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LT1581CT7

10A, Very Low Dropout Regulators

Linear Technology 

LT1581/LT1581-2.5

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 para-

sitic 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). The ESL of the output

capacitors produces a droop proportional to the rate of

change of the output current (V = L)(∆I/∆t). The output

capacitance produces a change in output voltage

proportional to the time until the regulator can respond

(∆V = ∆t)(∆I/C). These transient effects are illustrated in

Figure 4 .

+

VPOWER +

VCONTROL

CONTROL

POWER

OUTPUT

LT1581

SENSE

ADJ

VREF

+

R1

VOUT

IADJ = 60µA

( ) VOUT = VREF

1 + R2

R1

+ IADJ (R2)

R2

1581 F05

Figure 5. Setting Output Voltage

ESR

EFFECTS

ESL

EFFECTS

SLOPE,

V

t

=

∆I

C

POINT AT WHICH REGULATOR

TAKES CONTROL

CAPACITANCE

EFFECTS

1581 F04

Figure 4

minimum load current of 10mA. The current out of the

ADJUST pin adds to the current from R1. The ADJUST pin

current is small, typically 60µA. The output voltage contri-

bution of the ADJUST pin current is small and only needs

to be considered when very precise output voltage setting

is required. Note that the top of the resistor divider should

be connected directly to the SENSE pin for best regulation.

See the section on grounding and Kelvin sensing above.

The use of capacitors with low ESR, low ESL and good

high frequency characteristics is critical in meeting the

output voltage tolerances of these high speed micropro-

cessors. These requirements dictate a combination of

high quality, surface mount, tantalum and ceramic capaci-

tors. The location of the decoupling network is critical to

transient performance. Place the decoupling network as

close to the processor pins as possible because trace runs

from the decoupling capacitors to the processor pins are

inductive. The ideal location for the decoupling network is

actually inside the microprocessor socket cavity. In addi-

tion, use large power and ground plane areas to minimize

distribution drops.

Output Voltage

The adjustable version of the LT1581 develops a 1.25V

reference voltage between the SENSE pin and the ADJUST

pin (see Figure 5). Placing a resistor R1 between these two

terminals causes a constant current to flow through R1

and down through R2 to set the overall output voltage.

Normally, R1 is chosen so that this current is the specified

Protection Diodes

In normal operation the LT1581 does not require protec-

tion diodes. Older 3-terminal regulators require protection

diodes between the OUTPUT pin and the INPUT pin or

between the ADJUST pin and the OUTPUT pin to prevent

die overstress.

On the LT1581, internal resistors limit internal current

paths on the ADJUST pin. Therefore even with bypass

capacitors on the ADJUST pin, no protection diode is

needed to ensure device safety under short-circuit con-

ditions. The ADJUST pin can be driven on a transient

basis ±7V with respect to the output without any device

degradation.

A protection diode between the OUTPUT pin and the

VPOWER pin is usually not needed. An internal diode

between the OUTPUT pin and the VPOWER pin on the

LT1581 can handle microsecond surge currents of 50A to

100A. Even with large value output capacitors it is difficult

to obtain those values of surge currents in normal opera-

tion. Only with large values of output capacitance, such as

8

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