參數(shù)資料
型號(hào)： SC475A
廠商： Semtech Corporation
英文描述： Synchronous Buck Controller with Dual-Level VOUT Transition Support
中文描述： 同步降壓控制器與雙級(jí)VOUT電源支持過渡
文件頁(yè)數(shù)： 13/27頁(yè)
文件大?。?/td> 603K
代理商： SC475A
13
2006 Semtech Corp.
SC475A
www.semtech.com
POWER MANAGEMENT
Applications Information
(continued)
V
OUT
FB
R1
R2
D0
D0
RTN
750mV
(FB threshold)
FB
DH
DL
Final
VOUT
Initial
VOUT
Voltage overshoot
VOUT
R3
Figure 8
The overshoot can be approximated by the following
equation, where ICL is the current limit, V
is the desired
setpoint for the
fi
nal voltage, L is in
μ
H and COUT is in
μ
F.
VMAX =
√
( ICL
2
* L/COUT + V
FINAL
This overshoot can be eliminated by using a small RC
circuit to smooth the voltage seen at FB, see Figure 9.
The presence of Rs/Cs will prevent the rapid changes at
D0 from moving FB too quickly. The result is a gradual
change from VOUT
to VOUT
, to prevent the build-
up of high inductor current and reducing overshoot. Note
that Cs can be connected to either VOUT or GND. VOUT is
preferred because this results in higher ripple seen at the
FB pin, which improves stability.
2
)
RTN
750mV
(FB threshold)
FB
DH
DL
Final
VOUT
Initial
VOUT
VOUT
V
OUT
FB
R1
R2
RS
CS
VS
R3
D0
CS
D0
Figure 9
Note that Rs/Cs are part of the FB resistor divider and
therefore affect the output voltage sensing. To minimize
the effect of this, select Rs and Cs according to the
following guidelines:
The total of Rs + R3 should be chosen to give the correct
total resistance needed to adjust VOUT. Set Rs and R3 to
the same value.
Cs should be chosen to give a time constant equal to
approximately 18
μ
sec for 325 kHz operation. Note that
Cs is charging through a resistive network composed
of Rs/R1/R2/R3. The effective resistance seen by Cs
is roughly equal to the parallel combination of Rs and
R3. For example, if the values for R1/R2/R3/Rs are
10k/50k/15k/15k, then the effective resistance would
be (10k + 15k) paralleled with 15k, which is 9.4k. To set
a time constant of 18
μ
sec, CS should be approximately
18
μ
sec/9.4k or 2000pF.
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