LT3513

15

3513fa

Use a diode with a reverse voltage rating greater than the

input voltage. Table 3 lists several Schottky diodes and

their manufacturers.

Table 3. Schottky Diodes

PART NUMBER

On Semiconductor

MBRM120E

20

1

530

MBRM140

40

1

550

MBRS240

40

2

MBRA340

40

3

450

Diodes Inc.

B120

20

1

500

B240

40

2

500

B340A

40

3

450

BOOST PIN CONSIDERATIONS

The minimum operating voltage of an LT3513 application

is limited by the undervoltage lockout ~4V and by the

maximum duty cycle. The boost circuit also limits the

minimum input voltage for proper start-up. If the input

voltage ramps slowly or the LT3513 turns on when the

output is already in regulation, the boost capacitor may

not be fully charged. Because the boost capacitor charges

with the energy stored in the inductor, the circuit will rely

on some minimum load current to get the boost circuit

running properly. This minimum load will depend on input

and output voltages. The Typical Performance Character-

istics section shows a plot of the minimum load current

to start as a function of input voltage for a 3.3V output.

The minimum load current generally goes to zero once the

circuit has started. Even without an output load current, in

many cases the discharged output capacitor will present

a load to the switcher that will allow it to start.

INVERTER/STEP-UP 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:

RR

34

125

1

= ⎛

⎝⎜

⎞

⎠⎟

.

–

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

Regulating Negative Output Voltages

The LT3513 contains an inverting op amp with a gain of

1. The NFB4 pin works just as the other FB pins. Choose

the resistors according to:

R

VR

R

OUT

6

5

125

5

=

•

.

–

R5 should be 2.5k

Ω or less to avoid bias current errors.

R6

R5

NFB4

3513 A2

22

Duty Cycle Range

The maximum duty cycle (DC) of the LT3513 switching

regulator is 75% for SW2, and 84% for SW3 and SW4.

The duty cycle for a given application using the step-up

or charge pump topology is:

DC

VV

V

OUT

IN

OUT

=

–

The duty cycle for a given application using the inverter

or SEPIC is:

DC

V

VV

OUT

IN

OUT

=

+

The LT3513 can still be used in applications where the duty

cycle, as calculated above, is greater than the maximum.

However, the part must be operated in discontinuous mode

so that the actual duty cycle is reduced.

OPERATION