CS8156

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7

DEFINITION OF TERMS

Dropout Voltage − The input−output voltage differential

at which the circuit ceases to regulate against further

reduction in input voltage. Measured when the output

voltage has dropped 100 mV from the nominal value

obtained at 14 V input, dropout voltage is dependent upon

load current and junction temperature.

Input Voltage − The DC voltage applied to the input

terminals with respect to ground.

Input Output Differential − The voltage difference

between the unregulated input voltage and the regulated

output voltage for which the regulator will operate.

Line Regulation − The change in output voltage for a

change in the input voltage. The measurement is made under

conditions of low dissipation or by using pulse techniques

such that the average chip temperature is not significantly

affected.

Load Regulation − The change in output voltage for a

change in load current at constant chip temperature.

Long Term Stability − Output voltage stability under

accelerated life−test conditions after 1000 hours with

maximum rated voltage and junction temperature.

Output Noise Voltage − The rms AC voltage at the

output, with constant load and no input ripple, measured

over a specified frequency range.

Quiescent Current − The part of the positive input

current that does not contribute to the positive load current,

i.e., the regulator ground lead current.

Ripple Rejection − The ratio of the peak−to−peak input

ripple voltage to the peak−to−peak output ripple voltage.

change in output voltage for a thermal variation from room

temperature to either temperature extreme.

Figure 19. Typical Circuit Waveform

14 V

0 V

0 V

2.0 V

0.8 V

ENABLE

60 V

12 V

5.0 V

34 V

12 V

2.4 V

3.0 V

26 V

12 V

0 V

12 V

5.0 V

12 V

14V

0 V

Load

Dump

Line

Noise, Etc.

Short

Circuit

Thermal

Shutdown

Turn

Off

Turn

On

2.4 V

Short

Circuit

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

cheapest 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 capacitors C2 and C3 shown in

the test and applications circuit should work for most

applications, however it is not necessarily the best solution.

To determine acceptable values for C2 and C3 for a

particular application, start with a tantalum capacitor of the

recommended value and work towards a less expensive

alternative part for each output.

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

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