Datasheet

R5 x C10 =
(6V ± 2.84V) x 424 ns
0.05V
= 26.8 x 10
-6
R5 x C10 =
(V
IN
± V
A
) x t
ON
'V
Cff t
t
ON (max)
x 3
(R2//R3)
FB
SW
BST
VCC
ISEN
SGND
VIN
R1
28k
SS
RTN
Gnd
D1
A1
A2
On
Timer
Minimum
Off
Timer
Logic
Regulation
Comparator
B3
A3
Current Limit
Detect
D3
C3
0.2:
C1
IN
Gnd
C2
1PF
6V to 24 V
RON/SD
C6
0.022
PF
0.1
PF
C3
1PF
0.
1
PF
C4
B1
V
IN
V
OUT
C5
D1
D2
C1
2.49k
R3
787:
R2
2200 pF
PF10
PF10
3.3V
LM34919B
L1 8.2 PH
R
6 0:
C8C7
2.5V
0.022
PF
Cff
R
4
Output Ripple Control www.ti.com
5.2 Option B) Intermediate Ripple Configuration
This configuration generates less ripple at V
OUT
than Section 5.1 by the addition of one capacitor (Cff)
across R2, as shown in Figure 3.
Figure 3. Intermediate Ripple Configuration
Since the output ripple is passed by Cff to the FB pin with little or no attenuation, R4 can be reduced so
the minimum ripple at V
OUT
is ≊25 mVp-p. The minimum value for Cff is calculated from:
(2)
where t
ON(max)
is the maximum on-time (at minimum V
IN
), and R2//R3 is the parallel equivalent of the
feedback resistors, see Figure 8.
5.3 Option C) Minimum Ripple Configuration
To obtain minimum ripple at V
OUT
, R4 is set to 0Ω, and R5, C9, and C10 are added to generate the
required ripple for the FB pin. In this configuration, the output ripple is determined primarily by the ESR of
the output capacitance and the inductor’s ripple current.
The ripple voltage required by the FB pin is generated by R5, C10, and C9 since the SW pin switches
from -1V to V
IN
, and the right end of C10 is a virtual ground. The values for R5 and C10 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 C9. The following procedure is used to calculate values for R5, C10 and C9.
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 1V), and V
IN
is
the minimum input voltage. For this circuit, V
A
calculates to 2.84V. This is the approximate DC voltage at
the R5/C10 junction, and is used in Equation 4.
2) Calculate the R5 x C10 product:
(4)
where, t
ON
is the maximum on-time (≊424 ns), V
IN
is the minimum input voltage, and ΔV is the desired
ripple amplitude at the R5/C10 junction, 50 mVp-p for this example.
(5)
4
AN-2016 LM34919B Evaluation Board SNVA418A–May 2010–Revised April 2013
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