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LT1575/LT1577 Ultrafast Transient Response, Low Dropout Regulators Adjustable and Fixed
FEATURES
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DESCRIPTION
The LT(R)1575/LT1577 are single/dual controller ICs that drive low cost external N-channel MOSFETs as source followers to produce ultrafast transient response, low dropout voltage regulators. The LT1575/LT1577 achieve unprecedented transientload performance by eliminating expensive tantalum or bulk electrolytic output capacitors in the most demanding modern microprocessor applications. Precision-trimmed adjustable and fixed output voltage versions accommodate any required microprocessor power supply voltage. Selection of the N-channel MOSFET RDS(ON) allows very low dropout voltages to be achieved. Unique protection features include a high side current limit amplifier that activates a fault protection timer circuit. A multifunction Shutdown pin provides either current limit time-out with latchoff, overvoltage protection, thermal shutdown or a combination of these functions. The LT1575 is available in 8-pin SO or PDIP and the LT1577 is available in 16-pin narrow body SO.
, LTC and LT are registered trademarks of Linear Technology Corporation. UltraFast is a trademark of Linear Technology Corporation. Pentium is a registered trademark of Intel Corporation. PowerPC is a trademark of IBM Corporation.
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UltraFastTM Transient Response Eliminates Tantalum and Electrolytic Output Capacitors FET RDS(ON) Defines Dropout Voltage 1% Reference/Output Voltage Tolerance Over Temperature Typical Load Regulation: 1mV High Side Sense Current Limit Multifunction Shutdown Pin with Latchoff
APPLICATIONS
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Pentium(R) Processor Supplies PowerPCTM Supplies 5V to 3.XXV or 3.3V to 2.XXV Microprocessor Supplies GTL Termination Low Voltage Logic Supplies
Adjustable 1.5V Fixed 2.8V Fixed 3.3V Fixed 3.5V Fixed 5V Fixed Adjustable, Adjustable 3.3V Fixed, Adjustable 3.3V Fixed, 2.8V Fixed
LT1575CN8/LT1575CS8 LT1575CN8-1.5/LT1575CS8-1.5 LT1575CN8-2.8/LT1575CS8-2.8 LT1575CN8-3.3/LT1575CS8-3.3 LT1575CN8-3.5/LT1575CS8-3.5 LT1575CN8-5/LT1575CS8-5 LT1577CS-ADJ/ADJ LT1577CS-3.3/ADJ LT1577CS-3.3/2.8
Consult factory for additional output voltage combinations available in the LT1577.
TYPICAL APPLICATION
Ultrafast Transient Response 5V to 3.3V Low Dropout Regulator (For Schematic Including Current Limit, See Typical Applications)
12V 1 * FOR T < 45C: C6 = 24 x 1F Y5V CERAMIC SURFACE MOUNT CAPACITORS. FOR T > 45C: C6 = 24 x 1F X7R CERAMIC SURFACE MOUNT CAPACITORS. PLACE C6 IN THE MICROPROCESSOR SOCKET CAVITY 2 LT1575-3.3 SHDN IPOS INEG GATE COMP 8 7 6 5 R2 5 5V C5 220F
C2 VIN 1F 3 GND 4 OUT
+
Q1 IRFZ24
50mV/DIV
VOUT 3.3V 5A C6* 24F GND
1575/77 TA01
C3 10pF
R1 7.5k C4 1000pF
2A/DIV
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Transient Response for 0.2A to 5A Output Load Step
100s/DIV
1575/77 TA02
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LT1575/LT1577 ABSOLUTE
(Note 1)
AXI U
RATI GS
Junction Temperature (Note 2) ................ 0C to 100C Storage Temperature Range ................ - 65C to 150C Lead Temperature (Soldering, 10 sec)................. 300C
VIN, IPOS, INEG ...................................................... 22V SHDN ....................................................................... VIN Operating Ambient Temperature Range ..... 0C to 70C
PACKAGE/ORDER I FOR ATIO
TOP VIEW SHDN 1 VIN 2 GND 3 FB 4 N8 PACKAGE 8-LEAD PDIP 8 7 6 5 IPOS INEG GATE COMP SHDN 1 VIN 2 GND 3 OUT 4
S8 PACKAGE 8-LEAD PLASTIC SO
N8 PACKAGE 8-LEAD PDIP
TJMAX = 100C, JA = 100C/ W (N8) TJMAX = 100C, JA = 130C/ W (S8)
TJMAX = 100C, JA = 100C/ W (N8) TJMAX = 100C, JA = 130C/ W (S8)
ORDER PART NUMBER LT1575CN8 LT1575CS8
ORDER PART NUMBER LT1575CN8-1.5 LT1575CS8-1.5 LT1575CN8-2.8 LT1575CS8-2.8 LT1575CN8-3.3 LT1575CS8-3.3 LT1575CN8-3.5 LT1575CS8-3.5 LT1575CN8-5 LT1575CS8-5
S8 PART MARKING 1575
S8 PART MARKING 157515 157528 157533 157535 15755
TOP VIEW SHDN1 1 VIN1 2 GND1 3 OUT-3.3 4 SHDN2 5 VIN2 6 GND2 7 FB 8 16 IPOS1 15 INEG1 14 GATE1 13 COMP1 12 IPOS2 11 INEG2 10 GATE2 9 COMP2 SHDN1 1 VIN1 2 GND1 3 OUT-3.3 4 SHDN2 5 VIN2 6 GND2 7 OUT-2.8 8
S PACKAGE 16-LEAD PLASTIC NARROW SO
TJMAX = 100C, JA = 100C/ W
ORDER PART NUMBER LT1577CS-3.3/ADJ
Consult factory for Industrial and Military grade parts.
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TOP VIEW 8 7 6 5 IPOS INEG GATE COMP
SHDN1 1 VIN1 2 GND1 3 FB1 4 SHDN2 5 VIN2 6 GND2 7 FB2 8 TOP VIEW 16 IPOS1 15 INEG1 14 GATE1 13 COMP1 12 IPOS2 11 INEG2 10 GATE2 9 COMP2
S8 PACKAGE 8-LEAD PLASTIC SO
S PACKAGE 16-LEAD PLASTIC NARROW SO
TJMAX = 100C, JA = 100C/ W
ORDER PART NUMBER LT1577CS-ADJ/ADJ
TOP VIEW 16 IPOS1 15 INEG1 14 GATE1 13 COMP1 12 IPOS2 11 INEG2 10 GATE2 9 COMP2
S PACKAGE 16-LEAD PLASTIC NARROW SO
TJMAX = 100C, JA = 100C/ W
ORDER PART NUMBER LT1577CS-3.3/2.8
LT1575/LT1577
ELECTRICAL CHARACTERISTICS
TA = 25C, VIN = 12V, GATE = 6V, IPOS = INEG = 5V, SHDN = 0.75V unless otherwise noted.
SYMBOL IQ VFB VOUT PARAMETER Supply Current LT1575 Reference Voltage
q
CONDITIONS
q
MIN 5 - 0.6 - 1.0 - 0.6 - 1.0 - 0.6 - 1.0 - 0.6 - 1.0 - 0.6 - 1.0 - 0.6 - 1.0
TYP 12 1.210 1.210 1.500 1.500 2.800 2.800 3.300 3.300 3.500 3.500 5.000 5.000 0.01 - 0.6
MAX 19 0.6 1.0 0.6 1.0 0.6 1.0 0.6 1.0 0.6 1.0 0.6 1.0 0.03 - 4.0 1.5
UNITS mA % % % % % % % % % % % % %/V A mA dB dB dB dB dB dB
LT1575-1.5 Output Voltage
q
LT1575-2.8 Output Voltage
q
LT1575-3.3 Output Voltage
q
LT1575-3.5 Output Voltage
q
LT1575-5 Output Voltage
q
Line Regulation IFB IOUT AVOL FB Input Bias Current OUT Divider Current LT1575 Large-Signal Voltage Gain LT1575-1.5 Large-Signal Voltage Gain LT1575-2.8 Large-Signal Voltage Gain LT1575-3.3 Large-Signal Voltage Gain LT1575-3.5 Large-Signal Voltage Gain LT1575-5 Large-Signal Voltage Gain VOL VOH GATE Output Swing Low (Note 3) GATE Output Swing High IPOS + INEG Supply Current Current Limit Threshold Voltage
10V VIN 20V FB = VFB OUT = VOUT VGATE = 3V to 10V VGATE = 3V to 10V VGATE = 3V to 10V VGATE = 3V to 10V VGATE = 3V to 10V VGATE = 3V to 10V IGATE = 0mA IGATE = 0mA 3V IPOS 20V
q q q q q q q q q q q q q
0.5 69 67 60 60 60 56 VIN - 1.6 0.3 42 37
1.0 84 82 76 75 74 71 2.5 VIN - 1 0.625 50 50 - 0.20
3.0 1.0 58 63 - 0.50 8.0 - 23 0.25 2.20 1.240 150
V V mA mV mV %/V A A V V V mV
Current Limit Threshold Voltage Line Regulation SHDN Sink Current SHDN Source Current SHDN Low Clamp Voltage SHDN High Clamp Voltage SHDN Threshold Voltage SHDN Threshold Hysteresis
3V IPOS 20V Current Flows Into Pin Current Flows Out of Pin
q q q q q q q
2.5 -8 1.50 1.18 50
5.0 - 15 0.1 1.85 1.21 100
The q denotes specifications which apply over the full operating temperature range. Note 1: Absolute Maximum Ratings are those values beyond which the life of the device may be impaired. Note 2: TJ is calculated from the ambient temperature TA and power dissipation PD according to the following formulas: LT1575CN8: TJ = TA + (PD * 100CW) LT1575CS8: TJ = TA + (PD * 130CW) LT1577CS: TJ = TA + (PD * 100CW)
Because the LT1577 consists of two regulators in the package, the total LT1577 power dissipation must be used for its junction temperature calculation. The total LT1577 PD = PD (Regulator 1) + PD (Regulator 2). Note 3: The VGS(th) of the external MOSFET must be greater than 3V - VOUT.
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LT1575/LT1577 TYPICAL PERFORMANCE CHARACTERISTICS
Quiescent Current vs Temperature
19 18 17 16 15 VIN = 12V 14 VIN = 20V 13 12 VIN = 8V 11 10 9 8 7 6 5 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C)
1575/77 G01
FB INPUT BIAS CURRENT (A)
QUIESCENT CURRENT (mA)
REFERENCE VOLTAGE (V)
LT1575-1.5 VOUT vs Temperature
1.515 1.512
REFERENCE VOLTAGE (V)
1.509 1.506 1.503 1.500 1.497 1.494 1.491 1.488 1.485 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C)
1575/77 G04
OUTPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
LT1575-3.5 VOUT vs Temperature
3.535 3.530 3.525 3.520 3.515 3.510 3.505 3.500 3.495 3.490 3.485 3.480 3.475 3.470 3.465 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C)
1575/77 G07
OUT DIVIDER CURRENT (mA)
OUTPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
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Adjustable LT1575 VREF vs Temperature
1.222 1.220 1.218 1.216 1.214 1.212 1.210 1.208 1.206 1.204 1.202 1.200 1.198 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C)
1575/77 G02
FB Input Bias Current vs Temperature
4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 VIN = 20V VIN = 12V VIN = 8V
0 -75 - 50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C)
1575/77 G03
LT1575-2.8 VOUT vs Temperature
2.828 2.824 2.800 2.816 2.812 2.808 2.804 2.800 2.796 2.792 2.788 2.784 2.780 2.776 2.772 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C)
1575/77 G05
LT1575-3.3 VOUT vs Temperature
3.333 3.327 3.321 3.315 3.309 3.303 3.297 3.291 3.285 3.279 3.273 3.267 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C)
1575/77 G06
LT1575-5 VOUT vs Temperature
5.050 5.040 5.030 5.020 5.010 5.000 4.990 4.980 4.970 4.960 4.950 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C)
1575/77 G08
OUT Divider Current vs Temperature
1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 -75 - 50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C)
1575/77 G09
LT1575/LT1577 TYPICAL PERFORMANCE CHARACTERISTICS
VREF/VOUT Line Regulation vs Temperature
0.030 0.025
LARGE-SIGNAL VOLTAGE GAIN (dB)
120 115 110 105 100 95 90 85 80 75 70 -75 - 50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C)
1575/77 G11
ERROR AMPLIFIER GAIN AND PHASE
LINE REGULATION (%/V)
0.020 0.015 0.010 0.005 0 -75 - 50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C)
1575/77 G10
Gate Output Swing Low vs Temperature
3.00 2.75 ILOAD = 50mA
GATE OUTPUT SWING HIGH (V)
GATE OUTPUT SWING LOW (V)
2.5 2.0 1.5 1.0 NO LOAD 0.5 0 -75 - 50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C)
1575/77 G14
2.50 NO LOAD 2.25 2.00 1.75 1.50 1.25 1.00 -75 - 50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C)
1575/77 G13
IPOS + INEG SUPPLY CURRENT (A)
Current Limit Threshold Voltage vs Temperature
CURRENT LIMIT THRESHOLD VOLTAGE (mV)
65 60 55 50 IPOS = 20V 45 40 35 -75 - 50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C)
1575/77 G16
CURRENT LIMIT THRESHOLD VOLTAGE LINE REGULATION (%/V)
UW
Error Amplifier Large-Signal Voltage Gain vs Temperature
200
Gain and Phase vs Frequency
150 PHASE 100 GAIN 50
0 1k 10k 100k 1M FREQUENCY (Hz) 10M 100M
1575/77 G12
Gate Output Swing High vs Temperature
3.0
1000 900 800 700 600 500 400
IPOS + INEG Supply Current vs Temperature
ILOAD = 50mA
IPOS = INEG = 5V IPOS = INEG = 12V IPOS = INEG = 20V IPOS = INEG = 3V
300 -75 - 50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C)
1575/77 G15
Current Limit Threshold Voltage Line Regulation vs Temperature
0
- 0.1
IPOS = 5V IPOS = 3V
- 0.2
- 0.3
- 0.4
- 0.5 -75 - 50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C)
1575/77 G17
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LT1575/LT1577 TYPICAL PERFORMANCE CHARACTERISTICS
SHDN Sink Current vs Temperature
7.5 7.0
SHDN SOURCE CURRENT (A)
SHDN SINK CURRENT (A)
6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 -75 - 50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C)
1575/77 G18
- 12 - 13 - 14 - 15 - 16 - 17 - 18 - 19 - 20 -75 - 50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C)
1575/77 G19
SHDN LOW CLAMP VOLTAGE (V)
SHDN High Clamp Voltage vs Temperature
2.1
SHDN HIGH CLAMP VOLTAGE (V)
2.0
SHDN HYSTERESIS (mV)
1.9 1.8 1.7 1.6 1.5 -75 - 50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C)
1575/77 G21
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SHDN Source Current vs Temperature
-10 - 11 0.20 0.25
SHDN Low Clamp Voltage vs Temperature
0.15
0.10
0.05
0 - 75 - 50 - 25 0 25 50 75 100 125 150 175 TEMPERATURE (C)
1575/77 G20
SHDN Hysteresis vs Temperature
150 140 130 120 110 100 90 80 70 60 50 -75 - 50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C)
1575/77 G22
LT1575/LT1577
PIN FUNCTIONS
SHDN (Pin 1): This is a multifunction shutdown pin that provides GATE drive latchoff capability. A 15A current source, that turns on when current limit is activated, charges a capacitor placed in series with SHDN to GND and performs a current limit time-out function. The pin is also the input to a comparator referenced to VREF (1.21V). When the pin pulls above VREF, the comparator latches the gate drive to the external MOSFET off. The comparator typically has 100mV of hysteresis and the Shutdown pin can be pulled low to reset the latchoff function. This pin provides overvoltage protection or thermal shutdown protection when driven from various resistor divider schemes. VIN (Pin 2): This is the input supply for the IC that powers the majority of internal circuitry and provides sufficient gate drive compliance for the external N-channel MOSFET. The typical supply voltage is 12V with 12.5mA of quiescent current. The maximum operating VIN is 20V and the minimum operating VIN is set by VOUT + VGS of the MOSFET at max. IOUT + 1.6V (worst-case VIN to GATE output swing). GND (Pin 3): Analog Ground. This pin is also the negative sense terminal for the internal 1.21V reference. Connect external feedback divider networks that terminate to GND and frequency compensation components that terminate to GND directly to this pin for best regulation and performance. FB (Pin 4): This is the inverting input of the error amplifier for the adjustable voltage LT1575. The noninverting input is tied to the internal 1.21V reference. Input bias current for this pin is typically 0.6A flowing out of the pin. This pin is normally tied to a resistor divider network to set output voltage. Tie the top of the external resistor divider directly to the output voltage for best regulation performance. OUT (Pin 4): This is the inverting input of the error amplifier for the fixed voltage LT1575. The fixed voltage parts contain a precision resistor divider network to set output voltage. The typical resistor divider current is 1mA into the pin. Tie this pin directly to the output voltage for best regulation performance. COMP (Pin 5): This is the high impedance gain node of the error amplifier and is used for external frequency compensation. The transconductance of the error amplifier is 15 millimhos and open-loop voltage gain is typically 84dB. Frequency compensation is generally performed with a series RC network to ground. GATE (Pin 6): This is the output of the error amplifier that drives N-channel MOSFETs with up to 5000pF of "effective" gate capacitance. The typical open-loop output impedance is 2. When using low input capacitance MOSFETs (< 1500pF), a small gate resistor of 2 to 10 dampens high frequency ringing created by an LC resonance that is created by the MOSFET gate's lead inductance and input capacitance. The GATE pin delivers up to 50mA for a few hundred nanoseconds when slewing the gate of the N-channel MOSFET in response to output load current transients. INEG (Pin 7): This is the negative sense terminal of the current limit amplifier. A small sense resistor is connected in series with the drain of the external MOSFET and is connected between the IPOS and INEG pins. A 50mV threshold voltage in conjunction with the sense resistor value sets the current limit level. The current sense resistor can be a low value shunt or can be made from a piece of PC board trace. If the current limit amplifier is not used, tie the INEG pin to IPOS to defeat current limit. An alternative is to ground the INEG pin. This action disables the current limit amplifier and additional internal circuitry activates the timer circuit on the SHDN pin if the GATE pin swings to the VIN rail. This option provides the user with a "sense-less" current limit function. IPOS (Pin 8): This is the positive sense terminal of the current limit amplifier. Tie this pin directly to the main input voltage from which the output voltage is regulated. The typical input voltage is a 5V logic supply. This pin is also the input to a comparator on the fixed voltage versions that monitors the input/output differential voltage of the external MOSFET. If this differential voltage is less than 0.5V, then the SHDN timer is not allowed to start even if the GATE is at the VIN rail. This allows the regulator to start up normally as the input voltage is ramping up, even with very slow ramp rates.
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LT1575/LT1577
BLOCK DIAGRAM
I1 15A SW1 SHDN NORMALLY OPEN OR1 D1
I2 5A SW2 NORMALLY CLOSED VIN START-UP GND
+
FB ERROR AMP
-
COMP I3 100A Q1 R1 50k Q2 Q3
1575/77 BD1
Q4
8
-
VREF
1.21V
COMP3
+
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LT1575 Adjustable Voltage
VTH1 50mV IPOS
+
ILIM AMP
+ -
INEG
-
+
COMP1 R2 5k Q6
-
100mV HYSTERESIS
D2 COMP2
- +
OR2
VTH2 1V
+ -
GATE Q5
LT1575/LT1577
BLOCK DIAGRAM
I1 15A SW1 NORMALLY OPEN SHDN OR1 D1
I2 5A SW2 NORMALLY CLOSED OR2 COMP2
VIN START-UP GND VREF
1.21V Q7
COMP3
OUT
+
ERROR AMP R3* I3 100A R4* Q1 R1 50k Q2 Q3
1575/77 BD2
-
COMP Q4 Q5
*VOUT = (1 + R3/R4)VREF
-
+
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LT1575 Fixed Voltage
IPOS
+
ILIM AMP
-
VTH1 50mV
+ -
INEG
+
COMP1 R2 5k Q6
D2 COMP4
- +
VTH3 500mV
+ -
-
100mV HYSTERESIS
- +
VTH2 1V
+ -
GATE
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LT1575/LT1577
APPLICATIONS INFORMATION
Introduction The current generation of microprocessors place stringent demands on the power supply that powers the processor core. These microprocessors cycle load current from near zero to amps in tens of nanoseconds. Output voltage tolerances as low as 100mV include transient response as part of the specification. Some microprocessors require only a single output voltage from which the core and I/O circuitry operate. Other higher performance processors require a separate power supply voltage for the processor core and the I/O circuitry. These requirements mandate the need for very accurate, very high speed regulator circuits. Previously employed solutions included monolithic 3-terminal linear regulators, PNP transistors driven by low cost control circuits and simple buck converter switching regulators. The 3-terminal regulator achieves a high level of integration, the PNP driven regulator achieves very low dropout performance and the switching regulator achieves high electrical efficiency. However, the common trait manifested by these solutions is that transient response is measured in many microseconds. This fact translates to a regulator output decoupling capacitor scheme that requires several hundred microfarads of very low ESR bulk capacitance using multiple capacitors surrounding the CPU. This required bulk capacitance is in addition to the ceramic decoupling capacitor network that handles the transient load response during the first few hundred nanoseconds as well as providing microprocessor clock frequency noise immunity. The combined cost of all capacitors is a significant percentage of the total power supply cost. The LT1575/LT1577 family of single/dual controller ICs are unique, easy to use devices that drive external N-channel MOSFETs as source followers and permit a user to realize an extremely low dropout, ultrafast transient response regulator. These circuits achieve superior regulator bandwidth and transient load performance by completely eliminating expensive tantalum or bulk electrolytic capacitors in the most modern and demanding microprocessor applications. For example, a 200MHz Pentium processor can operate with only the recommended 24 1F ceramic capacitors. Users benefit directly by saving significant cost as all additional bulk capacitance is removed. The additional savings of insertion cost, purchasing/inventory cost and board space are readily apparent. Precision-trimmed adjustable and fixed output voltage versions accommodate any required microprocessor power supply voltage. Proper selection of the N-channel MOSFET RDS(ON) allows user-settable dropout voltage performance. The only output capacitors required are the high frequency ceramic decoupling capacitors. This regulator design provides ample bandwidth and responds to transient load changes in a few hundred nanoseconds versus regulators that respond in many microseconds. The ceramic capacitor network generally consists of 10 to 24 1uF capacitors for individual microprocessor requirements. The LT1575/LT1577 family also incorporates current limiting for no additional system cost, provides on/off control and overvoltage protection or thermal shutdown with simple external components. Therefore, the unique design of these new ICs combines the benefits of low dropout voltage, high functional integration, precision performance and ultrafast transient response, as well as providing significant cost savings on the output capacitance needed in fast load transient applications. As lower input/output differential voltage applications become increasingly prevalent, an LT1575-based solution achieves comparable efficiency performance with a switching regulator at an appreciable cost savings. The new LT1575/LT1577 family of low dropout regulator controller ICs step to the next level of performance required by system designers for the latest generation motherboards and microprocessors. The simple versatility and benefits derived from these circuits allow the power supply needs of today's high performance microprocessors to be met with ease. Block Diagram Operation The primary block diagram elements consist of a simple feedback control loop and the secondary block diagram elements consist of multiple protection functions. Examining the block diagram for the LT1575, a start-up circuit provides controlled start-up for the IC, including the precision-trimmed bandgap reference, and establishes all internal current and voltage biasing.
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LT1575/LT1577
APPLICATIONS INFORMATION
Reference voltage accuracy for the adjustable version and output voltage accuracy for the fixed voltage versions are specified as 0.6% at room temperature and as 1% over the full operating temperature range. This places the LT1575/LT1577 family among a select group of regulators with a very tightly specified output voltage tolerance. The accurate 1.21V reference is tied to the noninverting input of the main error amplifier in the feedback control loop. The error amplifier consists of a single high gain gm stage with a transconductance equal to 15 millimhos. The inverting terminal is brought out as the FB pin in the adjustable voltage version and as the OUT pin in fixed voltage versions. The gm stage provides differential-tosingle ended conversion at the COMP pin. The output impedance of the gm stage is about 1M and thus, 84dB of typical DC error amplifier open-loop gain is realized along with a typical 75MHz uncompensated unity-gain crossover frequency. Note that the overall feedback loop's DC gain decreases from the gain provided by the error amplifier by the attenuation factor in the resistor divider network which sets the DC output voltage. These attenuation factors are already built into the Open-Loop Voltage Gain specifications for the LT1575 fixed voltage versions in the Electrical Characteristics table to simplify user calculations. External access to the high impedance gain node of the error amplifier permits typical loop compensation to be accomplished with a series RC network to ground. A high speed, high current output stage buffers the COMP node and drives up to 5000pF of "effective" MOSFET gate capacitance with almost no change in load transient performance. The output stage delivers up to 50mA peak when slewing the MOSFET gate in response to load current transients. The typical output impedance of the GATE pin is typically 2. This pushes the pole due to the error amplifier output impedance and the MOSFET input capacitance well beyond the loop crossover frequency. If the capacitance of the MOSFET used is less than 1500pF, it may be necessary to add a small value series gate resistor of 2 to 10. This gate resistor helps damp the LC resonance created by the MOSFET gate's lead inductance and input capacitance. In addition, the pole formed by this resistance and the MOSFET input capacitance can be fine tuned. Because the MOSFET pass transistor is connected as a source follower, the power path gain is much more predictable than designs that employ a discrete PNP transistor as the pass device. This is due to the significant production variations encountered with PNP Beta. MOSFETs are also very high speed devices which enhance the ability to produce a stable wide bandwidth control loop. An additional advantage of the follower topology is inherently good line rejection. Input supply disturbances do not propagate through to the output. The feedback loop for a regulator circuit is completed by providing an error signal to the FB pin in the adjustable voltage version and the OUT pin in the fixed voltage version. In both cases, a resistor divider network senses the output voltage and sets the regulated DC bias point. In general, the LT1575 regulator feedback loop permits a loop crossover frequency on the order of 1MHz while maintaining good phase and gain margins. This unity-gain frequency is a factor of 20 to 30 times the bandwidth of currently implemented regulator solutions for microprocessor power supplies. This significant performance benefit is what permits the elimination of all bulk output capacitance. Several other unique features are included in the design that increase its functionality and robustness. These functions comprise the remainder of the block diagram. A high side sense, current limit amplifier provides active current limiting for the regulator. The current limit amplifier uses an external low value shunt resistor connected in series with the external MOSFET's drain. This resistor can be a discrete shunt resistor or can be manufactured from a Kelvin-sensed section of "free" PC board trace. All load current flows through the MOSFET drain and thus, through the sense resistor. The advantage of using high side current sensing in this topology is that the MOSFET's gain and the main feedback loop's gain remain unaffected. The sense resistor develops a voltage equal to IOUT(RSENSE). The current limit amplifier's 50mV threshold voltage is a good compromise between power dissipation in the sense resistor, dropout voltage impact and noise immunity. Current limit activates when the sense resistor voltage equals the 50mV threshold. Two events occur when current limit activates: the first is that the current limit amplifier drives Q2 in the block
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LT1575/LT1577
APPLICATIONS INFORMATION
diagram and clamps the positive swing of the COMP node in the main error amplifier to a voltage that provides an output load current of 50mV/RSENSE. This action continues as long as the output current overload persists. The second event is that a timer circuit activates at the SHDN pin. This pin is normally held low by a 5A active pull-down that limits to 100mV above ground. When current limit activates, the 5A pull-down turns off and a 15A pull-up current source turns on. Placing a capacitor in series with the SHDN pin to ground generates a programmable time ramp voltage. The SHDN pin is also the positive input of COMP1. The negative input is tied to the internal 1.21V reference. When the SHDN pin ramps above VREF, the comparator drives Q4 and Q5. This action pulls the COMP and GATE pins low and latches the external MOSFET drive off. This condition reduces the MOSFET power dissipation to zero. The time period until the latched-off condition occurs is typically equal to CSHUT(1.11V)/15A. For example, a 1F capacitor on the SHDN pin yields a 74ms ramp time. In short, this unique circuit block performs a current limit time-out function that latches off the regulator drive after a predefined time period. The time-out period selected is a function of system requirements including start-up and safe operating area. The SHDN pin is internally clamped to typically 1.85V by Q6 and R2. The comparator tied to the SHDN pin has 100mV of typical hysteresis to provide noise immunity. The hysteresis is especially useful when using the SHDN pin for thermal shutdown. Restoring normal operation after the load current fault is cleared is accomplished in two ways. One option is to recycle the nominal 12V LT1575 supply voltage as long as an external bleed path for the Shutdown pin capacitor is provided. The second option is to provide an active reset circuit that pulls the SHDN pin below VREF. Pulling the SHDN pin below VREF turns off the 15A pull-up current source and reactivates the 5A pull-down. If the SHDN pin is held below VREF during a fault condition, the regulator continues to operate in current limit into a short. This action requires being able to sink 15A from the SHDN pin at less than 1V. The 5A pull-down current source and the 15A pull-up current source are designed low enough in value so that an external resistor divider network can drive the SHDN pin to provide overvoltage protection or to provide thermal shutdown with the use of a thermistor in the divider network. Diode-ORing these functions together is simple to accomplish and provides multiple functionality for one pin. If the current limit amplifier is not used, two choices present themselves. The simplest choice is to tie the INEG pin directly to the IPOS pin. This action defeats current limit and provides the simplest, no frills circuit. An application in which the current limit amplifier is not used is where an extremely low dropout voltage must be achieved and the 50mV threshold voltage cannot be tolerated. However, a second available choice permits a user to provide short-circuit protection with no external sensing. This technique is activated by grounding the INEG pin. This action disables the current limit amplifier because Schottky diode D1 clamps the amplifier's output and prevents Q2 from pulling down the COMP node. In addition, Schottky diode D2 turns off pull-down transistor Q1. Q1 is normally on and holds internal comparator COMP3's output low. This comparator circuit, now enabled, monitors the GATE pin and detects saturation at the positive rail. When a saturated condition is detected, COMP3 activates the shutdown timer. Once the time-out period occurs, the output is shut down and latched off. The operation of resetting the latch remains the same. Note that this technique does not limit the FET current during the time-out period. The output current is only limited by the input power supply and the input/output impedance. Setting the timer to a short period in this mode of operation keeps the external MOSFET within its SOA (safe operating area) boundary and keeps the MOSFET's temperature rise under control. Unique circuit design incorporated into the LT1575 alleviates all concerns about power supply sequencing. The issue of power supply sequencing is an important topic as the typical LT1575 application has inputs from two separate power supply voltages. If the typical 12V VIN supply voltage is slow in ramping up, insufficient MOSFET gate drive is present and therefore, the output voltage does not come up. If the VIN supply voltage is present, but the typical 5V supply voltage tied to the IPOS pin has not started yet, then the feedback loop wants to drive the GATE pin to the positive VIN rail. This would result in a
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LT1575/LT1577
APPLICATIONS INFORMATION
very large current spike as soon as the 5V supply started to ramp up. However, undervoltage lockout circuit COMP2, which monitors the IPOS supply voltage, holds Q3 on and pulls the COMP pin low until the IPOS voltage increases to greater than the internal 1.21 reference voltage. The undervoltage lockout circuit then smoothly releases the COMP pin and allows the output voltage to come up in dropout from the input supply voltage. An additional benefit derived from the speed of the LT1575 feedback loop is that turn-on overshoot is virtually nonexistent in a properly compensated system. An additional circuit feature is built-in to the LT1575 fixed voltage versions. When the regulator circuit starts up, it must charge up the output capacitors. The output voltage typically tracks the input voltage supply as it ramps up with the difference in input/output voltage defined by the dropout voltage. Until the feedback loop comes into regulation, the circuit operation results in the GATE pin being at the positive VIN rail, which starts the timer at the SHDN pin if the current limit amplifier is disabled. However, internal comparator COMP4 monitors the input/output voltage differential. This comparator does not permit the shutdown timer to start until the differential voltage is greater than 500mV. This permits normal start-up to occur. One final benefit is derived in using an LT1575 fixed voltage version. Today's highest performance microprocessors dictate that precision resistors must be used with currently available adjustable voltage regulators to meet the initial set point tolerance. The LT1575 fixed voltage versions incorporate the precision resistor divider into the IC and still maintain a 1% output voltage tolerance over temperature. Thus, the LT1575 fixed voltage versions completely eliminate the requirement for precision resistors and this results in additional system cost savings. Applications Support Linear Technology invests an enormous amount of time, resources and technical expertise in understanding, designing and evaluating microprocessor power supply solutions for system designers. As processor speeds and power increase, the power supply challenges presented to the motherboard designer increase as well. Application Note 69, "Using the LT1575 Linear Regulator Controller," has been written and serves as an extremely useful guide for this new family of ICs. This Application Note covers topics including PC board layout for the LT1575/LT1577 family, MOSFET selection criteria, external component selection (capacitors) and loop compensation. Linear Technology welcomes the opportunity to discuss, design, evaluate and optimize a microprocessor power supply solution with a customer. For additional information, consult the factory.
TYPICAL APPLICATIONS N
UltraFast Transient Response 5V to 3.5V Low Dropout Regulator with Current Limit and Timer Latchoff
12V 1 RESET Q2 VN2222L C1 1F 2 LT1575-3.5 SHDN IPOS INEG GATE COMP 8 7 6 5 R2 5 R3* 0.007 5V
*R3 IS MADE FROM "FREE" PC BOARD TRACE **C6 = 24 x 1F X7R CERAMIC SURFACE MOUNT CAPACITORS. PLACE C6 IN THE MICROPROCESSOR SOCKET CAVITY
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C2 VIN 1F 3 GND 4 OUT
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Q1 IRFZ24
C5 220F VOUT 3.5V 5A
C3 10pF
R1 7.5k C4 1000pF
C6** 24F
GND
1575/77 TA11
13
LT1575/LT1577
TYPICAL APPLICATIONS N
Setting Output Voltage with the Adjustable LT1575
VOUT
R2 FB R1
VOUT = 1.21V(1 + R2/R1)
1575 TA03
Setting Current Limit
IPOS VCC RSENSE* INEG GATE Q2
VOUT *ILIM = 50mV/RSENSE RSENSE = DISCRETE SHUNT RESISTOR OR RSENSE = KELVIN-SENSED PC BOARD TRACE ACTIVATING CURRENT LIMIT ALSO ACTIVATES THE SHDN PIN TIMER 1575 TA05
Shutdown Time-Out with Reset
SHDN RESET 0V TO 5V R1 100k Q1 VN2222L C1*
*C1 = 15A(t)/1.11V t = SHUTDOWN LATCHOFF TIME
Shutdown Time-Out with Reset
R2 100k R3 100k SHDN Q2 2N3904 C2*
RESET 0V TO 5V
*C2 = 15A(t)/1.11V t = SHUTDOWN LATCH-OFF TIME
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Using "Sense-Less" Current Limit
R3 10 SHDN CT INEG GATE Q1 IPOS C1 10F VCC
VOUT
1575 TA04
Setting Current Limit with Foldback Limiting
IPOS R5 10 INEG D1 1N4148 D2 1N4148 R6 1.2k
1575 TA06
VCC R4
GATE
Q3 VOUT
Basic Thermal Shutdown
5V RT1 10k NTC SHDN RT1 = DALE NTHS-1206N02 THERMALLY MOUNT RT1 IN CLOSE PROXIMITY TO THE EXTERNAL N-CHANNEL MOSFET 1575 TA08
1575 TA07
R4 549
Overvoltage Protection
VOUT R6 SHDN R5
1575 TA09
1575 TA10
VOUT(uth) = 1.21(R6/R5) + 5A(R6) VOUT(lth) = 1.11(R6/R5) - 15A(R6)
LT1575/LT1577
TYPICAL APPLICATIONS N
Pentium (R) II Processor GTL+ Power Supply
VTT 1.5V
12V
VIN 3.3V
+
LT1575-1.5 RESET C2 0.22F C3 1F 1 2 3 4 SHDN VIN GND OUT IPOS INEG GATE COMP 8 7 6 5 R2 3.9 R1 0.005
C4 10pF
R3 4.99k C5 1000pF
NOTE: LTC RECOMMENDS CENTRALLY LOCATING THE LT1575-1.5 OUTPUT TO MINIMIZE VTT DISTRIBUTION DROPS AND USING SEPARATE VREF GENERATORS AT EACH BUS END
LT1262 1 C1 0.22F C2 0.22F 2 3 4 C1- C1+ C2- C2+ SHDN GND VOUT VCC 8 7 6 5 C4 4.7F 12V 25mA VCC 4.75V TO 5.5V C5 100F 10V VCC 4.75V TO 5.5V
C3 4.7F
VCC 4.75V TO 5.5V 74HC14 C7 100F 10V
+
C8 390pF
R1 2k
Pentium is a registered trademark of Intel Corporation.
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C1 220F 6.3V
R4 75 VREF R5 150 C6 0.1F
R8 100
R10 R11 100 100
R9 100
R6 75 VREF R7 150 C7 0.1F
Q1 IRFZ24 C8 TO C23 1F CERAMIC 0805 CASE
RX TX Q2 Q3
RX TX
RX TX Q4 * * * 142 TOTAL SIGNAL LINES Q5
RX TX
1575/77 TA12
Generating 12V Gate Drive from a 5V Power Supply
D1 1N5818 2 SW 12V 25mA
L1 33H
+
LT1109CZ-12 VOUT GND 1
3
+
C6 10F 25V
D2 BAT85 C9 0.22F D3 BAT85 D4 BAT85 D5 BAT85 12V 25mA x5 D6 BAT85 C10 0.22F C11 0.22F C12 0.22F
1575/77 TA13
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LT1575/LT1577
TYPICAL APPLICATIONS N
12V to 3.3V/9A (14A Peak) Hybrid Regulator
+
C11 150F 16V
+
C12 150F 16V
+
C13 150F 16V
+
C14 150F 16V 13 VIN EXTVCC COSC TG SW
9 1 C8, 68pF C7, 0.1F C10, 1000pF C9 1500pF R5 16.5k 2 3 4 5 6
RUN/SS BOOST LTC1435 12 ITH INTVCC SFB SGND VOS PGND 10 BG S+ S 11 8 D1, CMDSH-3 Q3
R8 15K
50mV/DIV
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LT1575 1 12V 2 3 C6 0.1F 4 SHDN VIN GND OUT IPOS INEG GATE COMP 8 7 6 5 Q1 IRLZ44
C21, 10pF 12V C22, 1000pF C17 1F C15 1F C16 1F R1 2.1k, 1% R2 1.21k 1% Q2 C3, 0.1F L1 4H R6 0.0075 C1, 470pF 1F X7R CERAMIC 0805 CASE x 40 R9 2k
VCORE 3.3V
16 14 15
R3 100
R4 100
+
C18 C20 1000F 1000F 10V 10V
+
C23 1F
+
C2, 1000pF
-7
C19 1000F 10V
+
C4, 4.7F
C5 0.1F
D2 MBRS330T3
R7 35.7k
1575/77 TA16
L1 =COILTRONICS CTX02-13199 Q2, Q3 =SILICONIX SUD50N03-10
Transient Response to a 10A Load Step
200s/DIV
1575/77 TA17
LT1575/LT1577
TYPICAL APPLICATIONS N
3.3V to 2.8V 100mV at 5.7A with Sense-Less Current Limit and Timer Latchoff
INPUT 3.3V
+
RTN
C1 330F 6.3V
+
C2 330F 6.3V 12V
PACKAGE DESCRIPTION
0.300 - 0.325 (7.620 - 8.255)
0.009 - 0.015 (0.229 - 0.381)
(
+0.035 0.325 -0.015 8.255 +0.889 -0.381
)
*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.010 INCH (0.254mm)
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FAULT RESET LT1575-2.8 1 2 3 C3 680pF C6 0.1F 4 SHDN VIN GND OUT IPOS INEG GATE COMP 8 7 6 5
R2 10
C7 10F
Q1 IRL3303 R1 4.7k C4 1000pF VCORE 2.8V
C5 22pF
+
1575/77 TA14
C8 TO C31* 1F
*X7R CERAMIC 0805 CASE
Dimensions in inches (millimeters) unless otherwise noted.
N8 Package 8-Lead PDIP (Narrow 0.300)
(LTC DWG # 05-08-1510)
0.400* (10.160) MAX 8 7 6 5
0.255 0.015* (6.477 0.381)
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2
3
4 0.130 0.005 (3.302 0.127)
0.045 - 0.065 (1.143 - 1.651)
0.065 (1.651) TYP 0.125 (3.175) 0.020 MIN (0.508) MIN 0.018 0.003 (0.457 0.076) N8 1197
0.100 0.010 (2.540 0.254)
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LT1575/LT1577
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
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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
0.228 - 0.244 (5.791 - 6.197)
0.150 - 0.157** (3.810 - 3.988)
1 0.053 - 0.069 (1.346 - 1.752)
2
3
4
0.004 - 0.010 (0.101 - 0.254)
0.014 - 0.019 (0.355 - 0.483)
0.050 (1.270) TYP
SO8 0996
LT1575/LT1577
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 16-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
0.386 - 0.394* (9.804 - 10.008) 16 15 14 13 12 11 10 9
0.228 - 0.244 (5.791 - 6.197)
0.150 - 0.157** (3.810 - 3.988)
1 0.053 - 0.069 (1.346 - 1.752)
2
3
4
5
6
7
8
0.004 - 0.010 (0.101 - 0.254)
0.014 - 0.019 (0.355 - 0.483)
0.050 (1.270) TYP
S16 0695
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LT1575/LT1577
TYPICAL APPLICATION
LT1577 Split Plane System
FAULT RESET INPUT 5V C1 330F 6.3V
+
1/2 LT1577 1 SHDN1 VIN1 GND1 IPOS1 INEG1 GATE1 16 15 14 13 R1 3.9 R2 3.9k C6 1500pF Q1 IRFZ24 VI/O 3.3V C9 TO C20* 1F 5 6 7 8 2 3 C3 0.33F 4 1/2 LT1577 SHDN2 VIN2 GND2 IPOS2 INEG2 GATE2 12 11 10 9
12V
OUT-3.3 COMP1
C4 1F
RELATED PARTS
PART NUMBER LTC1266 LTC1392 LTC1430 LTC1435 LTC1553 LTC1553L LT1573 LT1580 LT1585-1.5 DESCRIPTION Current Mode, Step-Up/Down Switching Regulator Controller Micropower Temperature, Power Supply and Differential Voltage Monitor High Power Step-Down Switching Regulator Controller High Efficiency, Low Noise Synchronous Step-Down Switching Regulator Digitally Controlled Synchronous Switching Regulator Controller Digitally Controlled Synchronous Switching Regulator Controller Low Dropout Regulator Driver 7A, Very Low Dropout Linear Regulator Fixed 1.5V, 5A Low Dropout Fast Response Regulator COMMENTS Synchronous N- or P-Channel FETs, Comparator/Low Battery Detector Temperature to Bits Control Voltage Mode, 5V to 3.xxV at >10A Current Mode with Wide Input Voltage Range Controller for Pentium II Processor, Buck Conversion from 5V or 12V Main Power Controller for Pentium II Processor, Buck Conversion from 5V Main Power Drives Low Cost PNP Transistor for High Power, Low Dropout Applications 0.54V Dropout at 7A, Fixed 2.5VOUT and Adjustable GTL+ Regulator
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 q FAX: (408) 434-0507 q www.linear-tech.com
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C2 330F 6.3V
OUT-2.8 COMP2
R5 3.9 R6 7.5k C8 1000pF
Q2 IRFZ24 VCORE 2.8V C21 TO C44* 1F
C5 10pF
C7 10pF
*X7R CERAMIC 0805 CASE
1575/77 TA15
15757f LT/TP 0598 4K * PRINTED IN THE USA
(c) LINEAR TECHNOLOGY CORPORATION 1996

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