LM3488QMM National Semiconductor, LM3488QMM Datasheet - Page 19

• Manufacturer Part Number: LM3488QMM
• Description: LM3488/LM3488Q High Efficiency Low-Side N-Channel Controller for Switching Regulators; Package: MINI SOIC; No of Pins: 8; Container: Reel
• Manufacturer: National Semiconductor

POWER MOSFET SELECTION
As in boost converter, the parameters governing the selection
of the MOSFET are the minimum threshold voltage, V
(MIN)
reverse transfer capacitance, C
to source voltage, V
SEPIC is given by:
The selected MOSFET should satisfy the condition:
The peak switch current is given by:
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
Similar to the boost converter, the average diode current is
equal to the output current. Schottky diodes are recommend-
ed.
SELECTION OF INDUCTORS L1 AND L2
Proper selection of the inductors L1 and L2 to maintain con-
stant current mode requires calculations of the following pa-
rameters.
Average current in the inductors:
, the on-resistance, R
V
SW(PEAK)
V
DS(MAX)
DS(MAX)
= V
DS(ON)
IN
. The peak switch voltage in a
> V
+ V
, the total gate charge, Q
RSS
SW(PEAK)
OUT
, and the maximum drain
+ V
DIODE
FIGURE 14. Typical SEPIC Converter
IN
+ V
g
, the
OUT
TH
.
19
Peak to peak ripple current, to calculate core loss if neces-
sary:
maintains the condition I
mode.
Peak current in the inductor, to ensure the inductor does not
saturate:
I
the current sense resistor.
The value of L1 can be increased above the minimum rec-
ommended to reduce input ripple and output ripple. However,
L1PK
must be lower than the maximum current rating set by
I
L2AVE
L
> Δi
= I
L
/2 to ensure constant current
OUT
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