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SELECTION OF COMPONENTS
• Nch MOS FET
The N-ch MOSFET for switching use should be rated for at least 20% more than the maximum output voltage.
To minimize continuity loss, use a FET with low R
and high frequency operation, on/off-cycle switching loss will be higher so that power dissipation must be
considered. In this application, the SANYO MCH3405 is used. Continuity loss, on/off switching loss, and total
loss are determined by the following formulas. The selection must ensure that peak drain current does not exceed
rated values.
Continuity loss : P
C
P
I
D 2
R
Duty
C
DS(ON)
On-cycle switching loss : P
V
I
D (Max)
D
P
S (ON)
6
Off-cycle switching loss : P
V
I
D (Max)
D (Max)
P
S (OFF)
6
Total loss : P
T
P
P
P
P
T
C
S (ON)
S (OFF)
Example: Using the SANYO MCH3405
Input voltage V
IN (Max)
f
500 kHz, L
6.8 H, drain-source on resistance R
OSC
Drain current (Max) : I
D (Max)
V
I
O
O
I
D (Max)
V
IN(Min)
9
0.25
2.4
: =
1.20 (A)
Drain current (Min) : I
D (Min)
V
I
O
O
I
D (Min)
V
IN(Min)
9
0.25
2.4
: =
0.68 (A)
S (ON)
tr
f
OSC
S (OFF)
tf
f
OSC
2.4 V, output voltage V
O
V
IN(Min)
ton
2L
2.4 (9 2.4)
2
6.8
10
6
9
V
IN(Min)
ton
2L
2.4 (9 2.4)
6
2
6.8
10
9
500
between the drain and source. For high output voltage
DS(ON)
9 V, drain current I
0.94 A, Oscillation frequency
D
: = 160 m , tr
DS (ON)
V
V
O
IN(Min)
ton
t
V
O
1
500
10
3
1
3
10
MB39A105
18 ns, tf
8 ns.
19
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