LT1576CS8 datasheet(9/28 Pages) LINER

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LT1576/LT1576-5

APPLICATIONS INFORMATION

Figure 2. Frequency and Current Limit Foldback

–

1.21V

VSW

GND

TO SYNC CIRCUIT

1576 F02

TO FREQUENCY

SHIFTING

OUTPUT

ERROR

AMPLIFIER

1.4V

LT1576

Table 1

OUTPUT

% ERROR AT OUTPUT

VOLTAGE

(NEAREST 1%)

DUE TO DISCREET 1%

(V)

Ω)(kΩ)

RESISTOR STEPS

4.99

7.32

– 0.50

3.3

4.99

8.66

+ 0.30

4.99

15.8

+ 0.83

4.99

19.6

– 0.62

4.99

28.0

– 0.01

4.99

36.5

+ 0.61

4.99

44.2

– 0.60

4.99

56.2

– 1.08

More Than Just Voltage Feedback

The feedback 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 Characteristics).

This is done to control power dissipation in both the IC and

the external diode and inductor during short-circuit con-

ditions. 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 (typically 2A for the LT1576,

folding back to less than 0.77A). Minimum switch on time

limitations would prevent the switcher from attaining a

sufficiently low duty cycle if switching frequency were

maintained at 200kHz, so frequency is reduced by about

5:1 when the feedback pin voltage drops below 0.7V (see

Frequency Foldback graph). This does not affect operation

with normal load conditions; one simply sees a gear shift

in switching frequency during start-up as the output

voltage rises.

In addition to lower switching frequency, the LT1576 also

operates at lower switch current limit when the feedback

pin voltage drops below 0.7V. Q2 in Figure 2 performs this

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

normal 2.1V upper clamp level. This

foldback current limit

greatly reduces power dissipation in the IC, diode and

inductor during short-circuit conditions. External synchro-

nization is also disabled to prevent interference with

foldback operation. Again, it is nearly transparent to the

user under normal load conditions. The only loads that 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 0.7V to defeat foldback current limit.

Caution: clamp-

ing 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 LT1576 to lose control

of current limit.

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