4

three main characteristics of a linear regulator: start-up

delay, load transient response and loop stability.

The selection of a capacitor value and type should be

based on cost, availability, size and temperature con-

straints. 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 provide this

information.

shown in Figure 3 should work for most applications, but it

is not necessarily the least expensive or the optimal solution.

Figure 3: Test and application circuit showing an output compensation

capacitor.

application, start with a tantalum capacitor of the recom-

mended value and work towards a less expensive alterna-

tive part.

Step 1:

Place the completed circuit with a tantalum capac-

itor of the recommended value in an environmental cham-

ber at the lowest specified operating temperature. Monitor

the outputs on the 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 oscil-

lations 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 condi-

tions.

Step 5:

If the capacitor is adequate, repeat steps 3 and 4

with the next smaller valued capacitor. (A smaller capaci-

tor will usually cost less and occupy less board space.) If

the capacitor oscillates within the range of expected oper-

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

standard capacitor value.

Adj

CS8271

10

mF

ENABLE

0.1

mF

(optional)

Selecting the Right Capacitor Value

Application Notes

The output stage is protected against overvoltage, short

circuit and thermal runaway conditions (Figure 2).

If the input voltage rises above 30V (e.g. load dump), the

output shuts down. This response protects the internal cir-

cuitry and enables the IC to survive unexpected voltage

transients up to 60V in magnitude.

Short circuit protection limits the amount of current the

output transistor can supply. In the case of a CS8271 under

a short circuit condition, the output transistor current is

limited to 100mA.

Figure 2: Typical Circuit Waveforms for Output Stage Protection.

Should the junction temperature of the power device exceed

180ûC (typ) the power transistor is turned off. Thermal shut-

down is an effective means to prevent die overheating since

the power transistor is the principle heat source in the IC.

The

switches the output transistor. When the

voltage on the

pin exceeds 2.0V typ, the output

pass transistor turns off, leaving a high impedance facing

the load. The IC will remain in Sleep mode, drawing only

20µA (typ), until the voltage on the

pin drops

below the

threshold.

ENABLE

ENABLE

ENABLE

ENABLE

ENABLE

Load

Dump

Short

Circuit

Thermal

Shutdown

> 30V

Output Stage Protection

Circuit Description: continued