LT1107CS8 datasheet(10/12 Pages) LINER

10 / 12 page

LT1107

VOUTshould be less than 6.2V. More negative output

voltages can be accommodated as in the prior section.

In Figure 4, 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.

Figure 4. Negative-to-Positive Converter

Using the ILIM Pin

The LT1107 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

LT1107 must operate at an 800mA peak switch current

with a 2V 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 “Maxi-

mum Switch Current vs RLIM” characteristic), the switch

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

increases.

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

5, 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

output capacitor and inductor. With the ILIM feature, the

switch turns off at the programmed current as shown in

Figure 6, keeping output ripple to a minimum.

Figure 6. Current Limit Keeps Inductor Current Under Control

Figure 5. No Current Limit Causes Large Inductor

Current Build-Up

1107 F05

OFF

SWITCH

1107 F06

OFF

SWITCH

PROGRAMMED CURRENT LIMIT

ILIM

VIN

SW2

GND

LT1107

1107 F04

+VOUT

–VIN

SW1

2N3906

VOUT =

1.25V + 0.6V

()R1

1N5818

ILIM

VIN

SW2

GND

LT1107

1107 F03

+VIN

–VOUT

SW1

Figure 3. Positive-to-Negative Converter

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