9

LT1107

S

APPLICATI

I FOR ATIO

I

V

L

t

PEAK

IN

ON

=( )

20

The usual step-up configuration for the LT1107 is shown

Figure 2. Step-Down Mode Hookup

+

D1

L1

R3

FB

SW1

SW2

GND

C1

LT1107

1107 F01

R2

R1

+

D1

1N5818

L1

R3

100

Ω

FB

SW2

GND

C1

LT1107

1107 F02

R2

R1

SW1

+

C2

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:

Step-Down (Buck Mode) Operation

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

a lower voltage. The usual hookup for an LT1107 based

step-down converter is shown in Figure 2.

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

ON time, the current in L1 is equal to:

Figure 1. Step-Up Mode Hookup

V

R

R

V







1

2

1

125

21

.( )

I

V

VV

L

t

PEAK

IN

SW

OUT

ON

=

−−

()

22

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 LT1107. Output voltage is deter-

mined by:

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 100

Ω resistor pro-

grams the switch to turn off when the current reaches

approximately 700mA. When using the LT1107 in step-

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

less. Higher output voltages can be accommodated by

inserting a 1N5818 diode in series with the SW2 pin

(anode connected to SW2).

Inverting Configurations

The LT1107 can be configured as a positive-to-negative

converter (Figure 3), or a negative-to-positive converter

(Figure 4). In Figure 3, 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

V

R

R

V







1

2

1

125

23

.( )

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

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