Datasheet

FB
SW
L1
LM5008A
Cff
R
FB2
R3
C2
V
OUT
R
FB1
Cff =
3 x t
ON (max)
(R
FB1
//R
FB2
)
LM5008A
SNVS583F –MARCH 2009–REVISED MARCH 2013 www.ti.com
Figure 13. Efficiency vs. Load Current and V
IN
Figure 14. Efficiency vs. V
IN
LOW OUTPUT RIPPLE CONFIGURATIONS
For applications where low output ripple is required, the following options can be used to reduce or nearly
eliminate the ripple.
a) Reduced ripple configuration: In Figure 15, Cff is added across R
FB2
to AC-couple the ripple at V
OUT
directly
to the FB pin. This allows the ripple at V
OUT
to be reduced to a minimum of 25 mVp-p by reducing R3, since the
ripple at V
OUT
is not attenuated by the feedback resistors. The minimum value for Cff is determined from:
where
• t
ON(max)
is the maximum on-time, which occurs at V
IN(min)
. The next larger standard value capacitor should be
used for Cff. (9)
Figure 15. Reduced Ripple Configuration
b) Minimum ripple configuration: If the application requires a lower value of ripple (<10 mVp-p), the circuit of
Figure 16 can be used. R3 is removed, and the resulting output ripple voltage is determined by the inductor’s
ripple current and C2’s characteristics. RA and CA are chosen to generate a sawtooth waveform at their junction,
and that voltage is AC-coupled to the FB pin via CB. To determine the values for RA, CA and CB, use the
following procedure:
Calculate V
A
= V
OUT
- (V
SW
x (1 - (V
OUT
/V
IN(min)
)))
where
• V
SW
is the absolute value of the voltage at the SW pin during the off-time (typically 1V). VA is the DC voltage
at the RA/CA junction, and is used in Equation 11. (10)
Calculate RA x CA = (V
IN(min)
- V
A
) x t
ON
/ΔV
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