Datasheet

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LT1581/LT1581-2.5

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Low Dropout, 430mV at 10A Output Current

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Fast Transient Response

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Remote Sense

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1mV Load Regulation

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Fixed 2.5V Output and Adjustable Output

■

No Supply Sequencing Problems in

Dual Supply Mode

The LT

1581 is a 10A low dropout regulator designed to

power the new generation of microprocessors. The drop-

out voltage of this device is 100mV at light loads rising to

just 430mV at 10A. To achieve this dropout a second low

current input voltage 1V greater than the output voltage is

required. The device can also be used as a single supply

device where dropout is comparable to an LT1584.

Several other new features have been added to the LT1581.

A remote SENSE pin is brought out. This feature virtually

eliminates output voltage variations due to load changes.

Typical load regulation for a load current step of 100mA to

10A, measured at the SENSE pin, is less than 1mV.

The LT1581 has fast transient response, equal to the

LT1584. On fixed voltage devices, the ADJUST pin is

brought out. A small capacitor on the ADJUST pin further

improves transient response.

This device is ideal for generating processor supplies of

2V to 3V on motherboards where both 5V and 3.3V

supplies are available.

10A, Very Low

Dropout Regulators

FEATURES

DESCRIPTION

, LTC and LT are registered trademarks of Linear Technology Corporation.

Pentium is a registered trademark of Intel Corporation.

PowerPC is a trademark of IBM Corporation.

APPLICATIONS

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Microprocessor Supplies

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Post Regulators for Switching Supplies

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High Current Regulators

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5V to 3.XXV for Pentium

Processors Operating

at 90MHz to 166MHz and Beyond

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3.3V to 2.9V for Portable Pentium Processor

■

PowerPC

Series Power Supplies

Dropout Voltage –

Minimum Power Voltage

TYPICAL APPLICATION

OUTPUT

SENSE

ADJGND

0.1µF

1581 TA01

10µF

TANT

330µF

OS-CON

2.5V/10A

LT1581-2.5

100µF TANT

AVX TPS

× 7

POWER

3.3V

10A

5V

0.2A

CONTROL

2.5V Microprocessor Supply

OUTPUT CURRENT (A)

MINIMUM POWER VOLTAGE (V)

POWER

– V

OUT

) (V)

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

1581 G03

DATA SHEET LIMIT

= 25°C

= 125°C

INDICATES GUARANTEED TEST POINTS

Summary of content (12 pages)

PAGE 1
LT1581/LT1581-2.5 10A, Very Low Dropout Regulators U DESCRIPTION FEATURES ■ ■ ■ ■ ■ ■ The LT ® 1581 is a 10A low dropout regulator designed to power the new generation of microprocessors. The dropout voltage of this device is 100mV at light loads rising to just 430mV at 10A. To achieve this dropout a second low current input voltage 1V greater than the output voltage is required. The device can also be used as a single supply device where dropout is comparable to an LT1584.
PAGE 2
LT1581/LT1581-2.5 U U W W W VPOWER Input Voltage ................................................ 6V VCONTROL Input Voltage ........................................... 13V Operating Junction Temperature Range Control Section ...................................... 0°C to 125°C Power Transistor ................................... 0°C to 150°C Storage Temperature ............................ – 65°C to 150°C Lead Temperature (Soldering, 10 sec)..................
PAGE 3
LT1581/LT1581-2.5 ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 2) PARAMETER Dropout Voltage (Note 3) CONDITIONS Minimum VCONTROL: LT1581-2.5 (VCONTROL – VOUT) VPOWER = 3.3V, ILOAD = 100mA VPOWER = 3.3V, ILOAD = 1A VPOWER = 3.3V, ILOAD = 4A VPOWER = 3.3V, ILOAD = 7A VPOWER = 3.
PAGE 4
LT1581/LT1581-2.5 U W TYPICAL PERFORMANCE CHARACTERISTICS Dropout Voltage — Minimum Control Voltage Control Pin Current vs Output Current 200 2 0.
PAGE 5
LT1581/LT1581-2.5 W BLOCK DIAGRA CONTROL POWER + – SENSE OUTPUT FOR FIXED VOLTAGE DEVICE 1581 BD ADJ GND U W U U APPLICATIONS INFORMATION The LT1581 is a low dropout regulator designed to power the new generation of microprocessors. Low dropout regulators have become more common in desktop computer systems as microprocessor manufacturers have moved away from 5V only CPUs. A wide range of supply requirements exists today with new voltages just over the horizon.
PAGE 6
LT1581/LT1581-2.5 U W U U APPLICATIONS INFORMATION processors cycle load current from several hundred milliamperes to several amperes in tens of nanoseconds. Output voltage tolerances are tighter and include transient response as part of the specification. The LT1581 is designed to meet the fast current load step requirements of these microprocessors and saves total cost by needing less output capacitance to maintain regulation.
PAGE 7
LT1581/LT1581-2.5 U U W U APPLICATIONS INFORMATION LT1581 can control the voltage at the load as long as the input/output voltage is greater than the total of the dropout voltage of the LT1581 plus the voltage drop across RP. 5V CONTROL 3.3V SENSE POWER Stability LT1581 + OUTPUT RP ADJ LOAD R1 VOUT R2 RP – 1581 F01 Figure 1. Conventional Load Sensing 5V CONTROL 3.3V SENSE POWER LT1581 + OUTPUT RP ADJ LOAD R1 VOUT R2 RP – 1581 F02 Figure 2.
PAGE 8
LT1581/LT1581-2.5 U W U U APPLICATIONS INFORMATION network must handle until the regulator throttles to the load current level. Capacitors are not ideal elements and contain parasitic resistance and inductance. These parasitic elements dominate the change in output voltage at the beginning of a transient load step change. The ESR of the output capacitors produces an instantaneous step in output voltage (∆V = ∆I)(ESR).
PAGE 9
LT1581/LT1581-2.5 U W U U APPLICATIONS INFORMATION 1000µF to 5000µF, and with the VPOWER pin instantaneously shorted to ground can damage occur. A crowbar circuit at the power input can generate those levels of current and a diode from output to power input is then recommended. This is shown in Figure 6. Normal power supply cycling or system “hot-plugging and unplugging” will not do any damage. A protection diode between the OUTPUT pin and the VCONTROL pin is usually not needed.
PAGE 10
LT1581/LT1581-2.5 U W U U APPLICATIONS INFORMATION where ICONTROL is equal to between IOUT/100 (typ) and IOUT/58 (max). Power dissipation under these conditions is equal to: Total Power Dissipation = PDRIVE + POUTPUT ICONTROL is a function of output current. A curve of ICONTROL vs IOUT can be found in the Typical Performance Characteristics curves. ICONTROL = IOUT/58 = 4A/58 = 69mA The power in the output transistor is equal to: PDRIVE = (5.25V – 2.
PAGE 11
LT1581/LT1581-2.5 U TYPICAL APPLICATION 2.5V/10A Regulator 5V 5 3.3V VCONT VPOWER SENSE LT1581 ADJ C3 22µF 25V + + C2 220µF 10V 1 VOUT 6 3 VCC R1 110Ω 1% C1 R2 110Ω 100µF 10V 1% C4 0.33µF VOUT = 2.5V 4 100µF 10V ×2 + + 1µF 25V × 10 MICROPROCESSOR SOCKET VSS RTN 1581 TA03 U PACKAGE DESCRIPTION Dimensions in inches (millimeters) unless otherwise noted. T7 Package 7-Lead Plastic TO-220 (Standard) (LTC DWG # 05-08-1422) 0.390 – 0.415 (9.906 – 10.541) 0.165 – 0.180 (4.191 – 4.
PAGE 12
LT1581/LT1581-2.5 U TYPICAL APPLICATION Dual Regulators Power Pentium Processor or Upgrade CPU R12 0.0075Ω* 5V + 12V R1 10k C8 0.1µF VOUT VIN C1 220µF 10V D1 1N4148 + C9 220µF 10V + C4 0.33µF 5V D2 1N4148 12V R13 0.005Ω* R4 178Ω 1% C6 0.01µF R10 10k R3 110Ω 1% R2 470Ω LT1006 – + LT1587 ADJ + 6 C11 + 22µF 35V C2 220µF 10V VCONT SENSE R11 10k R14, 2Ω 3 C10 1µF LT1581 5 VPOWER VOUT ADJ R7 107Ω 0.25% R6 89.8Ω 0.5% + R8 107Ω 0.35% C7 330µF 6.