US1176
BLOCK DIAGRAM
Vin
Vctrl
Vout
Vsense
1.25V
CURRENT
LIMIT
+
+
THERMAL
SHUTDOWN
1176blk1-1.0
Adj
SD
OUTPUT
SHUTDOWN
Figure 1 - Simplified block diagram of the US1176
APPLICATION INFORMATION
Introduction
The US1176 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 US1176 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 US1176 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 US1176 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 US1176 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:
R
2
V
OUT
=
V
REF
1
+ +
I
ADJ
×
R
2
R
1
Wehre : V
REF
=125 V Typically
.
I
ADJ
=50 uA Typically
R
1
& R
2
as shown in figure 2
Vin
Vin
Vout
Vout
US1176
Vctrl
Vctrl
SD
Open
IAdj = 50uA
R2
1176app2-1.0
Vsense
Adj
Vref
R1
Figure 2 - Typical application of the US1176 for
programming the output voltage.
2-4
Rev. 1.1
9/24/99
UNISEM [ UNISEM ]
