lm25085asdx National Semiconductor Corporation, lm25085asdx Datasheet - Page 19

• Manufacturer Part Number: lm25085asdx
• Description: 42v Constant On-time Pfet Buck Switching Controller With 0.9v Reference
• Manufacturer: National Semiconductor Corporation

Alternate Output Ripple
Configurations
The minimum ripple configuration, using C1, C2 and R3, used
in the example circuit Figure 4, results in a low ripple ampli-
tude at V
output capacitor and the ripple current in L1. This configura-
tion allows multiple ceramic capacitors to be used for V
the output voltage is provided to several places on the PC
board. However, if a slightly higher level of ripple at V
acceptable in the application, and distributed capacitance is
not used, the ripple required for the FB comparator pin can
be generated with fewer external components using the cir-
cuits shown below.
a) Reduced ripple configuration: In Figure 9, R3, C1 and
C2 are removed (compared to Figure 4). A low value resistor
(R4) is added in series with C
added across R
inductor’s ripple current flowing through R4, and that ripple
b) Lowest cost configuration: This configuration, shown in
Figure 10, is the same as Figure 9 except Cff is removed.
Since the ripple voltage at V
R
FB1
, the minimum ripple required at V
OUT
V
RIP(min)
determined mainly by the characteristics of the
FB2
= 25 mV x (R
. Ripple is generated at V
OUT
OUT
FB2
FIGURE 10. Lowest Cost Ripple Generating Configuration
is attenuated by R
, and a capacitor (Cff) is
+ R
OUT
FB1
FIGURE 9. Reduced Ripple Configuration
is equal to:
)/R
FB1
OUT
FB2
by the
OUT
OUT
and
is
if
19
voltage is passed to the FB pin via Cff. The ripple at V
be set as low as 25 mVp-p since it is not attenuated by R
and R
where I
minimum input voltage. The minimum value for Cff is deter-
mined from:
where t
imum VIN. The next larger standard value capacitor should
be used for Cff.
The minimum value for R4 is calculated from:
where I
minimum input voltage.
FB1
OR(min)
ON(max)
OR(min)
. The minimum value for R4 is calculated from:
is the minimum ripple current, which occurs at
is the minimum ripple current, which occurs at
is the maximum on-time, which occurs at min-
30085251
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OUT
can
FB2