MINIMUM LOAD

A minimum load current of 500 µA is required for proper op-

eration. The external resistor divider can provide the mini-

mum load, with the resistor from the adjust pin to ground set

to 2.4 k

Ω.

SETTING THE OUTPUT VOLTAGE

The output voltage of the LM2991 is set externally by a resis-

tor divider using the following equation:

V

where V

REF

= −1.21V. The output voltage can be pro-

grammed within the range of −3V to −24V, typically an even

greater range of −2V to −25V. The adjust pin current is about

60 nA, causing a slight error in the output voltage. However,

using resistors lower than 100 k

Ω makes the adjust pin cur-

rent negligible. For example, neglecting the adjust pin cur-

rent, and setting R2 to 100 k

Ω and V

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 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.

Typical Applications

Output Capacitor ESR Range

DS011260-5

Fully Isolated Post-Switcher Regulator

DS011260-6

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