Datasheet

FB
SW
L1
SM72485
R3
C2
V
OUT
R
FB2
R
FB1
SW
FB
SM72485
L1
C2
CB
CA
RA
V
OUT
R
FB2
R
FB1
SM72485
SNVS697D –JANUARY 2011–REVISED APRIL 2013 www.ti.com
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) (10)
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
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) (11)
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 16. Minimum Output Ripple Using Ripple Injection
c) Alternate minimum ripple configuration: The circuit in Figure 17 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.
Figure 17. Alternate Minimum Output Ripple
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