SC4624A View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC4624A
Semtech Corporation
'SC4624A' PDF : 19 Pages View PDF
SC4624A
POWER MANAGEMENT
OAppelriactaiotinon Information (Cont.)
switching actions. For the continuous conduction mode,
the RMS value of the input capacitor can be calculated
from:
,&,1 506
, u 20$;
9287 u 9,1 9 287
9 IN,1
5
D
92
u
5
5
4
3
C1
R1 C2
2
1
SC4624A
COMP
This current gives the capacitor’s power loss as follows:
Vout
3&,1
, u 5
&,1 506
&,1 (65
C
C8
FB
L1
PH
This capacitor’s RMS loss can be a significant part of the
R7
R8
R
C4
total loss in the converter and reduces the overall convert- B
er efficiency. The input ripple voltage mainly depends on
R9
the input capacitor’s ESR and its capacitance for a given
load, input voltage and output voltage. Assuming that the
input current of the converter is constant, the required
input capacitance for a given voltage ripple can be calcu- A
Figure 17. Compensation Network
lated by:
Provides 3 Poles and 2 Zeros
5
4
3
2
1
&,1
,20$; u I6 u
' u '
'9 , u 5 ∆ ,
20$;
&,1 (65
For voltage mode step down applications as shown in Fig-
ure 17, the power stage transfer function is:
where
D = VO/VI , duty ratio.
DVI = the given input voltage ripple.
Loop Compensation Design
For a DC/DC converter, it is usually required that the con-
verter has a loop gain of a high cross-over frequency for
fast load response, high DC and low frequency gain for low
steady state error, and enough phase margin for its oper-
ating stability. Often one can not have all these properties
at the same time. The purpose of the loop compensation
is to arrange the poles and zeros of the compensation
network to meet the requirements for a specific applica-
tion.
where
R = load resistance
RC = C4’s ESR.
The compensation network will have these characteris-
tics:
The SC4624A has an internal error amplifier and requires
the compensation network to connect among the COMP
pin and FB pin, GND, and the output as shown in Figure
17. The compensation network includes C1, C2, R1, R7,
R8 and C8. R9 is used to program the output voltage ac-
cording to:
1+ s 1+ s
GCOMP (s)
=
wI
s
⋅
1+
wZ1
s
⋅
wZ2
⋅1+ s
wP1
wP2
2008 Semtech Corp.
14
www.semtech.com
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