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LT1959

APPLICATIONS INFORMATION

FEEDBACK PIN FUNCTIONS

The feedback (FB) pin on the LT1959 is used to set output

voltage and provide several overload protection features.

The first part of this section deals with selecting resistors

to set output voltage and the remaining part talks about

foldback frequency and current limiting created by the FB

pin. Please read both parts before committing to a final

design.

The suggested value for the output divider resistor (see

Figure 2) from FB to ground (R2) is 2.5k or less, and a

formula for R1 is shown below. The output voltage error

caused by ignoring the input bias current on the FB pin is

less than 0.1% with R2 = 2.5k. Please read the following

if divider resistors are increased above the suggested

values.

RVOUT

21 21

121

−

()

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

in the external diode and inductor during short-circuit

conditions. A shorted output requires the switching regu-

lator 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 6A for the

LT1959, folding back to less than 3A). Minimum switch on

time limitations would prevent the switcher from attaining

a sufficiently low duty cycle if switching frequency were

maintained at 500kHz, so frequency is reduced by about

5:1 when the feedback pin voltage drops below 0.5V (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 LT1959 also

operates at lower switch current limit when the feedback

pin voltage drops below 0.8V. 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 0.75V with an external diode to defeat foldback cur-

rent 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 LT1959 to lose control of current limit.

Figure 2. Frequency and Current Limit Foldback

–

1.21V

VSW

GND

TO SYNC CIRCUIT

1959 F02

TO FREQUENCY

SHIFTING

2.5k

OUTPUT

ERROR

AMPLIFIER

LT1959

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