CS8128 datasheet(5/6 Pages) CHERRY

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VIN

Sense

Delay

Ref Gnd

SHUTDOWN

Pwr Gnd

VOUT

RRST

4.7 kΩ

CDelay

0.022µF

10µF

VOUT

(5V)

VIN

RIN

220Ω

CIN

0.022µF

TIP42B

ROUT

220Ω

Gnd

CS8128

RESET

RBIAS

560Ω

RESET

Test Circuit

Application Information

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.

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 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

“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.

ROUT 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:

ROUT =

x ßQ1***

***βQ1 = Pass transistor minimum β @ maximum output

current.

Typical start-up current and current limit can be calculat-

ed as follows:

ISTART ≈

+ 5mA

ILimit ≈

x ßQ1 @ Current Limit

For example, if the minimum input voltage is 6V, maxi-

mum output current is 1Amp, and minimum transistor

β @ 1Amp is 60, then ROUT can be calculated as follows:

ROUT ≈

x 60 = 300Ω

IStart ≈

+ 5mA = 18.3mA

With VIN = 14V, and a pass transistor β of 40 @ current

limit:

ILimit ≈

x 40 = 1.7Amps

14V – 1V

300Ω

6V – 1V

1Amp

VIN – 1V

ROUT

VIN(min) – 1V

IOUT(max)

External Component Selection

Thermal Shutdown

Overvoltage Shutdown

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