US1075
PIN DESCRIPTIONS
PIN #
1
2
3
PIN SYMBOL
Adj
Vout
Vin
PIN DESCRIPTION
A resistor divider from this pin to the Vout pin and ground sets the output voltage.
The output of the regulator. A minimum of 10uF capacitor must be connected
from this pin to ground to insure stability.
The input pin of the regulator. Typically a large storage capacitor is connected
from this pin to ground to insure that the input voltage does not sag below the
minimum drop out voltage during the load transient response. This pin must
always be 1.3V higher than Vout in order for the device to regulate properly.
BLOCK DIAGRAM
Vin 3
2 Vout
+
+
1.25V
CURRENT
LIMIT
THERMAL
SHUTDOWN
1075blk1-1.0
1 Adj
Figure 1 - Simplified block diagram of the US1050
APPLICATION INFORMATION
Introduction
The US1075 adjustable Low Dropout (LDO) regulator is
a 3 terminal device which can easily be programmed
with the addition of two external resistors to any volt-
ages within the range of 1.25 to 5.5 V.This regulator
unlike the first generation of the 3T regulators such as
LM117 that required 3V differential between the input
and the regulated output,only needs 1.3V differential to
maintain output regulation. This is a key requirement for
today’s microprocessors that need typically 3.3V sup-
ply and are often generated from the 5V supply. Another
major requirement of these microprocessors such as
the Intel P54C
™
is the need to switch the load current
from zero to several amps in tens of nanoseconds at
the processor pins ,which translates to an approximately
300 to 500 nS current step at the regulator . In addition,
the output voltage tolerances are also extremely tight
and they include the transient response as part of the
specification.For example Intel VRE
™
specification calls
for a total of
±
100mV including initial tolerance,load regu-
lation and 0 to 4.6A load step.
The US1075 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 output capacitors.
Rev. 1.0
9/14/99
2-35
UNISEM [ UNISEM ]
