12

LT1111

S

APPLICATI

I FOR ATIO

L1

LT1111 • F08

GND

SW2

FB

SW1

LIM

I

IN

V

D1

A0

R2

1.25V + 0.6V

R1

R2

( )

R1

2N3906

+

C1

LT1111

+

C2

Figure 8. Negative-to-Positive Converter

The LT1111 switch can be programmed to turn off at a set

switch current, a feature not found on competing devices.

This enables the input to vary over a wide range without

exceeding the maximum switch rating or saturating the

inductor. Consider the case wh

ere analysis shows the

LT1111 must operate at an 800mA peak switch current

will rise to 1.6A, exceeding the maximum switch current

rating. With the proper resistor selected (see the “Maxi-

current will be limited to 800mA, even if the input voltage

increases.

when the device goes into continuous mode operation.

This occurs in step-up mode when:

V

V

VV

DC

OUT

DIODE

IN

SW

+

−

<

−

1

1

25

()

When the input and output voltages satisfy this relation-

ship, inductor current does not go to zero during the

switch OFF time. When the switch turns on again, the

current ramp starts from the non-zero current level in the

inductor just prior to switch turn-on. As shown in Figure

9, the inductor current increases to a high level before the

comparator turns off the oscillator. This high current can

cause excessive output ripple and requires oversizing the

however, the switch current turns off at a programmed

level as shown in Figure 10, keeping output ripple to a

minimum.

LT1111 • F09

I

OFF

L

ON

SWITCH

Figure 9. No Current Limit Causes Large Inductor

Current Build-Up

LT1111 • F10

I

ON

L

OFF

SWITCH

PROGRAMMED CURRENT LIMIT

Figure 10. Current Limit Keeps Inductor Current Under Control

Figure 11 details current limit circuitry. Sense transistor

Q1, whose base and emitter are paralleled with power

switch Q2, is ratioed such that approximately 0.5% of

Q2’s collector current flows in Q1’s collector. This current

is passed through internal 80

Ω resistor R1 and out

oscillator, turning off the switch. Delay through this cir-

cuitry is approximately 1

µs. The current trip point be-

comes less accurate for switch ON times less than 3

µs.

Resistor values programming switch ON time for 1

µs or

less will cause spurious response in the switch circuitry

although the device will still maintain output regulation.

LT1111 • F11

SW2

SW1

DRIVER

OSCILLATOR

R1

80

Ω

(INTERNAL)

(EXTERNAL)

Q1

Q2

Q3

Figure 11. LT1111 Current Limit Circuitry