LM2991SX National Semiconductor, LM2991SX Datasheet - Page 6

• Manufacturer Part Number: LM2991SX
• Description: IC,VOLT REGULATOR,ADJUSTABLE,-2 TO -25V,BIPOLAR,SIP,5PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LM2991SX

• Rohs Compliant: NO

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Application Hints
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 .
SETTING THE OUTPUT VOLTAGE
Typical Applications
Output Capacitor ESR Range
(Continued)
Fully Isolated Post-Switcher Regulator
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6
The output voltage of the LM2991 is set externally by a resis-
tor divider using the following equation:
where V
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
rent negligible. For example, neglecting the adjust pin cur-
rent, and setting R2 to 100 k 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.
REF
V
OUT
= −1.21V. The output voltage can be pro-
= V
REF
x (1 + R
2
/R
makes the adjust pin cur-
1
) − (I
OUT
to −5V, results in an
ADJ
x R
2
)
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