CS8221

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4

CIRCUIT DESCRIPTION

VOLTAGE REFERENCE AND OUTPUT CIRCUITRY

Output Stage Protection

The output stage is protected against overvoltage, short

circuit and thermal runaway conditions (Figure 3).

Figure 3. Typical Circuit Waveforms for Output

Stage Protection

Load

Dump

Thermal

Shutdown

Short

Circuit

> 30 V

If the input voltage rises above 30 V, the output shuts

down. This response protects the internal circuitry and

enables the IC to survive unexpected voltage transients.

Should the junction temperature of the power device

exceed 180

°C (typ) the power transistor is turned off.

Thermal shutdown is an effective means to prevent die

overheating since the power transistor is the principle heat

source in the IC.

Figure 4. Application and Test Diagram

0.1

mF

10

mF

CS8221

GND

APPLICATION NOTES

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.

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.

4 should work for most applications, however it is not

necessarily the optimized solution.

application, start with a tantalum capacitor of the

recommended value and work towards a less expensive

alternative part.

Step 1: Place the completed circuit with a tantalum

capacitor of the recommended value in an environmental

chamber at the lowest specified operating temperature and

monitor the outputs with an oscilloscope. A decade box

connected in series with the capacitor will simulate the

higher ESR of an aluminum capacitor. Leave the decade box

outside the chamber, the small resistance added by the

longer leads is negligible.

Step 2: With the input voltage at its maximum value,

increase the load current slowly from zero to full load while

observing the output for any oscillations. If no oscillations

are observed, the capacitor is large enough to ensure a stable

design under steady state conditions.

Step 3: Increase the ESR of the capacitor from zero using

the decade box and vary the load current until oscillations

appear. Record the values of load current and ESR that cause

the greatest oscillation. This represents the worst case load

conditions for the regulator at low temperature.

Step 4: Maintain the worst case load conditions set in

step 3 and vary the input voltage until the oscillations

increase. This point represents the worst case input voltage

conditions.

Step 5: If the capacitor is adequate, repeat steps 3 and 4

with the next smaller valued capacitor. A smaller capacitor

will usually cost less and occupy less board space. If the

output oscillates within the range of expected operating

conditions, repeat steps 3 and 4 with the next larger standard

capacitor value.

Step 6: Test the load transient response by switching in

various loads at several frequencies to simulate its real

working environment. Vary the ESR to reduce ringing.

Step 7: Increase the temperature to your highest operating

temperature. Vary the load current as instructed in step 5 to

test for any oscillations.