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CS8127

5V Linear Controller/Driver

ON Semiconductor 

VIN

VOUT

(5V)

CO

10µF

Test Circuit

TIP42B

RIN

220Ω

CIN

0.022µF

ROUT

220Ω

VIN

VOUT

Sense

Pwr Gnd

CS8127

Delay

PULLUP

CDelay

0.022µF

RRST

4.7 kΩ

RESET

Ref Gnd

Gnd

RESET

Application Information

Overvoltage Shutdown

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 180°C. 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.

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 =

VIN(min) – 1V

IOUT(max)

x ßQ1***

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

current.

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

ed as follows:

ISTART ≈

4V

ROUT

+ 5mA

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

“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

ILimit ≈

VIN – 1V

ROUT

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 ≈

6V – 1V

1Amp

x 60 = 300Ω

IStart ≈

4V + 5mA = 18.3mA

300Ω

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

limit:

ILimit ≈

14V – 1V

300Ω x 40 = 1.7Amps

5

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