5

LT1581/LT1581-2.5

BLOCK DIAGRA

The LT1581 is a low dropout regulator designed to power

the new generation of microprocessors. Low dropout

regulators have become more common in desktop com-

puter 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. In many cases the input/output differential is very

small, effectively disqualifying many of the low dropout

regulators on the market today. The LT1581 is designed to

make use of multiple power supplies present in most

systems to reduce the dropout voltage. This 2-supply

approach maximizes efficiency.

The second supply, at least 1V greater than the output

voltage, is used to provide power for the control circuitry

and supply the drive current to the NPN output transistor.

This allows the NPN to be driven into saturation, thereby

conventional designs. The current requirement for the

–

+

POWER

CONTROL

SENSE

OUTPUT

1581 BD

FOR FIXED

VOLTAGE

DEVICE

GND

ADJ

control voltage is relatively small, equal to approximately

1% of the output current or about 100mA for a 10A load.

The bulk of this current is drive current for the NPN output

transistor. This drive current becomes part of the output

current.

The control voltage must be at least 1V greater than the

output voltage to obtain optimum performance. The maxi-

for fixed voltage devices is 6mA (typ) and is constant as a

function of load. ADJUST pin current for adjustable de-

vices is 60

µA at 25°C and varies proportional to absolute

temperature.

The LT1581 has improved frequency compensation which

permits the use of capacitors with very low ESR. This is

critical in addressing the needs of modern, low voltage,

high speed microprocessors. Current generation micro-