BD9305AFVM Datasheet, PDF (9/15 Pages) Rohm – Single-output Step-up,High-efficiency Switching Regulator(Controller Type)

BD9306AFVM, BD9305AFVM
Technical Note
(5) Setting of oscillation frequency
The angular wave oscillation frequency can be set by respectively connecting resistor and condenser to RT (1 pin) and CT
(2 pins). The currents to charge and discharge the condenser of CT are determined by RT.
Please refer to the following drawing for setting the RT’s resistor and the CT’s condenser.
RT:5~50kΩ, CT:100~1000pF, and the frequency range of 100kHz~800kHz are recommended.
Please pay attention to that, the switching will stop if your setting is off this range.
10000
1000
100
CT=100pF
CT=200pF
CT=470pF
CT=1000pF
Ta=25℃
VCC=12V
10
1
10
100
RT [kΩ]
Fig.27 Frequency Setting
(6)Selection of input condenser
For DC/DC converter, the condenser at the input side is also necessary because peak current is flowing between input
and output. Therefore, we recommend the low ESR condenser with over 10μF and below 100mΩ as the input condenser.
If a selected condenser is off this range, excessively large ripple voltage will overlaps with the input voltage, which may
cause IC malfunction.However, this condition varies with negative overcurrent, input voltage, output voltage, inductor’s
value, and switching frequency, so please be sure to do the margin check with actual devices.
(7)Selection of output rectifier diode
We recommend the Schottky barrier diode as the diode for rectification at the output stage of DC/DC converter. Please be
careful to choose the maximum inductor current, the maximum output voltage and the power supply voltage.
＜step-down DC/DC＞
Maximum inductor current
IOMAX
＋ ^⊿IL
2
＜ Diode’s rated current
Power supply voltage
＜step-up DC/DC＞
Maximum inductor current
VCC
I_INMAX ＋ ^⊿IL
2
＜ Diode’s rated voltage
＜ Diode’s rated current
Maximum output voltage
V_OMAX
＜ Diode’s rated voltage
Furthermore, each parameter has a deviation of 30%~40%, so please design in such a way that you have left a
sufficient margin for deviation in your design.
(8)Setting of Power FET
If step-down DC/DC is configured by BD9305AFVM, Pch FET is necessary; if step-up DC/DC is configured by
BD9306AFVM, Nch FET is necessary.
Please pay attention to the following conditions when you choose.
＜step-down DC/DC＞
Maximum inductor current
I_OMAX ＋
⊿IL
2
＜ FET’s rated current
Power supply voltage
VCC
Power supply voltage
Gate capacity (※)
＜step-up DC/DC＞
Maximum inductor current
＜ FET’s rated voltage
VCC
＞ FET’s gate ON voltage
C_GATE
＜ 2000pF
I_INMAX ＋ ^⊿IL
2
＜ FET’s rated current
Maximum output voltage
Power supply voltage
V_OMAX
VCC
＜ FET’s rated voltage
＞ FET’s gate ON voltage
Gate capacity (※)
C_GATE
＜ 2000pF
Furthermore, each parameter has a deviation of 30%~40%, so please design in such a way that you have left a sufficient
margin for deviation in your design.
(※) If Gate capacity becomes large, the switch’s switching speed gets slow, which may cause generation of heat and breakdown,
so please check thoroughly with actual devices.
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2009.05 - Rev.A