Datasheet

11
LT3437
3437fc
APPLICATIO S I FOR ATIO
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FEEDBACK PIN FUNCTIONS
The feedback (FB) pin on the LT3437 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
frequency foldback and soft-start features. Please read
both parts before committing to a final design.
Referring to Figure 2, the output voltage is determined by
a voltage divider from V
OUT
to ground which generates
1.25V at the FB pin. Since the output divider is a load on the
output, care must be taken when choosing the resistor
divider values. For light load applications the resistor
values should be as large as possible to achieve peak
efficiency in Burst Mode operation. Extremely large values
for resistor R1 will cause an output voltage error due to the
50nA FB pin input current. The suggested value for the
output divider resistor (see Figure 2) from FB to ground
(R2) is 100k or less. A formula for R1 is shown below. A
table of standard 1% values is shown in Table 1 for
common output voltages.
RR
V
RnA
OUT
12
125
125 2 50
=
+
•
–.
.•
Figure 2. Feedback Network
More Than Just Voltage Feedback
The FB pin is used for more than just output voltage
sensing. It also reduces switching frequency and con-
trols the soft-start voltage ramp rate when output voltage
is below the regulated level (see the Frequency Foldback
and Soft-Start Current graphs in Typical Performance
Characteristics).
Frequency foldback 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. As
a result, the average current through the diode and induc-
tor is equal to the short-circuit current limit of the switch
(typically 500mA for the LT3437). 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
10:1 when the FB pin voltage drops below 0.4V (see
Frequency Foldback graph). As the feedback voltage rises,
the switching frequency increases to 200kHz with 0.95V
on the FB pin. During frequency foldback, external syn-
chronization is disabled to prevent interference with fold-
back operation. Frequency foldback does not affect opera-
tion during normal load conditions.
In addition to lowering switching frequency, the soft-start
ramp rate is also affected by the feedback voltage. Large
capacitive loads or high input voltages can cause a
Table 1
OUTPUT R1 OUTPUT
VOLTAGE R2 NEAREST (1%) ERROR
(V) (kΩ, 1%) (kΩ)(%)
2.5 100 100 0
3 100 140 0
3.3 100 165 0.38
5 100 300 0
6 100 383 0.63
8 100 536 – 0.63
10 100 698 –0.25
12 100 866 0.63
SOFT-START
FOLDBACK
DETECT
200kHz
OSCILLATOR
+
–
ERROR
AMP
1.25V
V
C
7
FB
8
C
SS
V
OUT
5
SW
LT3437
C1
R1
R2
3437 F02
1