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LT1941

Triple Monolithic Switching Regulator

Linear Technology 

LT1941

APPLICATIONS INFORMATION

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 shutdown

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.

PARASITIC DIODE

D4

VIN

VIN

SW

LT1941

VOUT

1941 F04

Figure 4. Diode D4 Prevents a Shorted Input from

Discharging a Backup Battery Tied to the Output

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:

R3

=

R4

⎛

⎝⎜

VOUT

1.25V

–

1⎞⎠⎟

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

NFB should be tied to FB3.

VOUT

R3

FB3

R4

1941 AI01

14

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

according to:

R4 = R3 • VOUT

1.24V

–VOUT

R4

NFB

R3

FB3

1941 AI02

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 application

using the inverting topology is:

DC = VOUT

VIN + VOUT

The duty cycle for a given application using the boost

topology is:

DC = VOUT – VIN

VOUT

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

efficiency. When using coupled inductors, choose one that

1941fb

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