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MB39A105
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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
DS(ON) between the drain and source. For high output voltage
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
On-cycle switching loss : PS (ON)
Off-cycle switching loss : PS (OFF)
Total loss : PT
PT = PC + PS (ON) + PS (OFF)
Example: Using the SANYO MCH3405
Input voltage V
IN (Max) = 2.4 V, output voltage VO = 9 V, drain current ID = 0.94 A, Oscillation frequency
f
OSC = 500 kHz, L = 6.8 µH, drain-source on resistance RDS (ON) := 160 mΩ, tr = 18 ns, tf = 8 ns.
Drain current (Max) : I
D (Max)
Drain current (Min) : ID (Min)
PC = ID
2
× RDS(ON) × Duty
P
S (ON) =
VD (Max) × ID × tr × fOSC
6
P
S (OFF) =
VD (Max) × ID (Max) × tf × fOSC
6
I
D (Max) =
VO × IO
+
VIN(Min)
ton ton =
VO − VIN(Min)
t
VIN(Min) 2L VO
=
9 × 0.25
+
2.4× (9−2.4)
×
1
2.4 2 × 6.8 × 10
−6
× 9500 × 10
3
:= 1.20 (A)
ID (Min) =
VO × IO
−
VIN(Min)
ton
VIN(Min) 2L
=
9 × 0.25
−
2.4× (9−2.4)
×
1
2.4 2 × 6.8 × 10
−6
× 9 500 × 10
3
:= 0.68 (A)