LM2990

SNVS093D – JUNE 1999 – REVISED APRIL 2013

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APPLICATION HINTS

EXTERNAL CAPACITORS

The LM2990 regulator requires an output capacitor to maintain stability. The capacitor must be at least 10

μF

aluminum electrolytic or 1

μF solid tantalum. The output capacitor's ESR must be less than 10Ω, or the zero

added to the regulator frequency response by the ESR could reduce the phase margin, creating oscillations

(refer to the graph on the right). An input capacitor, of at least 1

μF solid tantalum or 10 μF aluminum electrolytic,

is also needed if the regulator is situated more than 6

″ from the input power supply filter.

FORCING THE OUTPUT POSITIVE

Due to an internal clamp circuit, the LM2990 can withstand positive voltages on its output. If the voltage source

pulling the output positive is DC, the current must be limited to 1.5A. A current over 1.5A fed back into the

LM2990 could damage the device. The LM2990 output can also withstand fast positive voltage transients up to

26V, without any current limiting of the source. However, if the transients have a duration of over 1 ms, the

output should be clamped with a Schottky diode to ground.

Figure 28. Output Capacitor ESR

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