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LT1375/LT1376

APPLICATIONS INFORMATION

–

2.4V

VSW

GND

1375/76 F02

TO FREQUENCY

SHIFTING

OUTPUT

ERROR

AMPLIFIER

1.6V

LT1375/LT1376

Figure 2. Frequency and Current Limit Foldback

Please read the following if divider resistors are increased

above the suggested values.

RVOUT

22 42

242

−

()

Table 1

OUTPUT

% ERROR AT OUTPUT

VOLTAGE

(NEAREST 1%)

DUE TO DISCREET 1%

(V)

Ω)(kΩ)

RESISTOR STEPS

4.99

1.21

+ 0.23

3.3

4.99

1.82

+ 0.08

4.99

5.36

+ 0.39

4.99

7.32

– 0.5

4.99

11.5

– 0.04

4.99

15.8

+ 0.83

4.99

19.6

– 0.62

4.99

26.1

+ 0.52

More Than Just Voltage Feedback

The feedback (FB) pin is used for more than just output

voltage sensing. It also reduces switching frequency and

current limit when output voltage is very low (see the

Frequency Foldback graph in Typical Performance Char-

acteristics). This is done to control power dissipation in

both the IC and in the external diode and inductor during

short-circuit conditions. A shorted output requires the

switching regulator to operate at very low duty cycles, and

the average current through the diode and inductor is

equal to the short-circuit current limit of the switch (typi-

cally 2A for the LT1376, folding back to less than 1A).

Minimum switch on time limitations would prevent the

switcher from attaining a sufficiently low duty cycle if

switching frequency were maintained at 500kHz, so fre-

quency is reduced by about 5:1 when the feedback pin

voltage drops below 1V (see Frequency Foldback graph).

This does not affect operation with normal load condi-

tions; one simply sees a gear shift in switching frequency

during start-up as the output voltage rises.

In addition to lower switching frequency, the LT1376 also

operates at lower switch current limit when the feedback

pin voltage drops below 1.7V. Q2 in Figure 2 performs this

function by clamping the VC pin to a voltage less than its

normal 2.3V upper clamp level. This

foldback current limit

greatly reduces power dissipation in the IC, diode and

inductor during short-circuit conditions. Again, it is nearly

transparent to the user under normal load conditions. The

only loads which may be affected are current source loads

which maintain full load current with output voltage less

than 50% of final value. In these rare situations the

Feedback pin can be clamped above 1.5V with an external

diode to defeat foldback current limit.

Caution: clamping

the feedback pin means that frequency shifting will also be

defeated, so a combination of high input voltage and dead

shorted output may cause the LT1376 to lose control of

current limit.

The internal circuitry which forces reduced switching

frequency also causes current to flow out of the feedback

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