CS8151
Application Notes
Output Stage Protection
The output stage is protected against overvoltage, short
circuit and thermal runaway conditions (see Figure 4).
> 30V
V
IN
V
OUT
I
OUT
Load
Dump
Short
Circuit
Thermal
Shutdown
Figure 4: Typical Circuit Waveforms for Output Stage Protection.
If the input voltage rises above 56V (e.g. load dump), the
output shuts down. This response protects the internal cir-
cuitry and enables the IC to survive unexpected voltage
transients.
Should the junction temperature of the power device
exceed 180ûC (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.
Stability Considerations
The output or compensation capacitor C
2
(see Figure 5)
helps determine three main characteristics of a linear regu-
lator: start-up delay, load transient response and loop sta-
bility.
V
IN
C
1
*
0.1mF
V
OUT
C
2
**
10mF
To determine an acceptable value for C
2
for a particular
application, start with a tantalum capacitor of the recom-
mended value and work towards a less expensive
alternative 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 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 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 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 stan-
dard 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:
Remove the unit from the environmental chamber
and heat the IC with a heat gun. Vary the load current as
instructed in step 5 to test for any oscillations.
Once the minimum capacitor value with the maximum
ESR is found, a safety factor should be added to allow for
the tolerance of the capacitor and any variations in regula-
tor performance. Most good quality aluminum electrolytic
capacitors have a tolerance of ±20% so the minimum value
found should be increased by at least 50% to allow for this
tolerance plus the variation which will occur at low tem-
peratures. The ESR of the capacitor should be less than
50% of the maximum allowable ESR found in step 3 above.
Calculating Power Dissipation
in a Single Output Linear Regulator
The maximum power dissipation for a single output regu-
lator (Figure 6) is:
P
D(max)
= {V
IN(max)
Ð V
OUT(min)
}I
OUT(max)
+ V
IN(max)
I
Q
(1)
where:
V
IN(max)
is the maximum input voltage,
V
OUT(min)
is the minimum output voltage,
I
OUT(max)
is the maximum output current for the applica-
tion, and
I
Q
is the quiescent current the regulator consumes at
I
OUT(max)
.
6
CS8151
RESET
R
RST
*C
1
required if regulator is located
far from the power supply filter.
**C
2
required for stability.
Figure 5. Test and application circuit showing output compensation.
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 instabili-
ty. The aluminum electrolytic capacitor is the least expen-
sive solution, but, if the circuit operates at low tempera-
tures (-25¡C to -40¡C), both the value and ESR of the
capacitor will vary considerably. The capacitor manufac-
turers data sheet usually provide this information.
The value for the output capacitor C
2
shown in the test
and applications circuit should work for most applica-
tions, however it is not necessarily the optimized solution.
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
