US1175

2-4

Rev. 1.2

11/29/99

APPLICATION INFORMATION

APPLICATION INFORMATION

Introduction

The US1175 adjustable regulator is a 5 terminal device

designed specificaly to provide extremely low dropout

voltages comparable to the PNP type without the disad-

vantage of the extra power dissipation due to the base

current associated with PNP regulators.This is done by

bringinging out the control pin of the regulator that pro-

vides the base current to the power NPN and connect-

ing it to a voltage that is grater than the voltage present

at the Vin pin.This flexebility makes the US1175 ideal

for applications where dual inputs are available such as

a computer motherboard with an ATX style power sup-

ply that provides 5V and 3.3V to the board.One such

application is the new graphic chip sets that require any-

where from 2.4V to 2.7V supply such as the Intel I740

chip set. The US1175 can easily be programmed with

the addition of two external resistors to any voltages

within the range of 1.25 to 5.5 V. Another major require-

ment of these graphic chips is the need to switch the

load current from zero to several amps in tens of nano-

seconds at the processor pins ,which translates to an

approximately 300 to 500 nS of current step at the regu-

lator . In addition, the output voltage tolerances are also

extremely tight and they include the transient response

as part of the specification.

The US1175 is specifically designed to meet the fast

current transient needs as well as providing an accurate

initial voltage , reducing the overall system cost with the

need for fewer number of output capacitors.Another fea-

ture of the device is its true remote sensing capability

which allows accurate voltage setting at the load rather

than at the device.

Output Voltage Setting

The US1175 can be programmed to any voltages in the

range of 1.25V to 5.5V with the addition of R1 and R2

external resistors according to the following formula:

Figure 2 - Typical application of the US1175 for

programming the output voltage.

BLOCK DIAGRAM

BLOCK DIAGRAM

V

V

R

R

I

R

Wehre : V

V Typically

I

lly

R

in figure

OUT

REF

ADJ

REF

ADJ

=

+









+

×

1

125

2

2

1

2

1

2

= .

=50 uA Typica

& R as shown

Figure 1 - Simplified block diagram of the US1175

Vout

1175app2-1.0

R1

R2

Vin

Vctrl

Vref

IAdj = 50uA

US1175

Vsense

Adj

Vout

Vctrl

Vin

Vctrl

Vin

Vsense

1175blk1-1.0

Adj

Vout

THERMAL

SHUTDOWN

CURRENT

LIMIT

1.25V

+

+