CS8321 datasheet(3/6 Pages) ONSEMI

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CS8321

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CIRCUIT DESCRIPTION AND APPLICATION NOTES

VOLTAGE REFERENCE AND OUTPUT CIRCUITRY

The CS8321 is a series pass voltage regulator. It consists

of an error amplifier, bandgap voltage reference, PNP pass

transistor with antisaturation control, and current limit.

As the voltage at the input, VIN, is increased (Figure 1),

QN is forward biased via R. QN provides base drive for QP.

As QP becomes forward biased, the output voltage, VOUT,

begins to rise as QP’s output current charges the output

capacitor. Once VOUT rises to a certain level, the error

amplifier becomes biased and provides the appropriate

amount of base current to QP. The error amplifier monitors

the scaled output voltage via an internal voltage divider, R1

and R2, and compares it to the bandgap voltage reference.

The error amplifier’s output is a current which is equal to the

error amplifier’s differential input voltage times its

transconductance. Therefore, the error amplifier varies the

base drive current to QN, which provides bias to QP, based

on the difference between the reference voltage and the

scaled output voltage, VOUT.

Antisaturation Protection

An antisaturation control circuit has also been added to

prevent the pass transistor from going into deep saturation,

which would cause excessive power dissipation due to large

bias currents lost to the substrate via a parasitic PNP

transistor, as shown in Figure 2.

Figure 2. The Parasitic PNP Transistor Which Is

Part of the Pass Transistor (QP) Structure

VOUT

VIN

QParasitic

Substrate

Current Limit Limit

The output stage is protected against short circuit

conditions. As shown in Figure 3, the output current will fold

back when the faulted load is continually increased. This

technique has been incorporated to limit the total power

dissipation across the device during a short circuit condition,

since the device does not contain overtemperature

shutdown.

STABILITY CONSIDERATIONS

The output or compensation capacitor helps determine

three main characteristics of a linear regulator: start−up

delay, load transient response and loop stability.

Figure 3. Typical Current Limit and Fold Back

Waveform

0.0

Output Voltage

0.34257

0.30831

0.27405

0.23980

0.20554

0.17128

0.13703

0.10277

0.06851

0.03426

0.51 1.02 1.52 2.03 2.54 3.05 3.56 4.06 4.57 5.08

Curve will vary with temperature

and process variation.

The capacitor value and type should be based on cost,

availability, size and temperature constraints. A tantalum or

aluminum electrolytic capacitor is best, since a film or

ceramic capacitor with almost zero ESR can cause

instability. The aluminum electrolytic capacitor is the least

expensive solution, but, if the circuit operates at low

temperatures (−25

°C to −40°C), both the value and ESR of

the capacitor will vary considerably. The capacitor

manufacturers data sheet usually provides this information.

The value for the output capacitor COUT shown in Figure

4 should work for most applications, however it is not

necessarily the best solution.

Figure 4. Test and Application Circuit Showing

Output Compensation

CS8321

VIN

CIN*

0.1 mF

VOUT

VOUTSense†

COUT**

0.1 mF

*CIN required if regulator is located far from the power

supply filter.

**COUT required for stability. Capacitor must operate at

minimum temperature expected.

†Pin internally shorted to VOUT in 3−pin applications.

To determine an acceptable value for COUT for a particular

application, start with a tantalum capacitor of the

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