MB39A106 Datasheet, PDF (29/46 Pages) Fujitsu Component Limited. – 2-Channel DC/DC Converter IC with Overcurrent Protection Symmetrical-Phase Type

MB39A106
■ SELECTION OF COMPONENTS
• N-ch MOS FET
The N-ch MOS FET for switching use should be rated for at least 20% more than the maximum input voltage.
To minimize continuity loss, use a FET with low RDS(ON) between the drain and source. For high input voltage and
high frequency operation, on/off-cycle switching loss will be higher so that power dissipation must be considered.
In this application, the IR IRF7901D1 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, and
also must be in accordance with overcurrent detection levels.
Continuity loss : PC
PC = ID2 × RDS (ON) × Duty
On-cycle switching loss : PS (ON)
PS (ON) =
VD (Max)
× ID × tr × fOSC
6
Off-cycle switching loss : PS (OFF)
PS (OFF) =
VD (Max)
× ID (Max)
6
× tf × fOSC
Total loss : PT
PT = PC + PS (ON) + PS (OFF)
Example: Using the IR IRF7901D1
CH1 Main side
Input voltage VIN (Max) = 15 V, output voltage VO = 3.3 V, drain current ID = 3 A, Oscillation frequency
fOSC = 300 kHz, L = 22 µH, drain-source on resistance RDS (ON) =: 33 mΩ, tr = 13.8 ns, tf = 8 ns.
Drain current (Max) : ID (Max)
ID (Max) = IO
+
VIN (Max) −VO
2L
ton
=3 +
15−3.3
2 × 22 × 10^−6
×
1
300 × 10³
× 0.22
=: 3.20 (A)
Drain current (Min) : ID (Min)
ID (Min) = IO
−
VIN (Max) −VO
2L
ton
=3 −
15−3.3
2 × 22 × 10^−6
×
1
300 × 10³
× 0.22
=: 2.80 (A)
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