Test Circuit
VIN
TIP42B
VOUT
(5V)
RIN
220W
ROUT
220W
CO
10mF
CIN
0.022mF
V
VIN
OUT
Sense
Pwr Gnd
PULLUP
CS-8127
Delay
CDelay
0.022mF
RRST
4.7 kW
Gnd
Ref Gnd
RESET
RESET
Application Information
may be increased or decreased for a particular application.
OUT Resistor - This resistor controls the drive current
Overvoltage Shutdown
R
The CS8127 includes an over voltage shutdown circuit.
Shutdown typically occurs at 36V.
available to the pass transistor. It also determines regula-
tor start-up current and short circuit current limit. For
bipolar pass transistors, it can be selected by use of the fol-
lowing formulae:
VIN(min) Ð 1V
Thermal Shutdown
x §Q1***
ROUT
=
IOUT(max)
The CS8127 includes a thermal shutdown circuit that dis-
ables the output when junction temperature exceeds
approximately 180ûC. This is a self-protection feature
designed to protect the CS8127. The thermal shutdown cir-
cuit does not monitor the temperature of the pass transis-
tor, which will probably be much hotter. To optimize ther-
mal shutdown, board design should minimize the differ-
ence in temperature of the CS8127 and the pass device.
***bQ1 = Pass transistor minimum b @ maximum output
current.
Typical start-up current and current limit can be calculat-
ed as follows:
4V
ROUT
+ 5mA
ISTART
Å
V
IN Ð 1V
ROUT
External Component Selection
I
Limit Å
x §Q1 @ Current Limit
External Pass Device - Select a pass device that will deliv-
er the desired output current, withstand the maximum
expected input voltage, and dissipate the resulting power.
The CS8127 is compatible with a wide variety of Bipolar
and FET pass transistors.
For example, if the minimum input voltage is 6V, maxi-
mum output current is 1Amp, and minimum transistor
b@ 1Amp is 60, then ROUT can be calculated as follows:
Output Capacitor - An output capacitor is required for sta-
bility in most applications. Though a 10µF capacitor should
be sufficient, regulator stability is dependent on the
characteristics of the pass transistor. Capacitor effective
series resistance (ESR) also factors in system stability. Some
bench work may be required to determine the capacitor
characteristics required for use in a particular application.
6V Ð 1V
1Amp
ROUT
Å
x 60 = 300½
4V
300½
IStart
Å
+ 5mA = 18.3mA
With VIN = 14V, and a pass transistor b of 40 @ current
limit:
BIAS Resistor - This resistor provides bias current for the
CS8127 output stage, and prevents the pass device from
ÒleakingÓ. It also speeds the turn-off of the pass device
during an overvoltage transient. For proper operation over
temperature, the recommended value is 560½, although it
14V Ð 1V
300½
ILimit
Å
x 40 = 1.7Amps
5
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
