Datasheet

LM1085
www.ti.com SNVS038G –JULY 1999–REVISED MARCH 2013
Figure 18. Best Load Regulation using Adjustable Output Regulator
PROTECTION DIODES
Under normal operation, the LM1085 regulator does not need any protection diode. With the adjustable device,
the internal resistance between the adjustment and output terminals limits the current. No diode is needed to
divert the current around the regulator even with a capacitor on the adjustment terminal. The adjust pin can take
a transient signal of ±25V with respect to the output voltage without damaging the device.
When an output capacitor is connected to a regulator and the input is shorted, the output capacitor will discharge
into the output of the regulator. The discharge current depends on the value of the capacitor, the output voltage
of the regulator, and rate of decrease of V
IN
. In the LM1085 regulator, the internal diode between the output and
input pins can withstand microsecond surge currents of 10A to 20A. With an extremely large output capacitor
(≥1000 µf), and with input instantaneously shorted to ground, the regulator could be damaged. In this case, an
external diode is recommended between the output and input pins to protect the regulator, shown in Figure 19.
Figure 19. Regulator with Protection Diode
OVERLOAD RECOVERY
Overload recovery refers to regulator's ability to recover from a short circuited output. A key factor in the recovery
process is the current limiting used to protect the output from drawing too much power. The current limiting circuit
reduces the output current as the input to output differential increases. Refer to short circuit curve in the Typical
Performance Characteristics section.
During normal start-up, the input to output differential is small since the output follows the input. But, if the output
is shorted, then the recovery involves a large input to output differential. Sometimes during this condition the
current limiting circuit is slow in recovering. If the limited current is too low to develop a voltage at the output, the
voltage will stabilize at a lower level. Under these conditions it may be necessary to recycle the power of the
regulator in order to get the smaller differential voltage and thus adequate start up conditions. Refer to Typical
Performance Characteristics section for the short circuit current vs. input differential voltage.
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