lm25085sdx National Semiconductor Corporation, lm25085sdx Datasheet - Page 19

• Manufacturer Part Number: lm25085sdx
• Description: 42v Constant On-time Pfet Buck Switching Controller
• 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 amplitude
at V
capacitor and the ripple current in L1. This configuration al-
lows multiple ceramic capacitors to be used for V
output voltage is provided to several places on the PC board.
However, if a slightly higher level of ripple at V
able 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 circuits
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
OUT
, the minimum ripple required at V
determined mainly by the characteristics of the output
V
RIP(min)
FB2
= 25 mV x (R
. Ripple is generated at V
OUT
OUT
FB2
is attenuated by R
, and a capacitor (Cff) is
+ R
OUT
FB1
FIGURE 9. Reduced Ripple Configuration
is equal to:
)/R
FB1
OUT
OUT
is accept-
OUT
FB2
by the
if the
and
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-
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OUT
can
FB2