BD9302FP-E Datasheet, PDF (10/17 Pages) Rohm – A wide input voltage range of 6 V to 18 V, Easy switching frequency setting in the range of 200 k to 2.5 MHz.

(3) Setting of feedback resistance constant
In order to make settings of feedback resistance, refer to the formula shown. It is recommended to make setting of
resistance in the range of 10 kΩ to 330 kΩ. Setting the resistance to 10 kΩ or less will result in degraded power
efficiency, while setting it to 330 kΩ or more will increase the offset voltage due to the input bias current of 0.4 μA (TYP)
of the internal error amplifier.
VO
Internal reference voltage: 0.6 V
R8
FB
R9
Fig.24
VO=
R8+R9
R9
Â¥ 0.6 [V]
(4) Setting of oscillation frequency
Connecting a resistor to the RT pin (pin 4) will allow for the setting of triangular wave oscillation frequency. The RT
determines the charge/discharge current to the internal capacitor, with which the frequency varies. Referring to Figure
shown below, make settings of the RT resistor. Recommended setting range is 10 to 100 kΩ. Be noted that any setting
outside of this range may turn OFF switching, thus impairing the operation guarantee.
100000
10000
1000
100
10
10
100
TIMING RESISTANCE : RT [kΩ]
Fig.25 RT vs. Switching frequency
(5) Setting of soft start time
The soft start function will be required to prevent an excessive increase in the coil current and overshoot of the output
voltage, while in startup operation. Figure below shows the relationship between the capacitor and the soft start time.
Referring to this Figure, make the capacitor setting.
100
10
1
0
0.01
0.10
1.00
SS CAPACITOR : CSS [μF]
Fig.26 SS capacitance vs. Delay time
It is recommended to make setting of capacitance value in the range of 0.01 to 10 μF. Setting the capacitance value to
0.01 μF or less may cause overshoot to the output voltage. If any startup-related function (sequence) of other power
supply is provided, use a high-accuracy product (e.g. ¥ 5R) or the like.
Furthermore, since the soft start time varies with the input voltage, output voltage, load, coil, output capacitor, or else,
be sure to check to be sure this soft start time on the actual system.
10/16