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LT1940

1940i

Soft-Start and Shutdown

The RUN/SS (Run/Soft-Start) pins are used to place the

individual switching regulators and the internal bias cir-

cuits in shutdown mode. They also provide a soft-start

function. To shut down either regulator, pull the RUN/SS

pin to ground with an open-drain or collector. If both

RUN/SS pins are pulled to ground, the LT1940 enters its

shutdown mode with both regulators off and quiescent

current reduced to ~30

µA. Internal 2µA current sources

pull up on each pin. If either pin reaches ~0.5V, the internal

bias circuits start and the quiescent current increases to

~3.5mA.

If a capacitor is tied from the RUN/SS pin to ground, then

the internal pull-up current will generate a voltage ramp on

this pin. This voltage clamps the VC pin, limiting the peak

switch current and therefore input current during start up.

A good value for the soft-start capacitor is COUT/10,000,

where COUT is the value of the output capacitor.

The RUN/SS pins can be left floating if the shutdown

feature is not used. They can also be tied together with a

single capacitor providing soft-start. The internal current

sources will charge these pins to ~2.5V.

Power Good Indicators and Output Sequence

The PG pin is the open collector output of an internal

comparator. PG remains low until the FB pin is within 10%

of the final regulation voltage. Tie the PG pin to any supply

with a pull-up resistor that will supply less than 250

µA.

Note that this pin will be open when the LT1940 is placed

in shutdown mode (both RUN/SS pins at ground) regard-

less of the voltage at the FB pin. Power good is valid when

the LT1940 is enabled (either RUN/SS pin is high) and VIN

is greater than ~2.4V.

The PG pin can be used to sequence the two switching

regulators. The circuit in Figure 6 provides soft-start and

sequencing with the fewest components. PG1 is tied to

VC2, preventing switcher 2 from operating until output 1 is

in regulation. A single capacitor provides the soft-start

ramp for both regulators.

Shorted Input Protection

If the inductor is chosen so that it won’t saturate exces-

sively, the LT1940 will tolerate a shorted output. There is

another situation to consider in systems where the output

will be held high when the input to the LT1940 is absent.

If the VIN and one of the RUN/SS pins are allowed to float,

then the LT1940’s internal circuitry will pull its quiescent

current through its SW pin. This is fine if your system can

tolerate a few mA of load in this state. With both RUN/SS

pins grounded, the LT1940 enters shutdown mode and

the SW pin current drops to ~30

µA. However, if the VIN pin

is grounded while the output is held high, then parasitic

diodes inside the LT1940 can pull large currents from the

output through the SW pin and the VIN pin. A Schottky

diode in series with the input to the LT1940 will protect the

LT1940 and the system from a shorted or reversed input.

Figure 6. The Power Good Comparator can be used to Sequence

the Two Regulators. Switcher 1 will Start First.

OFF

1940 F06

RUN/SS1

RUN/SS2

PG2

VC2

PG1

POWER GOOD

VOUT2

GND

VIN

VOUT

LT1940

PARASITIC DIODE

1940 F07

Figure 7.Diode D4 Prevents a Shorted Input from

Discharging a Backup Battery Tied to the Output.

APPLICATIO S I FOR ATIO

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