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SC4530 数据表(PDF) 14 Page - Semtech Corporation

部件名 SC4530
功能描述  30V, 300mA Output Micropower Step-Down Switching Regulator
PDF  23 Pages
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制造商  SEMTECH [Semtech Corporation]
网页  http://www.semtech.com
标志 SEMTECH - Semtech Corporation

SC4530 数据表(HTML) 14 Page - Semtech Corporation

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4
SC4530
Applications Information (Continued)
Feed-Forward Compensation
A feed-forward capacitor C
4 (connected across the upper
feedback resistor R
) is needed for stability. An initial
estimate of C
4 can be found using Equation (0) below:
1
6
4
R
10
8
.
6
C
(0)
The value of C
4 can be optimized empirically by observing
the inductor current and the output voltage during load
transient. Starting with the initial estimate, C
4 is tuned until
there is no excessive ringing or overshoot in the inductor
current or the output voltage during load transient.
Mode Transition and the FB Pin
If the upper feedback resistor R
 (Figure 3, page 9) is large
and is about the same magnitude as R
2, then fast switching
transients may couple into the FB pin, disturbing or
delaying the transition from light-load operating mode
to continuous-conduction mode (CCM). As described
previously, the output ripple voltage is very low in
continuous-conduction mode. Delayed CCM transition
extends the load range in which the converter produces
larger output voltage ripples. This disturbance becomes
more pronounced when V
IN is increased above 2V and
SW_OUT_IL_28V to 5V@174mA_C4=10pF_onset CCM=176mA
V
OUT
50mV/div
AC Coupled
I
L1
200mA/div
V
SW
20V/div
4ms/div
C
4 = 10pF, C5 not Placed
V
OUT = 5V, IOUT = 174mA
CCM Onset I
OUT = 176mA
(a)
SW_OUT_IL_28V to 5V@110mA_C4=10pF_C5=33pF_onset CCM=126mA
V
OUT
50mV/div
AC Coupled
I
L1
200mA/div
V
SW
20V/div
10ms/div
C
4 = 10pF, C5 = 33pF
V
OUT = 5V, IOUT = 110mA
CCM Onset I
OUT = 126mA
(b)
Figure 7. Switching Waveforms of a 28V to 5V
Converter Just Before It Enters Continuous-
Conduction Mode
Figure 8. Switching Waveforms of a 28V to 3.3V
Converter Just Before It Enters Continuous-
Conduction Mode
SW_OUT_IL_28V to 3.3V@145mA_C4=22pF_onset CCM=150mA
V
OUT
50mV/div
AC Coupled
I
L1
200mA/div
V
SW
20V/div
4ms/div
C
4 = 22pF, C5 not Placed
V
OUT = 3.3V, IOUT = 145mA
CCM Onset I
OUT = 150mA
(a)
SW_OUT_IL_28V to 3.3V@118mA_C4=22pF_C5=47pF_onset CCM=121mA
10ms/div
V
OUT
50mV/div
AC Coupled
I
L1
200mA/div
V
SW
20V/div
C
4 = 22pF, C5 = 47pF
V
OUT = 3.3V, IOUT = 118mA
CCM Onset I
OUT = 121mA
(b)



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