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www.fairchildsemi.com
Features
Versatile —
Inverting function (+ to -)
Step-down function
Adjustable output voltage
Regulates supply changes
Micropower —
Low quiescent current — 170
m
A
Wide supply range — 4V to 30V
Description
Fairchild Semiconductor’s RC4391 is a monolithic switch
mode power supply controller for micropower circuits. The
RC4391 integrates all the active functions needed for low
power switching supplies, including oscillator, switch, refer-
ence and logic, into a small package. Also, the quiescent
supply current drawn by the RC4391 is extremely low; this
combination of low supply current, function, and small pack-
age make it adaptable to a variety of miniature power supply
applications.
The RC4391 complements another Fairchild Semiconductor
switching regulator IC, the RC4190. The RC4190 is dedi-
cated to step-up (V
OUT
> V
IN
) applications, while the
RC4391 was designed for inverting (V
OUT
= -V
IN
) and
step-down (V
OUT
< V
IN
) applications. Between the two
devices the ability to create all three basic switching regula-
tor configurations is assured. Refer to the RC4190 data sheet
for information on step-up applications.
The functions provided are:
Squarewave oscillator (adjustable externally)
Bandgap voltage reference
High current PNP switch transistor
Feedback comparator
Logic for gating the comparator
Circuitry for detecting a discharged battery condition
(in battery powered systems)
Few external components are required to build a complete
DC-to-DC converter:
Inductor
Low value capacitor to set the oscillator frequency
Electrolytic filter capacitor
Steering diode
Two resistors
Block Diagram
C2
C1
OSC
+1.25V
REF/Bias
LBR
C
X
C
X
Gnd
RC4391
LBD
65-3471-01
V
REF
V
REF
V
FB
+V
S
Q1
Q2
High performance —
High switch current — 375 mA
High efficiency — 70% typically
Low battery detection capability
8-lead mini-DIP or S.O. package
RC4391
Inverting and S tep-Down S witc hing Regulator
Rev. 1.1.1