BD9540EFV Datasheet, PDF (13/18 Pages) Rohm – 2ch Switching Regulators for Desktop PC

BD9540EFV
Technical Note
7. Relationship between output voltage and Ton duration
Both 1ch and 2ch of BD9540EFV are synchronous rectification type of switching controllers operated at fixed-frequency.
The Ton duration for each channel depends on the output voltage settings, as described by the following formulas.
Ton1 =
VOUT1
VIN
×2μ+130n [ns]・・・(14)
(1ch)
Ton2 =
VOUT2
VIN
×0.9μ+130n [ns]・・・(15)
Thus from the above Ton duration, the frequency of the applied condition is
(2ch)
Frequency =
VOUT
VIN ^×
1
Ton
[kHz]・・・(16)
However with actual applications, there exists a rising and falling time of the SW due to the gate capacitance of the
external MOSFET and the switching speed, which may vary the above parameters. Thus please also verify those
parameters experimentally.
8. Relationship between output current and frequency
BD9540EFV is a fixed-Ton type of switching controller. When the output current increases, the switching loss of the coil
and MOSFET also increases and hence the switching frequency speeds up.
The loss of the coil and MOSFET is determined as
① Loss of coil = IOUT² × DCR
② Loss of high-side MOSFET = IOUT² × Ronh ×
VOUT
VIN
③ Loss of low-side MOSFET =
IOUT² × Ronn × (1-
VOUT ₎
VIN
(Ronh : on-resistance of high-side MOSFET, Ronn : on resistance of low-side MOSFET)
Taking the above losses into the frequency equation, then T (=1/Freq) becomes
T (=1/Freq) =
VIN × IOUT × Ton
VOUT × IOUT + ① + ② + ③
・・・(17)
However since the parasitic resistance of the layout pattern exists in actual applications and affects the parameter, please
also verify experimentally.
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13/17
2009.04 - Rev.B