CS8321
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3
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, V
IN
, is increased (Figure 1),
Q
N
is forward biased via R. Q
N
provides base drive for Q
P
.
As Q
P
becomes forward biased, the output voltage, V
OUT
,
begins to rise as Q
P
’s output current charges the output
capacitor. Once V
OUT
rises to a certain level, the error
amplifier becomes biased and provides the appropriate
amount of base current to Q
P
. 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 Q
N
, which provides bias to Q
P
, based
on the difference between the reference voltage and the
scaled output voltage, V
OUT
.
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 (Q
P
) Structure
V
OUT
V
IN
Q
P
Q
Parasitic
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: startup
delay, load transient response and loop stability.
Figure 3. Typical Current Limit and Fold Back
Waveform
0.0
0.0
Output Voltage
0.34257
Load Current
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 C
OUT
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
V
IN
C
IN
*
0.1 mF
V
OUT
V
OUT
Sense†
C
OUT
**
0.1 mF
*C
IN
required if regulator is located far from the power
supply filter.
**C
OUT
required for stability. Capacitor must operate at
minimum temperature expected.
†Pin internally shorted to V
OUT
in 3pin applications.
To determine an acceptable value for C
OUT
for a particular
application, start with a tantalum capacitor of the
