Datasheet

SW
FB
LM5006
L1
C2
CB
CA
RA
V
OUT
R
FB2
R
FB1
FB
SW
L1
LM5006
Cff
R
FB2
R3
C2
V
OUT
R
FB1
LM5006
www.ti.com SNVS646B –FEBRUARY 2011–REVISED MARCH 2013
Figure 38. Reduced Ripple Configuration
b) Minimum ripple configuration: If the application requires a lower value of ripple (<10 mVp-p), the circuit of
Figure 39 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 (21)
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. If a MOSFET is used for synchronous rectification,
the off-time voltage is in the range of 50 mV to 200 mV, depending on the R
DS(on)
of the selected device. 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
where
• t
ON
is the maximum on-time (at minimum input voltage)
• ΔV is the desired ripple amplitude at the RA/CA junction (typically 40-50 mV) (22)
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 10 kΩ to 300 kΩ. CB is then chosen large compared to CA, typically 0.1 µF.
Figure 39. Minimum Output Ripple Using Ripple Injection
c) Alternate minimum ripple configuration: The circuit in Figure 40 is the same as that in the Block Diagram,
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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