LT1073

9

APPLICATIO S I FOR ATIO

V

R

R

mV







1

2

1

212

Step-Down (Buck Mode) Operation

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

a lower voltage. It is short-circuit protected because the

switch is in series with the output. Step-down converters

are characterized by low output voltage ripple but high

input current ripple. The usual hookup for an LT1073-

based step-down converter is shown in Figure 7.

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

ON time, the current in L1 is equal to

i

VV

V

L

t

PEAK

IN

SW

OUT

ON

=

––

When the switch turns off the SW2 pin falls rapidly and

actually goes below ground. D1 turns on when SW2

reaches 0.4V below ground.

D1 MUST BE A SCHOTTKY

DIODE. The voltage at SW2 must never be allowed to go

below –0.5V. A silicon diode such as the 1N4933 will allow

SW2 to go to –0.8V, causing potentially destructive power

dissipation inside the LT1073. Output voltage is deter-

mined by

V

R

R

mV







1

2

1

212

R3 programs switch current limit. This is especially im-

portant in applications where the input varies over a wide

range. Without R3, the switch stays on for a fixed time

each cycle. Under certain conditions the current in L1 can

build up to excessive levels, exceeding the switch rating

and/or saturating the inductor. The 220

Ω resistor pro-

grams the switch to turn off when the current reaches

approximately 400mA. When using the LT1073 in step-

down mode, output voltage should be limited to 6.2V or

less.

Inverting Configurations

The LT1073 can be configured as a positive-to-negative

converter (Figure 8), or a negative-to-positive converter

(Figure 9). In Figure 8, the arrangement is very similar to

a step-down, except that the high side of the feedback is

referred to ground. This level shifts the output negative. As

in the step-down mode, D1 must be a Schottky diode, and

Figure 7. Step-Down Mode Hookup

+

R3

220

Ω

L1

D1

1N5818

C1

+

C2

R2

R1

1073 FO7

LT1073

SW2

FB

GND

Figure 8. Positive-to-Negative Converter

Figure 9. Negative-to-Positive Converter

+

R3

L1

D1

1N5818

C1

+

C2

R1

R2

1073 FO8

LT1073

SW2

FB

GND

+

L1

D1

R1

C1

+

C2

R2

1073 F09

LT1073

SW1

FB

AO

SW2

GND

2N3906

R1

R2