LM3478MM/NOPB National Semiconductor, LM3478MM/NOPB Datasheet - Page 19

IC CTRLR SW REG N-CH 8MSOP

LM3478MM/NOPB

Manufacturer Part Number
LM3478MM/NOPB
Description
IC CTRLR SW REG N-CH 8MSOP
Manufacturer
National Semiconductor
Type
Step-Up (Boost), Flyback, Sepicr
Datasheet

Specifications of LM3478MM/NOPB

Internal Switch(s)
No
Synchronous Rectifier
No
Number Of Outputs
1
Voltage - Output
Adjustable
Current - Output
1A
Frequency - Switching
100kHz ~ 1MHz
Voltage - Input
2.97 ~ 40 V
Operating Temperature
-40°C ~ 125°C
Mounting Type
Surface Mount
Package / Case
8-MSOP, Micro8™, 8-uMAX, 8-uSOP,
For Use With
LM3478EVAL - BOARD EVALUATION LM3478
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
Power - Output
-
Other names
LM3478MM
LM3478MMTR

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POWER MOSFET SELECTION
As in a boost converter, 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 inductors L1 and L2 to maintain continu-
ous current conduction mode requires calculations of the
following parameters.
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 13. Typical SEPIC Converter
IN
+ V
g
, the
OUT
TH
.
19
In the above equation, V
FET, Q, and V
Peak to peak ripple current, to calculate core loss if neces-
sary:
Maintaining the condition I
rent conduction yields:
Peak current in the inductor, to ensure the inductor does not
saturate:
DIODE
is the forward voltage drop of the diode.
Q
I
L2AVE
is the on-state voltage of the MOS-
L
> Δi
= I
L
/2 to ensure continuous cur-
OUT
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