LT1173CS8 datasheet(10/16 Pages) LINER

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LT1173

LT1173 • TA13

GND

SW2

SW1

LIM

+VOUT

1.245V + 0.6V

OUT

( )

2N3906

–VIN

LT1173

Figure 8. Negative-to-Positive Converter

Using the ILIM Pin

The LT1173 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

LT1173 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 “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

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

output capacitor and inductor. With the ILIM feature,

however, the switch current turns off at a programmed

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

minimum.

APPLICATI

I FOR ATIO

LT1173 • TA14

OFF

SWITCH

Figure 9. No Current Limit Causes Large Inductor

Current Build-Up

LT1173 • TA15

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 through

the ILIM pin. The value of the external resistor connected

between ILIM and VIN sets the current limit. When suffi-

cient switch current flows to develop a VBE across R1 +

RLIM, Q3 turns on and injects current into the oscillator,

turning off the switch. Delay through this circuitry is

approximately 2

µs. The current trip point becomes less

accurate for switch ON times less than 4

µs. Resistor

values programming switch ON time for 2

µs or less will

cause spurious response in the switch circuitry although

the device will still maintain output regulation.

LT1173 • TA28

SW2

SW1

DRIVER

OSCILLATOR

VIN

ILIM

Ω

(INTERNAL)

RLIM

(EXTERNAL)

Figure 11. LT1173 Current Limit Circuitry

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