Datasheet

SW
FB
LM34919C
L1
R1
R2
C2
CB
CA
RA
V
OUT
RA x CA =
(V
IN(min)
- V
A
) x t
ON
'V
FB
SW
L1
LM34919C
Cff
R1 R3
R2 C2
V
OUT
LM34919B
LM34919B-Q1
www.ti.com SNVS623A –MAY 2010–REVISED FEBRUARY 2013
Figure 32. Reduced Ripple Configuration
b) Minimum ripple configuration: The circuit of Figure 33 provides minimum ripple at V
OUT
, determined
primarily by C2’s characteristics and the inductor’s ripple current since R3 is removed. 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)
))) (18)
where V
SW
is the absolute value of the voltage at the SW pin during the off-time (typically 1V). V
A
is the DC
voltage at the RA/CA junction, and is used in the next equation.
(19)
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 50 mV. RA and CA are then chosen from standard value components to satisfy the
above product. Typically CA is 3000 pF to 5000 pF, and RA is 10 kΩ 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 2 kΩ to 10 kΩ range.
Figure 33. Minimum Output Ripple Using Ripple Injection
c) Alternate minimum ripple configuration: The circuit in Figure 34 is the same as that in Figure 29, except
the output voltage is taken from the junction of R3 and C2. The ripple at V
OUT
is determined by the inductor’s
ripple current and C2’s characteristics. However, R3 slightly degrades the load regulation. This circuit may be
suitable if the load current is fairly constant.
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