10

LT1108

S

APPLICATI

I FOR ATIO

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 6, the inductor

current increases to a high level before the comparator

turns off the oscillator. This high current can cause exces-

sive output ripple and requires oversizing the output ca-

switch current turns off at a programmed level as shown in

Figure 7, keeping output ripple to a minimum.

provided by the PNP transistor, supplies proper polarity

feedback information to the regulator.

Figure 4. Positive-to-Negative Converter

Figure 5. Negative-to-Positive Converter

LT1108 • F04

+

C2

+

C1

D1

1N5818

R1

R2

L1

GND

SW2

SW1

R3

FB

LT1108

L1

LT1108 • F05

GND

SW2

FB

SW1

D1

AO

R2

1.245V + 0.6V

R1

R2

R1

2N3906

+

C1

LT1108

+

C2

()

The LT1108 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 where analysis shows the

LT1108 must operate at an 800mA peak switch current with

rise to 1.6A, exceeding the maximum switch current rating.

With the proper resistor selected (see the “Maximum

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

LT1108 • F06

OFF

ON

SWITCH

LT1108 • F07

ON

OFF

SWITCH

PROGRAMMED CURRENT LIMIT

Figure 7. Current Limit Keeps Inductor Current Under Control

Figure 6. No Current Limit Causes Large Inductor

Current Build-Up