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CS8156 Datasheet PDF : 11 Pages
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CS8156
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.
Temperature Stability of VOUT − The percentage
change in output voltage for a thermal variation from room
temperature to either temperature extreme.
60 V
34 V
26 V
14V
VIN 14 V
3.0 V
ENABLE 2.0 V
0.8 V
VOUT1 0 V
VOUT2
0V
12 V
12 V
2.4 V
5.0 V
2.4 V
12 V
0V
12 V
12 V
0V
5.0 V
Turn
On
Load
Dump
Low VIN
Line
VOUT1
Noise, Etc. Short
Circuit
Figure 19. Typical Circuit Waveform
VOUT2
Short
Circuit
VOUT1
Thermal
Shutdown
Turn
Off
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
connected in series with the capacitor C2 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
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