C8 =
(R2//R3)
t
ON(max)
FB
SW
LM2695
BST
VCC
C6
D1
ISEN
SGND
VIN
SS
RTN
10V
GND
C3
13
11
10
9
6
5
4
2
3
12
GND
C1
C2
2.2
P
F
2.2
P
F
C5
0.022
P
F
0.1
P
F
0.022
P
F
C4
0.1
P
F
L1 100
P
H
RON/SD
R1
200k
R2
7.5k
R3
2.5k
R4
0.55
:
C7
22
P
F
C8
1200 pF
12V - 30V
Input
V
OUT2
V
OUT1
Output Ripple Control
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5.2
Intermediate Ripple Level Configuration
This configuration generates more ripple at V
OUT1
than the above configuration, but uses one less
capacitor. If some ripple can be tolerated in the application, this configuration is slightly more economical,
and simpler. R4 and C8 are used instead of R6, C9, and C10, as shown in
Figure 3
.
Figure 3. Intermediate Ripple Level Configuration Using C8 and R4
R4 is chosen to generate
≥
25 mV - 30 mVp-p at V
OUT1
, knowing that the minimum ripple current in this
circuit is 50 mAp-p at minimum V
IN
. C8 couples that ripple to the FB pin without the attenuation of the
feedback resistors. C8's minimum value is calculated from
Equation 5
:
(5)
where t
ON(max)
is the maximum on-time (at minimum V
IN
), and R2//R3 is the equivalent parallel value of the
feedback resistors. For this evaluation board t
ON(max)
is approximately 2300 ns, and R2//R3 = 1.875 k
Ω
, and
C8 calculates to a minimum of 1200 pF. The resulting ripple at V
OUT1
ranges from 27 mVp-p to 105 mVp-p
over the input voltage range.
4
AN-1444 LM2695 Evaluation Board
SNVA147A – February 2006 – Revised April 2013
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