LTM8021

10

8021fb

APPLICATIONS INFORMATION

Figure 2. To Soft-Start the LTM8021, Add a

Resisitor and Capacitor to the RUN/SS Pin

Soft-Start

The RUN/SS pin can be used to soft-start the LTM8021,

reducing the maximum input current during start-up.

The RUN/SS pin is driven through an external RC filter

to create a voltage ramp at this pin. Figure 2 shows the

soft-start circuit. By choosing a large RC time constant,

the peak start-up current can be reduced to the current

that is required to regulate the output, with no overshoot.

Choose the value of the resistor so that it can supply 80μA

when the RUN/SS pin reaches 2V.

Figure 3. The Input Diode Prevents a Shorted Input from

Discharging a Backup Battery Tied to the Output. It Also Protects

the Circuit from a Reversed Input. The LTM8021 Runs Only When

the Input is Present

RUN/SS

BIAS

GND

LTM8021

8021 F03

RT

4V TO 36V

Shorted Input Protection

Care needs to be taken in systems where the output will

be held high when the input to the LTM8021 is absent.

This may occur in battery charging applications or in

battery backup systems where a battery or some other

supply is diode ORed with the LTM8021’s output. If the

the LTM8021’s internal circuitry will pull its quiescent

current through its internal power switch. This is fine if

your system can tolerate a few milliamps in this state. If

the RUN/SS pin is grounded, the internal power switch

is grounded while the output is held high, then parasitic

diodes inside the LTM8021 can pull large currents from

pin. Figure 3 shows a circuit that will run only when the

input voltage is present and that protects against a shorted

or reversed input.

8021 F02

RUN/SS

GND

0.22μF

RUN

15k