Datasheet

FB
SW
L1
LM34923
Cff
R
FB2
R3
C2
V
OUT
R
FB1
Cff =
3 x t
ON (max)
(R
FB1
//R
FB2
)
LM34923
SNVS695A –MARCH 2011–REVISED FEBRUARY 2013 www.ti.com
Figure 34. Efficiency vs. Load Current and V
IN
Figure 35. 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 36, 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:
(19)
where t
ON(max)
is the maximum on-time, which occurs at the minimum input voltage. The next larger standard
value capacitor should be used for Cff.
Figure 36. Reduced Ripple Configuration
b) Minimum ripple configuration: If the application requires a lower value of ripple (<10 mVp-p), the circuit of
Figure 37 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)
))) (20)
where V
SW
is the absolute value of the voltage at the SW pin during the off-time. If a Schottky diode is used for
the flyback function, the off-time voltage is in the range of 0.5V to 1V, depending on the specific diode used, and
the maximum load current. V
A
is the DC voltage at the RA/CA junction, and is used in the next equation.
- Calculate RA x CA = (V
IN(min)
- V
A
) x t
ON
/ΔV (21)
18 Submit Documentation Feedback Copyright © 2011–2013, Texas Instruments Incorporated
Product Folder Links: LM34923