LT1110 datasheet(11/16 Pages) LINER

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LT1110

Converter” section with the following conservative ex-

pression for VSW:

SAT

≈

( )

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

drive to Q1. R4 and R5 set output voltage.

Inverting Configurations

The LT1110 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

VOUTshould be less than 6.2V. More negative out-

put voltages can be accommodated as in the prior section.

LT1110 • TA03

–VOUT

1N5818

+VIN

GND

SW2

SW1

LIM

LT1110

Figure 7. Positive-to-Negative Converter

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.

dissipation inside the LT1110. Output voltage is deter-

mined by

OUT =+







 ()

220

)

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 800mA. When using the LT1110 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).

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

equals a VBE, the switch turns off. For temperature com-

pensation a Schottky diode can be inserted in series with

the ILIM pin. This also lowers the maximum drop across R1

to VBE – VD, increasing efficiency. As shown, switch

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

based on formulas in the “Inductor Selection Step-Down

APPLICATI

I FOR ATIO

Figure 6. Q1 Permits Higher-Current Switching.

LT1110 Functions as Controller.

LT1110 • TA16

1N5821

VOUT

VIN

25V

MAX

0.3

Ω

220

MJE210 OR

ZETEX ZTX789A

330

VOUT = 220mV (1 +

)

LT1110

GND

SW2

SW1

VIN

Figure 8. Negative-to-Positive Converter

LT1110 • TA04

GND

SW2

SW1

LIM

+VOUT

VOUT =

220mV + 0.6V

( )

2N3906

–VIN

LT1110

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