Datasheet

BD9540EFV
Technical Note
13/17
www.rohm.com
2009.04 - Rev.B
© 2009 ROHM Co., Ltd. All rights reserved.
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 =
Ton2 =
Thus from the above Ton duration, the frequency of the applied condition is
Frequency =
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
Taking the above losses into the frequency equation, then T (=1/Freq) becomes
However since the parasitic resistance of the layout pattern exists in actual applications and affects the parameter, please
also verify experimentally.
VOUT1
VIN
×2+130n [ns]・・・(14) (1ch)
VOUT2
VIN
×0.9+130n [ns]・・・(15) (2ch)
VOUT
VIN
① Loss of coil = IOUT
2
× DCR
② Loss of high-side MOSFET = IOUT
2
× Ronh ×
VIN
③ Loss of low-side MOSFET = IOUT
2
× Ronn × (1-
VOUT
)
VIN × IOUT × Ton
VOUT × IOUT + ① + ② + ③
T (=1/Freq) =
・・・(17)
(Ronh : on-resistance of high-side MOSFET, Ronn : on resistance of low-side MOSFET)
VOUT
VIN
×
1
Ton
[kHz]・・・(16)