using resistors lower than 100 k

Ω makes the adjust pin

current negligible. For example, neglecting the adjust pin

current, and setting R2 to 100 k

Ω and V

an output voltage error of only 0.16%.

ON/OFF PIN

The LM2991 regulator can be turned off by applying a TTL or

CMOS level high signal to the ON/OFF pin (see Adjustable

Current Sink Application).

FORCING THE OUTPUT POSITIVE

Due to an internal clamp circuit, the LM2991 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 LM2991 could damage

the device. The LM2991 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.

Typical Applications

20158701

*Required if the regulator is located further than 6 inches from the power supply filter capacitors.A1µF 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 10

Ω over the same operating temperature range as the regulator.

Fully Isolated Post-Switcher Regulator

20158706

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