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LT1959 Datasheet(PDF) 8 Page - Linear Technology

Part No. LT1959
Description  4.5A, 500kHz Step-Down Switching Regulator
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Maker  LINER [Linear Technology]
Homepage  http://www.linear.com
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LT1959 Datasheet(HTML) 8 Page - Linear Technology

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8
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.
R
RVOUT
1
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
VC
GND
TO SYNC CIRCUIT
1959 F02
TO FREQUENCY
SHIFTING
R3
1k
R4
1k
R1
R2
2.5k
OUTPUT
5V
R5
5k
ERROR
AMPLIFIER
FB
1V
Q1
LT1959
Q2
+


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