Datasheet

LM3481
V
IN
I
SEN
COMP
FB
AGND
UVLO
V
CC
DR
PGND
FA/SYNC/SD
+
+
V
IN
= 3.0V to 24V
V
OUT
= 5V, 1A
C
IN
15 PF, 35V x2
C
OUT
100 PF, 10V
1 PF, ceramic
R
SEN
0.05:
L1
10 PH
L2
10 PH
0.47 µF
MBRS130LT3
R
FA
40 k:
C
SEN
1 nF
Q1
IRF7807
C
S
R
C
4.7 k:
C
C
0.1 PF
R
F1
60 k:
R
F2
20 k:
R7
10 k:
D2
5.1V
D1
I
SWPEAK
= I
L1(AVG)
+ I
OUT
+
'I
L1
+ 'I
L2
2
LM3481
www.ti.com SNVS346E –NOVEMBER 2007–REVISED APRIL 2012
Power MOSFET Selection
As in a boost converter, the parameters governing the selection of the MOSFET are the minimum threshold
voltage, V
TH(MIN)
, the on-resistance, R
DS(ON)
, the total gate charge, Q
g
, the reverse transfer capacitance, C
RSS
,
and the maximum drain to source voltage, V
DS(MAX)
. The peak switch voltage in a SEPIC is given by:
V
SW(PEAK)
= V
IN
+ V
OUT
+ V
DIODE
The selected MOSFET should satisfy the condition:
V
DS(MAX)
> V
SW(PEAK)
The peak switch current is given by:
Where ΔI
L1
and ΔI
L2
are the peak-to-peak inductor ripple currents of inductors L1 and L2 respectively.
The rms current through the switch is given by:
Power Diode Selection
The Power diode must be selected to handle the peak current and the peak reverse voltage. In a SEPIC, the
diode peak current is the same as the switch peak current. The off-state voltage or peak reverse voltage of the
diode is V
IN
+ V
OUT
. Similar to the boost converter, the average diode current is equal to the output current.
Schottky diodes are recommended.
Figure 34. Typical SEPIC Converter
Selection of Inductors L1 and L2
Proper selection of the inductors L1 and L2 to maintain constant current mode requires calculations of the
following parameters.
Average current in the inductors:
I
L2AVE
= I
OUT
Copyright © 2007–2012, Texas Instruments Incorporated Submit Documentation Feedback 21
Product Folder Links: LM3481