Application Notes
low temperatures. The ESR of the capacitor should be less
Stability Considerations
than 50% of the maximum allowable ESR found in step 3
above.
The output or compensation capacitor helps determine
three main characteristics of a linear regulator: start-up
delay, load transient response and loop stability.
Calculating Power Dissipation
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 instabil-
ity. 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 least
expensive will vary considerably. The capacitor manufac-
turers data sheet usually provides this information.
in a Single Output Linear Regulator
The maximum power dissipation for a single output regu-
lator (Figure 1) is:
PD(max)= VIN(max)ÐVOUT(min)}
I
OUT(max)+VIN(max) Q
I
(1)
{
where
VIN(max) is the maximum input voltage,
VOUT(min) is the minimum output voltage,
The value for the output capacitor COUT shown in the test
and applications circuit should work for most applica-
tions, however it is not necessarily the optimized solution.
IOUT(max) is the maximum output current for the applica-
tion, and
To determine an acceptable value for COUT for a particular
application, start with a tantalum capacitor of the recom-
mended value and work towards a less expensive alterna-
tive part.
IQ is the quiescent current the regulator consumes at
IOUT(max)
.
Once the value of PD(max) is known, the maximum permis-
sible value of RQJA can be calculated:
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.
150¡C - TA
(2)
RQJA
=
PD
The value of RQJA can then be compared with those in
the package section of the data sheet. Those packages
with RQJA's less than the calculated value in equation 2
will keep the die temperature below 150¡C.
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.
In some cases, none of the packages will be sufficient to
dissipate the heat generated by the IC, and an external
heatsink will be required.
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.
I
IN
I
OUT
V
IN
Smart
Regulator
V
OUT
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 output oscillates within the range of expected operat-
ing conditions, repeat steps 3 and 4 with the next larger
standard capacitor value.
Control
Features
}
I
Q
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.
Figure 1: Single output regulator with key performance parameters
labeled.
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
6
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