參數(shù)資料
型號(hào): SC415
廠商: Semtech Corporation
英文描述: Dual Synchronous Buck Controller
中文描述: 雙同步降壓控制器
文件頁(yè)數(shù): 19/30頁(yè)
文件大小: 942K
代理商: SC415
SC415
19
Applications Information (continued)
The result is a well-defined sawtooth waveform, as shown
in Figure 11.
R1
R2
To FB
COUT
VOUT
LOAD
LX
ESR
Ripple Current
VOUT ripple from ESR
FB ripple
750 mV
(to MOSFETs)
Figure 11
In real applications, the output capacitor also has some
series inductance (ESL), and this can have a large effect
on ripple. The ripple current creates voltage across the
ESL; this is a square wave similar to the LX waveform. The
result is shown below. Note the fast rising and falling
edges created by the ESL as show in Figure 12.
R1
R2
To FB
COUT
VOUT
LOAD
(to MOSFETs)
ESR
Ripple Current
VOUT ripple from ESR
FB ripple
750 mV
ESL
VOUT ripple from ESL
VOUT ripple from ESL
LX
Figure 12
In addition to the ESL, most applications also have a small
capacitor in parallel with COUT; this is typically a small
ceramic capacitor intended to absorb high frequency
noise not filtered by the output capacitor, as shown by CB
in Figure 13.
R1
R2
To FB
COUT
VOUT
LOAD
LX
ESR
Ripple Current
VOUT ripple from ESR
FB ripple
750 mV
ESL
VOUT ripple from ESL
VOUT ripple from ESL
CB
(to MOSFETs)
Figure 13
This capacitor CB can have a large effect on the ripple
waveform. The switch transitions are fast, typically
10-30nsec. At this high speed, the output capacitor
impedance is dominated by ESL. In parallel with this is
CBP. The effective circuit is a parallel L-C filter, with ESL
in parallel with CPB. Thus the choice of CB can have
significant effect on the ripple waveform. The parallel LC
circuit can ring at high levels, and this can cause the VOUT
and therefore FB ripple to go below the trip point too
early. Since the FB waveform goes below the threshold
soon after the DH pulse is finished, there is the potential
for double-pulsing.
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