10

LT1111

S

APPLICATI

I FOR ATIO

Diode Selection

Speed, forward drop, and leakage current are the three

main considerations in selecting a catch diode for LT1111

converters. General purpose rectifiers such as the 1N4001

are

unsuitable for use in any switching regulator applica-

tion. Although they are rated at 1A, the switching time of

a 1N4001 is in the 10

µs to 50µs range. At best, efficiency

will be severely compromised when these diodes are

used; at worst, the circuit may not work at all. Most

LT1111 circuits will be well served by a 1N5818 Schottky

diode, or its surface mount equivalent, the MBRS130T3.

The combination of 500mV forward drop at 1A current,

fast turn ON and turn OFF time, and 4

µA to 10µA leakage

current fit nicely with LT1111 requirements. At peak

switch currents of 100mA or less, a 1N4148 signal diode

may be used. This diode has leakage current in the 1nA to

5nA range at 25

°C and lower cost than a 1N5818. (You can

also use them to get your circuit up and running, but

beware of destroying the diode at 1A switch currents.)

Step-Up (Boost Mode) Operation

A step-up DC/DC converter delivers an output voltage

higher than the input voltage. Step-up converters are not

short-circuit protected since there is a DC path from input

to output.

The usual step-up configuration for the LT1111 is shown

I

V

L

t

PEAK

IN

ON

=

()

20

Immediately after switch turn-off, the SW1 voltage pin

starts to rise because current cannot instantaneously stop

inductor current flows through D1 into C1, increasing

R2 set the output voltage according to the formula

V

R

R

V







1

2

1

125

21

.( )

Step-Down (Buck Mode) Operation

A step-down DC/DC converter converts a higher voltage

to a lower voltage. The usual hookup for an LT1111 based

step-down converter is shown in Figure 5.

L1

LT1111 • F04

GND

SW2

SW1

LIM

I

IN

V

D1

R3*

LT1111

+

R2

R1

C1

*OPTIONAL

FB

Figure 4. Step-Up Mode Hookup.

Refer to Table 1 for Component Values.

Note 1: This simple expression neglects the effect of switch and coil

resistance. This is taken into account in the “Inductor Selection” section.

causing a current to build up in L1. At the end of the switch

ON time, the current in L1 is equal to:

I

V

VV

L

t

PEAK

IN

SW

OUT

ON

=

−−

()

22

LT1111 • F05

GND

SW2

SW1

LIM

I

IN

V

R3

100

FB

+

C2

+

C1

D1

1N5818

R2

R1

L1

Ω

LT1111

Figure 5. Step-Down Mode Hookup