Fujitsu MB39A105 manual Selection of Components

PS (ON)

MB39A105

■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 RDS(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 : PC

PC = ID 2 ⋅ RDS(ON) ⋅ Duty

On-cycle switching loss : PS (ON)

= VD (Max) ⋅ ID ⋅ tr ⋅ fOSC

6

Off-cycle switching loss : PS (OFF)

PS (OFF) =	VD (Max) ⋅ ID (Max) ⋅ tf ⋅ fOSC
	6

Total loss : PT

PT = PC + PS (ON) + PS (OFF)

Example: Using the SANYO MCH3405

Input voltage VIN (Max) = 2.4 V, output voltage VO = 9 V, drain current ID = 0.94 A, Oscillation frequency fOSC = 500 kHz, L = 6.8 ∝H, drain-source on resistance RDS (ON) =: 160 mΩ, tr = 18 ns, tf = 8 ns.

Drain current (Max)

: ID (Max)

ID (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 ⋅ 9

500

⋅ 103

=:

1.20 (A)

Drain current (Min)

: ID (Min)

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 ⋅ 103

=:

0.68 (A)

19