, the SHDN pin current increases and is limited by an internal 20k resistor. GND (Pin 4): Circuit Ground. DRIVE (Pin 5): The DRIVE pin is connected to the collector of the main drive transistor of the LT1573. This drive transistor sinks the base current of the external PNP output transistor. A resistor is normally inserted between the base of the external PNP output transistor and the DRIVE pin. This resistor is sized to allow the LT1573 to sink the appropriate amount of base current for a given application and to activate the overcurrent latch in a fault condition. VIN (Pin 6): This pin provides power to all internal circuitry of the LT1573 including bias, start-up, thermal limit, error amplifier and all overcurrent latch circuitry. VOUT (Pin 7): The VOUT pin is the input to comparator C1 shown in Block Diagram. This pin is normally connected to the output. The comparator C1 is used to disable the overcurrent latch during start-up when the output transistor is saturated. For fixed voltage devices the top of the internal resistor divider that sets the output voltage is connected to this pin. COMP (Pin 8): A compensation network is inserted between the VOUT and COMP pins to obtain optimal transient response. Under normal condition, the DC voltage of the COMP pin sits at one VBE above ground.
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5
LT1573
BLOCK DIAGRAM
VIN 6
SHDN 3
+
C3
GND 4 VSHDNTH
- +
DRIVE IDISCHRG 5
+
ILATCH NORMALLY OFF S4 2 LATCH ICHRG
S1 NORMALLY ON
C2 Q2 Q1 VDRSAT Q5
+
C4 VLATCHTH
Q4
-
+
EXTERNAL CAPACITOR CEXT
Q3
1.265V FOR FIXED VOLTAGE VERSION
THERMAL SHUTDOWN
4 GND
FUNCTIONAL DESCRIPTIO
The basic block diagram of the LT1573 is shown above. The regulating loop consists of a 1.265V reference, an error amplifier, a Darlington driver and an external PNP pass transistor. The 1.265V reference feeds the noninverting input of the error amplifier. The error amplifier drives the Darlington connected transistor pair Q1 and Q2. The collector of Q1 comes out to the DRIVE pin and is used to drive the base of an external PNP power transistor as shown in Figure 1. The error amplifier will adjust the drive current to the external PNP power transistor to maintain the feedback pin voltage at 1.265V. The LT1573 provides overcurrent protection by means of a timed latch function. Base current to the external PNP transistor is limited by placing a resistor between the base of the transistor and the DRIVE pin. When the DRIVE pin drops below VDRSAT (the DRIVE pin saturation voltage) the output of the comparator C2 switches high; S1, which is normally closed, opens and the external capacitor connected to the LATCH pin is allowed to charge. Discharge current IDISCHRG is equal to approximately 28A and charging current ICHRG is equal to approximately 7A. If the fault condition goes
away before CEXT charges to the latch threshold, C2 will switch back low, S1 will close and Q4 will discharge CEXT. If the fault condition persists long enough for CEXT to charge up to the latch threshold (VLATCHTH), comparator C4 will switch high and S4 will close and latch the output off. The device will stay latched because latching current ILATCH is greater than the pull-down current of Q4. Thermal shutdown circuitry will close S4 and latch the device off with no timing delay. Comparator C1 is used to override the latching function during start-up. If the difference between the output voltage and the input voltage is less than the input-output differential threshold (VIODTH), comparator C1 output goes high which closes S2. Current source I2 then drives base of Q4 which prevents CEXT from charging. Comparator C3 is used for system shutdown and latch reset. If SHDN pin voltage is higher than shutdown threshold VSDTH, the comparator C3 output goes high, shutting down the regulator and closing switch S3. Current I3 will drive Q4 to discharge CEXT, resetting the latch.
6
+
ERROR AMP
-
-
+
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VOUT
+
I3 NORMALLY OFF S3 S2
+
I2 NORMALLY OFF C1
7
+
VIODTH VIN 6 COMP 8 FB 1
-
+
R1
R2
1573 BD
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LT1573
APPLICATIONS INFORMATION
The LT1573 is designed to be used in conjunction with an external PNP transistor. The overall specifications of a regulator circuit using the LT1573 and an external PNP will be heavily dependent on the specifications of the external PNP. While there are a wide variety of PNP transistors available that can be used with the LT1573, the specifications given in a typical transistor data sheet are of little use in determining overall circuit performance. In the following discussion the critical requirements of the PNP transistors are noted. Design equations are given and examples are shown using a readily available discrete PNP transistor. This device is inexpensive, available from multiple sources and can be used for a wide range of applications. For applications using other PNP transistors, the regulator specifications can be derived by the same method. Basic Regulator Circuit The basic regulator circuit is shown in Figure 2. The adjustable output LT1573 senses the regulator output voltage from its feedback pin via the output voltage divider, R1 and R2, and drives the base of the external PNP transistor to maintain the regulator output at the desired value. For fixed output versions of the LT1573, the regulator output voltage is sensed from the feedback pin via an internal voltage divider. The resistor RD is required for the overcurrent latch-off function. RD is also used to limit the drive current available to the external PNP transistor and to limit the power dissipation in the LT1573. Limiting the drive current to the external PNP transistor will limit the output current of the regulator which minimizes the stress
CC FB LATCH COMP VOUT VIN RD GND DRIVE RB QOUT
+
CTIME
LT1573 SHDN
Figure 2. Basic Regulator Circuit
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on the regulator circuit under overload conditions. The resistor RD is chosen based on the operating requirements of the circuit, primarily the dropout voltage and the output current. The dropout voltage of an LT1573-based regulator circuit is determined by the VCE saturation voltage of the discrete external PNP transistor when it is driven with a base current equal to the available drive current of the LT1573. External PNP Transistor Selection Criteria The selection of an appropriate external PNP transistor depends on the regulator application specifications. The critical PNP transistor selection criteria include: 1. The maximum output current of the PNP transistor 2. The dropout voltage at the maximum output current 3. The gain-bandwidth product fT of the transistor The PNP transistor must be able to supply the specified maximum regulator output current to be qualified for the regulator application. The VCE saturation voltage of the transistor at the maximum output current determines the dropout voltage of the circuit. The dropout voltage determines the minimum regulator input voltage for a certain specified output voltage. The gain-bandwidth product fT of the transistor determines how fast the voltage regulator can follow an output load change without losing voltage regulation. The D45H11 from Motorola and the KSE45H11TU from Samsung can be used in all LT1573 regulator circuits with current ratings up to 5A. The D45H11 can supply 5A of
RC
+
VIN CIN COUT1
VOUT R1
+
+
COUT2 R2
LOAD
GND
1573 F02
7
LT1573
APPLICATIONS INFORMATION
output current with dropout voltage as low as 0.35V. The gain-bandwidth product fT of the D45H11 is typically 40MHz which enables the regulator, composed of this PNP transistor and the LT1573, to handle the load changes of several amps in a few hundred nanoseconds with a minimum amount of output capacitance. The following sections describe how specifications can be determined for the basic regulator based on the LT1573 and D45H11 from Motorola. To determine the specifications for regulators formed by the LT1573 and other PNP transistors, a similar method can be used. Dropout Voltage The dropout voltage of an LT1573-based regulator circuit is determined by the VCE saturation voltage of the discrete external PNP transistor when it is driven with a base current equal to the available drive current of the LT1573. The LT1573 is guaranteed to sink 250mA of base current (440mA typ). The available drive current of the LT1573 can be reduced by adding a resistor (RD in Figure 2) in series with the DRIVE pin. Table 1 lists some useful operating points for the D45H11. These points were empirically determined using a sampling of devices.
Table 1. D45H11 Dropout Voltage
DRIVE CURRENT (mA) 20 20 40 40 60 60 80 100 100 150 200 150 200 250 OUTPUT CURRENT (A) 1 2 2 3 3 4 4 4 5 5 5 6 6 7 TYPICAL DROPOUT VOLTAGE (V) 0.20 0.50 0.25 0.50 0.25 0.70 0.45 0.35 0.70 0.40 0.35 0.65 0.45 0.50
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Current Limit For regulator circuits using the LT1573, current limiting is achieved by limiting the base drive current to the external PNP pass transistor. This means that the actual system current limit will be a function of both the current limit of the LT1573 and the Beta of the external PNP. Motorola provides the following Beta information for the D45H11. The minimum Beta of the D45H11 is 60 when VCE = 1V and IC = 2A. The minimum Beta is 40 when VCE = 1V and IC = 4A. For other PNP transistors, the user should first find out the Beta information from the external PNP transistor manufacturer to determine the appropriate LT1573 base drive current limit. The current limit of the regulator system then can be achieved by selecting the appropriate amount of resistance RD in Figure 2. Selecting RD Resistor RD can be used to limit the available drive current to the external PNP transistor. In order to select RD, the user should first choose the value of the drive current that will give the required value of output current and dropout voltage. For a circuit using the D45H11 as a pass transistor this can be done using Table 1. For circuits using transistors other than D45H11, the user must characterize the transistor to determine the drive current requirements for the specified output current and dropout voltage. In general, it is recommended that the user choose the lowest value of drive current that will satisfy the output current requirements. This will minimize the stress on circuit components during overload conditions. The formula used to determine the resistor RD is: RD = (VIN - VBE - VDRIVE)/(IDRIVE + IRB) where, VIN = the minimum input voltage to the circuit VBE = the maximum emitter/base voltage of the PNP pass transistor IDRIVE = the minimum PNP base current required IRB = the current through RB = VBE/RB VDRIVE = the DRIVE pin saturation voltage when the DRIVE pin current equals (IDRIVE + IRB)
(1)
LT1573
APPLICATIONS INFORMATION
Resistor RB helps to turn off the PNP (QOUT in Figure 2). Smaller values for RB turn off the PNP faster but will increase input current. The recommended value for RB is 50. For circuits that do not require high output current or fast transient response, the value of RB can be increased up to 200. For the D45H11, the emitter-base voltage is a function of base and collector current. Table 2 lists some useful operating points for the D45H11. These points were empirically determined using a sampling of devices.
Table 2. D45H11 VBE
IB (mA) 1 7 23 45 66 100 IC (A) 0.2 1 2 3 4 5 VBE AT 25C (V) 0.65 0.75 0.80 0.85 0.90 0.95
Design Example Given the following operating requirements: 4.5V < VIN < 5.5V IOUT(MAX) = 5A VOUT = 3.3V 1. The first step is to determine the required drive current for the D45H11. Dropout voltage must be less than 1.2V at 5A output current. From Table 1, a drive current of 100mA will give 0.7V dropout voltage at an output current of 5A. This satisfies the operating requirements. 2. The next step is to determine the value of RD. Assume RB is 50. From Table 2, the maximum emitter-base voltage for this design is 0.95V. The current through RB is: IRB = VBE/RB = 0.95/50 = 19mA VDRIVE is the DRIVE pin saturation voltage when the DRIVE pin current equals 119mA, which can be read from the typical performance characteristics curve to be 0.39V. Resistor RD now can be calculated from Eq (1):
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RD = (4.5 - 0.95 - 0.39)V/(100 + 19)mA = 26.6 The next lowest 5% value is 24. Overcurrent Latch-Off In addition to limiting the base drive current, the resistor RD is included in the circuit for the overcurrent protection latch-off function. There is a minimum value for this resistance. It is calculated by Equation 1 with the drive current IDRIVE set to the minimum available drive current (= 250mA) from the LT1573. At high currents, RD also limits the power dissipation in the LT1573. In some conditions, resistor RD can be replaced with a short. This is possible in circuits where an overload is unlikely and the input voltage and drive requirements are low. If resistor RD is not included in the circuit, the regulator is protected against the overcurrent condition only by the thermal shutdown function. After the resistor RD is determined, a certain amount of base drive current is available to the external PNP transistor. An overcurrent or output short condition will demand a base drive current greater than the LT1573 can supply. The internal drive transistor will saturate. A time-out latch will be triggered by this overcurrent condition to turn off the regulator system. The time-out period is determined by an external capacitor connected between the LATCH and GND pins. The timeout period is equal to the time it takes for the capacitor to charge from 0V to the latch threshold which is equal to 2VBE. The latch charging current is set by an internal current source and is a function of input voltage and temperature as shown in the typical performance characteristics curve. At 25C, the typical latch charging current ranges from 7.2A with 3V input to 8A with 7V input. If the overcurrent or output short condition persists longer than the time-out period, the regulator will be shut down. Otherwise, the regulator will function normally. In the latch-off mode, some extra current is drawn from the input to maintain the latch. The latching current is a function of input voltage and temperature as shown in the typical performance characteristic curve. At 25C, the typical latching current ranges from 0.3mA with 3V input to 9.5mA with 7V input. The latch can be reset by recycling input power, by grounding the LATCH pin or by putting the device into shutdown.
9
LT1573
APPLICATIONS INFORMATION
Thermal Considerations The thermal characteristics of several components need to be considered; the LT1573, the pass transistor and resistor RD. Power dissipation should be calculated based on the worst-case conditions seen by each component during normal operation. 1. Power Dissipation of the LT1573: The worst-case power dissipation in the LT1573 is a function of drive current, supply voltage and the value of RD. Worst-case dissipation for the LT1573 occurs when the drive current is equal to approximately one half of its maximum value. The worst-case power dissipation in the LT1573 can be calculated by the following formula: 2. Power Dissipation of the Resistor RD: The worst-case power dissipation in resistor RD needs to be calculated so that the power rating of the resistor can be determined. The worst-case power dissipation in this resistor will occur when the drive current is at a maximum. The power dissipation can be calculated from the following formula:
(VIN - VBE) PD =
where,
2
(2)
4RD RD > minimum RD for latch - off function
VIN = the maximum input voltage to the circuit VBE = the minimum emitter/base voltage of the PNP pass transistor Following the previous design example for selecting resistor RD, the power dissipation of LT1573 is calculated from Eq (2): PD =
(5.5 - 0.65) 4(24)
2
= 0.25W
For some operating conditions RD may be replaced with a short. This is possible in applications where the operating requirements (input voltage and drive current) are at the low end and the output will not be shorted. For RD = 0, the following formula may be used to calculate the maximum power dissipation in the LT1573: PD = (VIN - VBE)(IDRIVE) (3) where, VIN = the maximum input voltage VBE = the minimum emitter/base voltage of the PNP IDRIVE = the required maximum drive current
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(VIN - VBE - VDRIVE) PRD =
RD
2
(4)
where, VIN = the maximum input voltage VBE = the minimum emitter/base voltage of the PNP VDRIVE = the voltage at the LT1573 DRIVE pin = VSAT of the DRIVE pin in the worst case Following the previous design example, the power dissipation of resistor RD is calculated from Eq (4):
(5.5 - 0.65 - 0.39) PRD =
24
2
= 0.83W
3. Power Dissipation of the PNP Transistor: The worstcase power dissipation in the PNP pass transistor is simply equal to: PPNP = (VIN - VOUT)(IOUT) where, VIN = the maximum input voltage IOUT = the maximum output current Following the previous design example, the power dissipation of PNP transistor is calculated from Eq (5): PPNP = (5.5 - 3.3)(5) = 11W The LT1573 series regulators have internal thermal limiting designed to protect the device during overload conditions. For continuous normal load conditions, the maximum junction temperature rating of 125C must not be exceeded. It is important to give careful consideration to all sources of thermal resistance from junction to ambient. For surface mount devices, heat sinking is accomplished by using the heat spreading
(5)
LT1573
APPLICATIONS INFORMATION
capabilities of the PC board and its copper traces. Table 3 lists some typical values for the thermal resistance of the LT1573. Measured values of thermal resistance for a specific board size with different copper areas are listed. All measurements were taken in still air on 3/32" FR-4 board with 2oz copper. It is possible to achieve significantly lower values with thinner multilayer boards.
Table 3. LT1573 Thermal Resistance
COPPER AREA TOPSIDE* 2500mm2 1000mm2 225mm2 BACKSIDE 2500mm2 2500mm2 2500mm2 THERMAL RESISTANCE BOARD AREA (JUNCTION-TO-AMBIENT) 2500mm2 2500mm2 2500mm2 80C/W 80C/W 85C/W
*Device is mounted on topside.
We can find out the maximum junction temperature of the LT1573 during normal load operation after we calculate the maximum power dissipation of the LT1573 from Eq (2). From the previous design example, the maximum power dissipation of the LT1573 is 0.2W. From Table 3, we know the thermal resistance from junction-to-ambient is around 85C/W. The temperature difference between junction and ambient is: (0.25W)(85C/W) = 21.25C If the maximum ambient temperature is specified at 50C, the maximum junction temperature will be: TJMAX = 50C + 21.25C = 71.25C The maximum junction temperature must not exceed the specified 125C for safe continuous regulator operation. Thermal Limiting The thermal shutdown temperature of the LT1573 is approximately 150C. The thermal limit of the LT1573 can be used to protect both the LT1573 and the external PNP pass transistor. This is accomplished by thermally coupling the LT1573 to the PNP power transistor by locating the LT1573 as close to the PNP transistor as possible. In this case, the power dissipation of the power transistor must be considered in the LT1573 maximum junction temperature calculation.
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U
Compensation In order to improve the transient response to regulator output load variation, a capacitor in series with a resistor can be inserted between the VOUT and COMP pins. For the microprocessor power supply regulator system based on the LT1573 and the PNP transistor D45H11 with 24 1F surface mount ceramic capacitors in parallel with one 220F surface mount tantalum capacitor at the output as shown in Figure 1, a 100pF capacitor in series with a 1k resistor is recommended. In theory, the output capacitor forms the dominant pole of the regulator system. An internal compensation capacitor forms another pole. The external compensation capacitor and resistor form a zero which adds phase margin to the regulator system to prevent high frequency oscillation. The LT1573 has an internal pole at approximately 5kHz. An external compensation zero between 10kHz and 100kHz is usually required to stabilize the regulator. The zero frequency is primarily determined by the compensation capacitor and can be roughly calculated by the following equation: fZERO = 40kHz
(
30 (pF) ) CCOMP (pF) ,10 CCOMP 100
A compensation resistor between 1k and 10k is suggested. A compensation resistor of 5k works for most cases. In some cases, a greater compensation resistor is needed to stop oscillation above 1MHz. In some cases, the output capacitor may have enough equivalent series resistance (ESR) to generate the required zero and the external compensation zero may not be needed. Output Capacitor The LT1573 is designed to be used with an external PNP transistor with a high gain-bandwidth product fT to make a regulator with a very fast transient response, which can minimize the size of the output capacitor. For a regulator made of an LT1573 and a D45H11, only one 10F surface mount ceramic capacitor at the output is enough for the regulator to handle the output load varying up to 5A in a few hundred nanoseconds interval and to remain stable with a 30pF capacitor in series with a 7.5k resistor between the VOUT and COMP pins. If tighter voltage regulation is
11
LT1573
APPLICATIONS INFORMATION
needed during output transients, more capacitance can be added to the regulator output. If more capacitance is added to the output, the bandwidth of the regulator is lowered. A large value compensation capacitor may be needed to lower the frequency of the compensation zero to avoid high frequency oscillation. Equal value output capacitors with different ESR can have different output transient response. High frequency performance will be strongly affected by parasitics in the output capacitor and board layout. Some experimentation with the external compensation will be required for optimum results. Shutdown Function The regulator can be shut down by pulling the SHDN pin voltage higher than the shutdown threshold (about 1.3V). The regulator will restart itself if the SHDN is pulled below the shutdown threshold.The SHDN pin should be tied to ground if it is not used. The SHDN pin voltage can be higher than the input voltage. When the SHDN pin voltage is higher than 2V, the SHDN pin current increases and is limited by a 20k resistor. Momentarily putting the device into shutdown also resets the overcurrent latch. Lower Dropout Voltage or Higher Output Current Capability Lower dropout voltage or higher output current capability can be achieved by paralleling several output PNP transistors as shown in Figure 3. By paralleling output PNP transistors, the equivalent resistance between the emitters (VIN) and collectors (VOUT) is lowered or each PNP transistor sharing the output current now runs at a lower collector current, which causes the dropout voltage to decrease. Because the PNP transistors are running at a lower collector current where the transistor beta is higher, much more output current can be obtained at a given base drive current. When paralleling two or more output transistors, a separate resistor is needed for RB and RD for each output transistor. This allows the base drive current to be split evenly between output transistors, which promotes equal output current sharing. In the specific example drawn in Figure 3 with two output transistors, the resistance of RB1 and RB2 is now twice the value of the resistance of RB in Figure 2, and the resistance of RD1 and RD2 is twice the value of the resistance of RD in Figure 2. In case of n PNP transistors in parallel, the resistance RB
FB LATCH
COMP VOUT VIN RB1 RB2 QOUT1 RD1 RD2 VOUT R1 LOAD R2 GND
1573 F03
+
CTIME GND
LT1573 SHDN
DRIVE
+
CIN VIN COUT1
Figure 3. Reduced Dropout Voltage or Increased Output Current by Paralleling Output PNP Transistors
12
U
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CC
RC
QOUT2
+
LT1573
APPLICATIONS INFORMATION
equals the resistance of RB1, RB2, ..., and RBn in parallel, and the resistance RD equals the resistance of RD1, RD2, ..., and RDn in parallel. Voltage Feedback Resistor Divider Table Voltage feedback resistor divider is provided for convenience for the most possibly used output voltages in Table 4.
Table 4. LT1573 Thermal Resistance
OUTPUT VOLTAGE (V) 1.5 1.8 2.0 2.2 2.5 2.8 3.0 3.3 3.5 3.8 4.0 4.5 5.0 R2 () 1k 1k 1k 1k 1k 1k 1k 1k 1k 1k 1k 1k 1k R1 () (NEAREST 1%) 187 422 576 732 976 1210 1370 1620 1780 2000 2150 2550 2940
TYPICAL APPLICATIONS
3.3V/5A Microprocessor Supply
LT1573 FB LATCH CTIME 0.5F COMP VOUT VIN
+
SHDN
GND
DRIVE
CIN 5V
+
VIN 5V
COUT1 = 24 x 1F SURFACE MOUNT CERAMIC CAPACITOR (FOR T < 45C, COUT1 = 24 x 1F Y5V CERAMIC SURFACE MOUNT CAPACITORS, FOR T > 45C, COUT1 = 24 x 1F X7R CERAMIC SURFACE MOUNT CAPACITORS) PLACE COUT1 IN THE MICROPROCESSOR SOCKET CAVITY CIN, COUT2 = 220F SURFACE MOUNT TANTALUM CAPACITOR CTIME = 0.5F FOR 100ms TIME OUT AT ROOM TEMPERATURE SHDN (ACTIVE HIGH) PIN SHOULD BE TIED TO GROUND IF IT IS NOT USED
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CC 100pF
RC 1k 1/8W
RD 24 1/2W
RB 50 1/8W
QOUT MOTOROLA D45H11 R1 1.6k 1/8W COUT2 R2 1k 1/8W GND
1573 TA01
VOUT
+
+
COUT1
+
LOAD
13
LT1573
TYPICAL APPLICATIONS
3.3V to 2.5/2A Voltage Regulator
FB LATCH CTIME 0.5F
+
SHDN
GND
CIN
CIN = 22F SURFACE MOUNT TANTALUM CAPACITOR COUT1 = 10F SURFACE MOUNT CERAMIC CAPACITOR COUT2 = 15F SURFACE MOUNT TANTALUM CAPACITOR CTIME = 0.5F FOR 100ms TIME OUT AT ROOM TEMPERATURE SHDN (ACTIVE HIGH) PIN SHOULD BE TIED TO GROUND IF IT IS NOT USED
CTIME 0.5F
+
C IN = 150 F (SANYO SURFACE MOUNT ELECTROLYTIC, 10V, PART #10CV150BS) OR 10 F LOW ESR TANTALUM CAPACITOR C OUT = 47 F (SANYO SURFACE MOUNT ELECTROLYTIC, 25V, PART #25CV47BS) OR 150 F (SANYO SURFACE MOUNT ELECTROLYTIC, 10V, PART #10CV150BS) C TIME = 0.5 F FOR 100ms TIME OUT AT ROOM TEMPERATURE SHDN (ACTIVE HIGH) PIN SHOULD BE TIED TO GROUND IF IT IS NOT USED
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FB
LT1573 COMP VOUT VIN RD 39 1/2W
CC 30pF
RC 1k 1/8W
DRIVE
RB 200 1/8W
QOUT MOTOROLA D45H11 R1 976 1/8W COUT2 R2 1k 1/8W GND
1573 TA02
VOUT
+
VIN 3.3V
+
+
COUT1
+
LOAD
5V/2A Output from 6V to 9V Wall Adapter Input
LT1573 COMP VOUT VIN RD 130 1/2W RB 200 1/8W
LATCH SHDN
GND
DRIVE
MOTOROLA D45H11 R1 2.94k 1/8W COUT R2 1k 1/8W GND
1573 TA03
VOUT
VIN 6V to 9V
+
+
CIN
+
LOAD
LT1573
TYPICAL APPLICATIONS
3.3V to 2.85V/1A Voltage Regulator
CTIME 0.5F
+
CIN, COUT = AVX 100F/10V SURFACE MOUNT TANTALUM CAPACITOR CTIME =0.5 F FOR 100ms TIME OUT AT ROOM TEMPERATURE SHDN (ACTIVE HIGH) PIN SHOULD BE TIED TO GROUND IF IT IS NOT USED
High Efficiency 2.5V to 1.5V Converter at 6A Output Current
CTIME 0.5F
+
CIN1, COUT = AVX 100F/10V SURFACE MOUNT TANTALUM CAPACITOR CIN2 =AVX 15 F/10V SURFACE MOUNT TANTALUM CAPACITOR CTIME =0.5 F FOR 100ms TIME OUT AT ROOM TEMPERATURE SHDN (ACTIVE HIGH) PIN SHOULD BE TIED TO GROUND IF IT IS NOT USED
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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FB
LT1573 COMP VOUT VIN RD 91 1/2W RB 200 1/8W
LATCH SHDN
GND
DRIVE
MOTOROLA D45H11 R1 1.24k 1/8W COUT R2 1k 1/8W GND
1573 TA04
VOUT
VIN 3.3V
+
+
CIN
+
LOAD
LT1573 FB LATCH SHDN COMP VOUT VIN RD 6 1/2W RB 200 1/8W
+
CIN1
+
VIN1 2.5V
GND
DRIVE
MOTOROLA D45H11 R1 186 1/8W COUT R2 1k 1/8W GND
1573 TA05
VOUT
VIN1 3.3V TO 7V
+
+
CIN2
+
LOAD
15
LT1573
TYPICAL APPLICATIONS
High Efficiency 2.5V to 1.8V Converter at 5A Output Current
LT1573 FB LATCH CTIME 0.5F COMP VOUT VIN RD 6.2 1/2W RB 200 1/8W
+
CIN1, COUT = AVX 100F/10V SURFACE MOUNT TANTALUM CAPACITOR CIN2 =AVX 15 F/10V SURFACE MOUNT TANTALUM CAPACITOR CTIME =0.5 F FOR 100ms TIME OUT AT ROOM TEMPERATURE SHDN (ACTIVE HIGH) PIN SHOULD BE TIED TO GROUND IF IT IS NOT USED
PACKAGE DESCRIPTION
0.010 - 0.020 x 45 (0.254 - 0.508) 0.008 - 0.010 (0.203 - 0.254) 0- 8 TYP
0.053 - 0.069 (1.346 - 1.752)
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
RELATED PARTS
PART NUMBER LT1083/LT1084/LT1085 LT1529 LT1553 LT1575 LT1580/LT1581 LT1584/LT1585/LT1587 DESCRIPTION 7.5A/5A/3A Low Dropout Regulators 3A Micropower Low Dropout Regulator 5-Bit Programmable Synchronous Switching Regulator Low Dropout N-Channel MOSFET Regulator Driver 7A, 10A Very Low Dropout Linear Regulators 7A/4.6A/3A Low Dropout, Fast Response Regulators COMMENTS Maximum 1.5V Dropout, Adjustable and Fixed Outputs 50A Quiescent Current, 0.5V Dropout, Shutdown 1.8V to 3.5V Fixed Output Voltage, Meets Intel VRM 8.1 Ultrafast Transient, Adjustable/Fixed Output, Current Limiting For High Current 3.3V to 2.xV Applications For High Performance Microprocessors
16
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417q (408)432-1900 FAX: (408) 434-0507q TELEX: 499-3977 q www.linear-tech.com
U
U
+
CIN1
+
VIN1 2.5V
SHDN
GND
DRIVE
MOTOROLA D45H11 R1 420 1/8W COUT R2 1k 1/8W GND
1573 TA06
VOUT
VIN2 3.3V TO 7V
+
+
CIN2
+
LOAD
Dimensions in inches (millimeters) unless otherwise noted.
S8 Package 8-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
8 0.004 - 0.010 (0.101 - 0.254) 0.228 - 0.244 (5.791 - 6.197) 0.150 - 0.157** (3.810 - 3.988) 0.189 - 0.197* (4.801 - 5.004) 7 6 5
0.014 - 0.019 (0.355 - 0.483)
0.050 (1.270) BSC
1
2
3
4
SO8 0695
1573f LT/TP 1297 4K * PRINTED IN USA
(c) LINEAR TECHNOLOGY CORPORATION 1997

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