參數(shù)資料
型號(hào)： LT3437EFE
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： RADIATION HARDENED HIGH EFFICIENCY, 5 AMP SWITCHING REGULATORS
中文描述： 0.9 A SWITCHING REGULATOR, 240 kHz SWITCHING FREQ-MAX, PDSO16
封裝： 4.40 MM, PLASTIC, TSSOP-16
文件頁(yè)數(shù)： 9/24頁(yè)
文件大?。?/td> 523K
代理商： LT3437EFE
LT3437
9
3437f
APPLICATIU
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.
W
U
U
R
R
V
R
nA
OUT
+
1
2
2 50
1 25
.
=
–1 25
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
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
foldback operation. Frequency foldback does not affect
operation 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
high input current surge during start-up. The soft-start
Table 1
OUTPUT
VOLTAGE
(V)
2.5
3
3.3
5
6
8
10
12
R1
OUTPUT
ERROR
(%)
0
0
0.38
0
0.63
–0.63
–0.25
0.63
R2
NEAREST (1%)
(k
)
100
140
165
300
383
536
698
866
(k
, 1%)
100
100
100
100
100
100
100
100
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