BD9040FV_09 Datasheet, PDF (11/19 Pages) Rohm – Step-down,High-efficiency Switching Regulators (Controller type)

BD9040FV, BD9045FV
Technical Note
(4) Feedback resistor design
Please refer to the following equation in determining the proper feedback resistance.
The recommended setting is in a range between 1kΩ and 330kΩ. Resistance less than 1kΩ risks decreased power
efficiency, while setting the resistance value higher than 330kΩ will result in an internal error amp input bias current of
0.4uA increasing the offset voltage.
Vo
Internal reference 0.9V
R1
FB
R2
R1 +R2
Vo =
× 0.9 [V] ・・・(7)
R2
(5) Setting switching frequency
The triangular wave switching frequency can be set by connecting a resistor to the RT pin. The RT sets the frequency by
adjusting the charge/discharge current in relation to the internal capacitor. Refer to the figure below in determining proper
RT resistance. Settings outside this range may render the switching function inoperable, and proper operation of the
controller overall cannot be guaranteed when unsupported resistance values are used.
750
700
650
600
550
500
450
400
350
300
250
200
0
20
40 60
80 100 120 140 160
RT [kΩ]
(6) Setting the soft start delay
The soft start function is necessary to prevent an inrush of coil current and output voltage overshoot at startup.
The figure below shows the relation between soft start delay time and capacitance, which can be calculated using
equation (8) at right.
Css
TSL = 0.8 ×
[sec]
Iss（10µA typ）
TSH = 0.7 ×
Vo
×
Vcc
Css
[sec]・・・（8）
Iss（10µA typ）
If the capacitance value is reduced (to no more than 0.01 uF or so), the overshoot in output may be caused.
Please use high accuracy components (such as X5R) when implementing sequential startups involving other power
sources. Be sure to test the actual devices and applications to be used, since the soft start time varies, depending on input
voltage, output voltage, load, coil, output capacitance and phase compensation constant etc.
(7) Setting the EN_SS (output delay function)(in the case of BD9040FV)
If EN_SS is made to be L, the output voltage’s OFF time can be made delayed. The calculating formula for delay time is
shown in the following equation.
TDIS=
Vss_MAX(1.95VTyp)-(0.8+0.7×
Vo
Css
)×
[sec]・・・(9)
Vcc
Idis(3.3uA,Typ)
VCC
EN_SS
VO
TSL TSH
Tdis
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11/18
2009.05 - Rev.A