Typical Applications

The LM2940 is a positive 1A low dropout regulator; refer to its datasheet for further information.

Application Hints

EXTERNAL CAPACITORS

The LM2990 regulator requires an output capacitor to main-

tain 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 10

Ω, or the zero added to the regu-

lator 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 posi-

tive 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 di-

ode to ground.

Post Regulator for an Isolated Switching Power Supply

DS010801-5

Fixed Current Sink

DS010801-7

Adjustable Current Sink

DS010801-10

Output Capacitor ESR

DS010801-9

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