SC4624EVB-MLPQ View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC4624EVB-MLPQ
Semtech Corporation
'SC4624EVB-MLPQ' PDF : 20 Pages View PDF
SC4624
POWER MANAGEMENT
OAppelriactaiotinon Information (Cont.)
SC4624 regulates the inductor current to a new value dur- network to meet the requirements for a specific applica-
ing a load transient, the output capacitor delivers all the tion.
additional current needed by the load.
The SC4624 has an internal error amplifier and requires
The ESR and ESL of the output capacitor, the loop para- the compensation network to connect among the COMP
sitic inductance between the output capacitor and the pin and FB pin, GND, and the output as shown in Figure
load combined with inductor ripple current are all major 17. The compensation network includes C1, C2, R1, R7,
contributors to the output voltage ripple.
R8 and C8. R9 is used to program the output voltage ac-
cording to:
Input Capacitor Selection
The input capacitor selection is based on its ripple cur-
rent level, required capacitance and voltage rating. This
92
u
5
5
5
4
3
2
1
capacitor must be able to provide the ripple current by the
switching actions. For the continuous conduction mode,
D
the RMS value of the input capacitor can be calculated
from:
C1
SC4624
D
R1
C2
COMP
FB
,&,1 506
, u 20$;
9287 u 9,1 9 287
9IN,1
Vout
C
C8
L1
PH
C
R7
R8
R
C4
This current gives the capacitor’s power loss as follows:
B
B
R9
3&,1
, u 5
&,1 506
&,1 (65
This capacitor’s RMS loss can be a significant part of the A
A
total loss in the converter and reduces the overall convert-
er efficiency. The input ripple voltage mainly depends on
5
Figure 417. Compens3ation Networ2k
1
the input capacitor’s ESR and its capacitance for a given
Provides 3 Poles and 2 Zeros
load, input voltage and output voltage. Assuming that the
input current of the converter is constant, the required For voltage mode step down applications as shown in Fig-
input capacitance for a given voltage ripple can be calcu- ure 17, the power stage transfer function is:
lated by:
&,1
,20$; u I6 u
' u '
'9 , u 5 ∆ ,
20$;
&,1 (65
where
D = VO/VI , duty ratio.
DVI = the given input voltage ripple.
where
R = load resistance
RC = C4’s ESR.
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
The compensation network will have these characteris-
tics:
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
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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