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LT1941

1941f

APPLICATIO S I FOR ATIO

Converter with Backup Output Regulator

There is another situation to consider in systems where

the output will be held high when the input to the LT1941

is absent. If the VIN and one of the RUN/SS pins are allowed

to float, then the LT1941’s internal circuitry will pull its

quiescent current through its SW pin. This is acceptable if

the system can tolerate a few mA of load in this state. With

both RUN/SS pins grounded, the LT1941 enters shut-

down mode and the SW pin current drops to ~50mA.

However, if the VIN pin is grounded while the output is held

high, then parasitic diodes inside the LT1941 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

LT1941, as shown in Figure 4, will protect the LT1941 and

the system from a shorted or reversed input.

Regulating Negative Output Voltages

The LT1941 contains an inverting op-amp with its nonin-

verting terminal tied to ground and its output connected to

the FB3 pin. Use this op-amp to generate a voltage at FB3

that is proportional to VOUT. Choose the resistors accord-

ing to:

OUT

124

•

Figure 4. Diode D4 Prevents a Shorted Input from

Discharging a Backup Battery Tied to the Output

VIN

VOUT

LT1941

PARASITIC DIODE

1941 F04

FB3

1941 AI01

VOUT

NFB

FB3

1941 AI02

–VOUT

INVERTER/BOOST CONSIDERATIONS

Regulating Positive Output Voltages

The output voltage is programmed with a resistor divider

between the output and the FB pin. Choose the resistors

according to:

OUT

125









–

R4 should be 10k or less to avoid bias current errors.

NFB should be tied to FB3.

Use 10k or larger, up to 20k for R3.

Duty Cycle Range

The maximum duty cycle (DC) of the LT1941 inverter/

boost regulator is 77%. The duty cycle for a given applica-

tion using the inverter topology is:

OUT

The duty cycle for a given application using the boost

topology is:

OUT

–

The LT1941 can still be used in applications where the DC,

as calculated above, is above 77%; however, the part must

be operated in discontinuous mode so that the actual duty

cycle is reduced.

Inductor Selection

Several inductors that work well with the LT1941 inverter/

boost regulator are listed in Table 4. Besides these, many

other inductors will work. Consult each manufacturer for

detailed information and for their entire selection of related

parts. Use ferrite core inductors to obtain the best effi-

ciency. When using coupled inductors, choose one that

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