Datasheet

IN OUT
1
D =
1 V / V+
V
OUT
V
IN
=
D'
D
Battery or Power
Source
L1
10PH
D0-1608
D
(Schottky)
C
IN
22PF
Ceramic
C
OUT
10PF
Ceramic
R
FB2
12.1k
C
C
680pF
R
C
10k
1 4
3
5
6
7
2
FSLCT
FB
GND
SHDN
V
IN
SW
2.2V - 12V
R
FB1
20k
3.3V
L2
10PH
D0-1608
C
SEPIC
1PF
V
C
LM2698
LM2698
www.ti.com
SNVS153E MAY 2001REVISED APRIL 2013
Figure 21. 3.3V SEPIC Converter
3.3V SEPIC
The LM2698 can be used to implement a SEPIC technology. The advantages of the SEPIC topology are that it
can step up or step down an input voltage, and it has low input current ripple.
The conversion ratio for the SEPIC is :
(22)
where
D' = 1D (23)
Solving for D yields:
(24)
To avoid subharmonic oscillations, it is recommended that inductors L1 and L2 be the same inductance. Currents
conducted by the inductors are:
I
1
= I
OUT
(V
OUT
/V
IN
)
Δi
1
= V
IN
D/(2*L1*fs)
I
2
= I
OUT
Δi
1
= V
IN
D/(2*L2*fs)
The switch sees a maximum current of I
1
+ I
2
+ Δi
1
+ Δi
2
. If L1 = L2 = L, the maximum switch current is given by:
I
OUT
(1 + V
OUT
/V
IN
) + V
IN
D/(L*fs) (25)
The maximum load current is limited by this relationship to the switch current.
The polarity of C
SEPIC
will change between each cycle, so a ceramic capacitor should be used here. A high
quality, low ESR capacitor will directly improve efficiency because all the load current passes through C
SEPIC
.
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