CS8127
Test Circuit
V
IN
TIP42B
V
OUT
(5V)
R
IN
220W
C
O
10mF
R
OUT
220W
C
IN
0.022mF
V
IN
V
OUT
Sense
Pwr Gnd
CS-8127
Delay
C
Delay
0.022mF
R
RST
4.7 kW
RESET
Ref Gnd
Gnd
PULLUP
RESET
Application Information
Overvoltage Shutdown
may be increased or decreased for a particular application.
R
OUT
Resistor
- This resistor controls the drive current
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:
V
IN(min)
Ð 1V
x §
Q1
***
R
OUT
=
I
OUT(max)
***b
Q1
= Pass transistor minimum
b
@ maximum output
current.
Typical start-up current and current limit can be calculat-
ed as follows:
4V
+ 5mA
I
START
Å
R
OUT
I
Limit
Å
V
IN
Ð 1V
x §
Q1
@ Current Limit
R
OUT
The CS8127 includes an over voltage shutdown circuit.
Shutdown typically occurs at 36V.
Thermal Shutdown
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.
External Component Selection
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.
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.
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 560�½, although it
5
For example, if the minimum input voltage is 6V, maxi-
mum output current is 1Amp, and minimum transistor
b
@ 1Amp is 60, then R
OUT
can be calculated as follows:
R
OUT
Å
I
Start
Å
6V Ð 1V
x 60 = 300�½
1Amp
4V
300�½
+ 5mA = 18.3mA
With V
IN
= 14V, and a pass transistor
b
of 40 @ current
limit:
14V Ð 1V
x 40 = 1.7Amps
I
Limit
Å
300�½
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
