LM3940
SNVS114E –MAY 1999–REVISED MARCH 2013
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APPLICATION HINTS
EXTERNAL CAPACITORS
The output capacitor is critical to maintaining regulator stability, and must meet the required conditions for both
ESR (Equivalent Series Resistance) and minimum amount of capacitance.
MINIMUM CAPACITANCE:
The minimum output capacitance required to maintain stability is 33 μF (this value may be increased without
limit). Larger values of output capacitance will give improved transient response.
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 Figure 19. It is essential that the output capacitor meet these
requirements, or oscillations can result.
Figure 19. ESR Limits
It is important to note that for most capacitors, ESR is specified 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.
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.
Solid tantalum capacitors have a more stable ESR over temperature, but are more expensive than aluminum
electrolytics. 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 two capacitors are paralleled, the effective ESR is the parallel of the two individual values. The “flatter” ESR of
the Tantalum will keep the effective ESR from rising as quickly at low temperatures.
HEATSINKING
A heatsink may be required depending on the maximum power dissipation and maximum ambient temperature of
the application. Under all possible operating conditions, the junction temperature must be within the range
specified under Absolute Maximum Ratings.
To determine if a heatsink is required, the power dissipated by the regulator, PD, must be calculated.
Figure 20 shows the voltages and currents which are present in the circuit, as well as the formula for calculating
the power dissipated in the regulator:
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