LT1108CS8-12 datasheet(10/12 Pages) LINER

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LT1108

APPLICATI

I FOR ATIO

Another situation where the ILIM feature is useful occurs

when the device goes into continuous mode operation. This

occurs in step-up mode when

OUT

DIODE

−

)

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-

pacitor and inductor. With the ILIM feature, however, the

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

1N5818

VIN

GND

SW2

SW1

LT1108

ILIM

VIN

–VOUT

LT1108 • F05

GND

SW2

SW1

VOUT =

1.245V + 0.6V

2N3906

LT1108

ILIM

VIN

()

VOUT

–VIN

USING THE ILIM PIN

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

a 2.0V input. If VIN rises to 4V, the peak switch current will

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

With the proper resistor selected (see the “Maximum

Switch Current vs RLIM” characteristic), the switch current

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

LT1108 • F06

OFF

SWITCH

LT1108 • F07

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

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