Typical Applications
20158601
*Required if the regulator is located further than 6 inches from the power supply filter capacitors. A 1 μF solid tantalum or a 10 μF aluminum electrolytic capacitor
is recommended.
**Required for stability. Must be at least a 10 μF aluminum electrolytic or a 1 μF solid tantalum to maintain stability. May be increased without bound to maintain
regulation during transients. Locate the capacitor as close as possible to the regulator. The equivalent series resistance (ESR) is critical, and should be less than
10Ω over the same operating temperature range as the regulator.
Post Regulator for an Isolated Switching Power Supply
20158605
The LM2940 is a positive 1A low dropout regulator; refer to
its datasheet for further information.
gin, creating oscillations (refer to the graph on the right). An
input capacitor, of at least 1 μF solid tantalum or 10 μF alu-
minum electrolytic, is also needed if the regulator is situated
more than 6″ from the input power supply filter.
Fixed Current Sink
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
diode to ground.
20158607
Adjustable Current Sink
Output Capacitor ESR
20158610
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 10Ω, or the zero added to the regulator
frequency response by the ESR could reduce the phase mar-
20158609
7
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