HEATSINKING
Application Hints
A heatsink may be required depending on the maximum pow-
er dissipation and maximum ambient temperature of the ap-
plication. Under all possible operating conditions, the junction
temperature must be within the range specified under Abso-
lute Maximum Ratings.
EXTERNAL CAPACITORS
The output capacitor is critical to maintaining regulator stabil-
ity, and must meet the required conditions for both ESR
(Equivalent Series Resistance) and minimum amount of ca-
pacitance.
To determine if a heatsink is required, the power dissipated
by the regulator, PD, must be calculated.
MINIMUM CAPACITANCE:
The figure below shows the voltages and currents which are
present in the circuit, as well as the formula for calculating the
power dissipated in the regulator:
The minimum output capacitance required to maintain stabil-
ity is 33 μF (this value may be increased without limit). Larger
values of output capacitance will give improved transient re-
sponse.
ESR LIMITS:
The ESR of the output capacitor will cause loop instability if it
is too high or too low. The acceptable range of ESR plotted
versus load current is shown in the graph below. It is essen-
tial that the output capacitor meet these requirements, or
oscillations can result.
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IIN = IL + IG
PD = (VIN − VOUT) IL + (VIN) IG
FIGURE 2. Power Dissipation Diagram
The next parameter which must be calculated is the maximum
allowable temperature rise, TR (max). This is calculated by
using the formula:
TR (max) = TJ (max) − TA (max)
Where: TJ (max)
is the maximum allowable junction tem-
perature, which is 125°C for commercial
grade parts.
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TA (max)
is the maximum ambient temperature
which will be encountered in the applica-
tion.
FIGURE 1. ESR Limits
It is important to note that for most capacitors, ESR is speci-
fied only at room temperature. However, the designer must
ensure that the ESR will stay inside the limits shown over the
entire operating temperature range for the design.
Using the calculated values for TR(max) and PD, the maxi-
mum allowable value for the junction-to-ambient thermal re-
sistance, θ(JA), can now be found:
θ(JA) = TR (max)/PD
For aluminum electrolytic capacitors, ESR will increase by
about 30X as the temperature is reduced from 25°C to −40°
C. This type of capacitor is not well-suited for low temperature
operation.
IMPORTANT: If the maximum allowable value for θ(JA) is
found to be ≥ 60°C/W for the TO-220 package, ≥ 80°C/W for
the TO-263 package, or ≥174°C/W for the SOT-223 package,
no heatsink is needed since the package alone will dissipate
enough heat to satisfy these requirements.
Solid tantalum capacitors have a more stable ESR over tem-
perature, but are more expensive than aluminum electrolyt-
ics. A cost-effective approach sometimes used is to parallel
an aluminum electrolytic with a solid Tantalum, with the total
capacitance split about 75/25% with the Aluminum being the
larger value.
If the calculated value for θ(JA)falls below these limits, a
heatsink is required.
HEATSINKING TO-220 PACKAGE PARTS
The TO-220 can be attached to a typical heatsink, or secured
to a copper plane on a PC board. If a copper plane is to be
used, the values of θ(JA) will be the same as shown in the next
section for the TO-263.
If two capacitors are paralleled, the effective ESR is the par-
allel of the two individual values. The “flatter” ESR of the
Tantalum will keep the effective ESR from rising as quickly at
low temperatures.
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