CS8128
Test Circuit
V
IN
R
BIAS
560Ω
TIP42B
V
OUT
(5V)
R
IN
220Ω
C
O
10µF
R
OUT
220Ω
C
IN
0.022µF
V
IN
V
OUT
Sense
Pwr Gnd
CS8128
Delay
C
Delay
0.022µF
R
RST
4.7 kΩ
RESET
Ref Gnd
Gnd
SHUTDOWN
RESET
Application Information
Overvoltage Shutdown
The CS8128 includes an over voltage shutdown circuit
which is activated by connecting the SHUTDOWN lead to
the input. Shutdown typically occurs at 36V. Grounding
the SHUTDOWN lead disables this function. With the
overvoltage shutdown disabled, the CS8128 will continue
to regulate during an overvoltage condition.
Thermal Shutdown
“leaking”. It also speeds the turn-off of the pass device
during an overvoltage transient. For proper operation over
temperature, the recommended value is 560Ω, although it
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)
– 1V
x ß
Q1
***
R
OUT
=
I
OUT(max)
***β
Q1
= Pass transistor minimum
β
@ maximum output
current.
Typical start-up current and current limit can be calculat-
ed as follows:
4V
+ 5mA
I
START
≈
R
OUT
I
Limit
≈
V
IN
– 1V
x ß
Q1
@ Current Limit
R
OUT
The CS8128 includes a thermal shutdown circuit that dis-
ables the output when junction temperature exceeds
approximately 180˚C. This is a self-protection feature
designed to protect the CS8128. 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 CS8128 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 CS8128 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
CS8128 output stage, and prevents the pass device from
5
For example, if the minimum input voltage is 6V, maxi-
mum output current is 1Amp, and minimum transistor
β
@ 1Amp is 60, then R
OUT
can be calculated as follows:
R
OUT
≈
I
Start
≈
6V – 1V
x 60 = 300Ω
1Amp
4V
300Ω
+ 5mA = 18.3mA
With V
IN
= 14V, and a pass transistor
β
of 40 @ current
limit:
14V – 1V
x 40 = 1.7Amps
I
Limit
≈
300Ω
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
