Datasheet

R1
280k
0.1 PF
C5
LM5008A
C4
D1
L1
220 PH
V
OUT
GND
VIN
RT/SD
RCL
VCC
BST
SW
FB
RTN
V
IN
GND
SHUTDOWN
(TP1SD)
R6
8V to 75V
Input
0:
1 PF
C1
R2
715k
0.1 PF
C3
0.47 PF
R3
3.01k
RA
64.9k
R5
0:
C2
22 PF
5V
R4
3.01k
CA 4700 pF
CB
0.1 PF
RA x CA =
(8V - 4.78V) x 4.85 Ps
0.05V
= 3.12 x 10
-4
RA x CA =
(V
IN
± V
A
) x t
ON
'V
Output Ripple Control www.ti.com
The ripple voltage required by the FB pin is generated by RA, and CA since the SW pin switches from -1V
to V
IN
, and the right end of CA is a virtual ground. The values for RA and CA are chosen to generate a 50-
100 mVp-p triangle waveform at their junction. That triangle wave is then coupled to the FB pin through
CB. The following procedure is used to calculate values for RA, CA and CB:
1) Calculate the voltage V
A
:
V
A
= V
OUT
– (V
SW
x (1 – (V
OUT
/V
IN
))) (3)
where V
SW
is the absolute value of the voltage at the SW pin during the off-time (typically 0.6V), and V
IN
is
the minimum input voltage. For this circuit, V
A
calculates to 4.78V. This is the approximate DC voltage at
the RA/CA junction, and is used in the next equation.
2) Calculate the RA x CA product:
(4)
where t
ON
is the maximum on-time (≊4.85 µs), V
IN
is the minimum input voltage, and ΔV is the desired
ripple amplitude at the RA/CA junction, 50 mVp-p for this example.
(5)
RA and CA are then chosen from standard value components to satisfy the above product. Typically CA is
3000 to 10000 pF, and RA is 10 kΩ to 300 kΩ. CB is chosen large compared to CA, typically 0.1 µF. The
ripple at V
OUT
is typically less than 10 mVp-p. See Figure 4 and Figure 8.
Figure 4. Minimum Output Ripple Configuration
4
AN-1925 LM5008A Evaluation Board SNVA380C–March 2009–Revised April 2013
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