11

LT1111

S

APPLICATI

I FOR ATIO

Figure 6. Q1 Permits Higher Current Switching.

LT1111 Functions as Controller.

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 LT1111. Output voltage is deter-

mined by:

V

R

R

V







1

2

1

125

23

.(

)

R3 programs switch current limit. This is especially impor-

tant 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 programs the

switch to turn off when the current reaches approximately

700mA. When using the LT1111 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 con-

nected to SW2).

Higher Current Step-Down Operation

Output current can be increased by using a discrete PNP

pass transistor as shown in Figure 6. R1 serves as a

current limit sense. When the voltage drop across R1

pensation a Schottky diode can be inserted in series with

current is limited to 2A. Inductor value can be calculated

based on formulas in the “Inductor Selection — Step-

Down Converter” section with the following conservative

VV

V

V

SW

R

Q SAT

=+

≈

11

10

24

.(

)

R2 provides a current path to turn off Q1. R3 provides base

drive to Q1. R4 and R5 set output voltage. A PMOS FET can

LT1111 • TA08

D1

1N5821

+

+

30V

MAX

L1

R1

0.3

Ω

R2

220

Q1

MJE210 OR

ZETEX ZTX749

R3

330

R4

R5

C1

)

R4

R5

LT1111

GND

SW2

SW1

FB

C2

In Figure 8, the input is negative while the output is

positive. In this configuration, the magnitude of the input

voltage can be higher or lower than the output voltage. A

level shift, provided by the PNP transistor, supplies proper

polarity feedback information to the regulator.

Inverting Configurations

The LT1111 can be configured as a positive-to-negative

converter (Figure 7), or a negative-to-positive converter

(Figure 8). In Figure 7, 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

put voltages can be accommodated as in the prior section.

LT1111 • F07

+

C2

+

C1

D1

1N5818

R1

R2

L1

GND

SW2

SW1

LIM

I

IN

V

R3

FB

LT1111

Figure 7. Positive-to-Negative Converter