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LT1977 ๋ฐ์ดํ„ฐ์‹œํŠธ(HTML) 10 Page - Linear Technology

๋ถ€ํ’ˆ๋ช… LT1977
์ƒ์„ธ๋‚ด์šฉ  High Voltage 1.5A, 500kHz Step-Down Switching Regulator with 100uA Quiescent Current
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์ œ์กฐ์‚ฌ  LINER [Linear Technology]
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LT1977 ๋ฐ์ดํ„ฐ์‹œํŠธ(HTML) 10 Page - Linear Technology

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LT1977
10
1977f
APPLICATIO S I FOR ATIO
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 2 for
common output voltages.
RR
V
RnA
OUT
12
125
125
2 50
=
+
โ€ข
โ€“.
.โ€ข
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 used 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 2A for the LT1977). Minimum switch on time
limitations would prevent the switcher from operating at a
sufficiently low duty cycle if switching frequency were
maintained at 500kHz, so frequency is reduced by about
4:1 when the FB pin voltage drops below 0.4V (see
Frequency Foldback graph). In addition, if the current in
the switch exceeds 1.5 times the current limitations speci-
fied by the VC pin, due to minimum switch on time, the
LT1977 will skip the next switch cycle. As the feedback
voltage rises, the switching frequency increases to 500kHz
with 0.95V on the FB pin. During frequency foldback,
external synchronization is disabled to prevent interfer-
ence 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 func-
tion reduces input current surge by regulating switch
current via the VC pin to maintain a constant voltage ramp
rate (dV/dt) at the output. A capacitor (C1 in Figure 2) from
the CSS pin to the output determines the maximum output
dV/dt. When the feedback voltage is below 0.4V, the VC pin
will rise, resulting in an increase in switch current and
output voltage. If the dV/dt of the output causes the current
through the CSS capacitor to exceed ICSS the VC voltage is
reduced resulting in a constant dV/dt at the output. As the
feedback voltage increases ICSS increases, resulting in an
increased dV/dt until the soft-start function is defeated
with 0.9V present at the FB pin. The soft-start function
does not affect operation during normal load conditions.
However, if a momentary short (brown out condition) is
present at the output which causes the FB voltage to drop
below 0.9V, the soft-start circuitry will become active.
Table 2
OUTPUT
R1
OUTPUT
VOLTAGE
R2
NEAREST (1%)
ERROR
(V)
(kโ„ฆ, 1%)
(kโ„ฆ)
(%)
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
โ€“ 0.63
10
100
698
โ€“ 0.25
12
100
866
0.63
SOFT-START
FOLDBACK
DETECT
500kHz
OSCILLATOR
โ€“
+
ERROR
AMP
1.25V
VC
11
FB
12
CSS
VOUT
9
SW
LT1977
C1
R1
R2
1977 F02
2
Figure 2. Feedback Network


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