AN734
Vishay Siliconix
Document Number: 71337
31-Oct-00
www.vishay.com
1
Design Guidelines for the Low-Noise
Low-Dropout Regulator—Si9181
Nitin Kalje
INTRODUCTION
This application note discusses the operation and features of
the Si9181, a 350-mA CMOS low-dropout (LDO), low-noise
voltage regulator. Providing very low ground current to extend
battery life, the Si9181 also offers superior line/load transient
response and ripple rejection. Designed to maintain regulation
while delivering currents from 0 mA to 600 mA, the Si9181
regulator also features an “out of regulation” error flag signal
and ON/OFF control input for complete shutdown.
The Si9181 was designed for applications requiring a regu-
lated voltage equal to or less than the voltage of the source or
battery, and requiring very low output noise. Such applications
include digital signal processors, baseband circuits, mixers, A-
to-D converters, or intermediate amplifiers. The Si9181’s LDO
characteristics are determined by its voltage, ground current,
power-dissipation, peak current capability, noise and ripple re-
jection specifications. The Si9181 is designed to meet the cri-
teria for high quality on each of these determinants, and is
suitable for portable and noise–sensitive appliances such as
cellular phones.
–
+
C
IN
2.2 C0109F
V
IN
4
6
SD
8
ON
OFF
3
GND
ADJ
–
+
60 mV
5% V
REF
+
1.215 V
V
REF
–
1C
NOISE
–
+
+
2.2 C0109A
To V
IN 5
C
OUT
2.2 C0109F
2
7
C
DELAY
0.1 C0109F
R
EXT
V
OUT
ERROR
FIGURE 1. 350-mA CMOS LDO Regulator (Adjustable Output)
6 MC0087
R
2
R
1
V
ADJ
U1
U2
U3
Q1
Q2
C
NOISE
S19181