Datasheet

SW
FB
LM34914
L1
R1
R2
C2
CB
CA
RA
V
OUT
FB
SW
L1
LM34914
Cff
R1 R3
R2 C2
V
OUT
Cff =
t
ON (max)
(R1//R2)
LM34914
SNVS453B –MAY 2006–REVISED MARCH 2013 www.ti.com
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 11, Cff is added across R1 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:
(16)
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. R1 and R2 should each be towards the upper end of the 1kΩ to 10kΩ range.
Figure 11. Reduced Ripple Configuration
b) Minimum ripple configuration: If the application requires a lower value of ripple (<10 mVp-p), the circuit of
Figure 12 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)
))) (17)
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 the next equation.
Calculate RA x CA = (V
IN(min)
- V
A
) x t
ON
/ΔV (18)
where t
ON
is the maximum on-time (at minimum input voltage), and ΔV is the desired ripple amplitude at the
RA/CA junction (typically 40-50 mV). RA and CA are then chosen from standard value components to satisfy the
above product. Typically CA is 1000 pF to 5000 pF, and RA is 100kΩ to 300 kΩ. CB is then chosen large
compared to CA, typically 0.1 µF. R1 and R2 should each be towards the upper end of the 1kΩ to 10kΩ range.
Figure 12. Minimum Output Ripple Using Ripple Injection
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