BD9305AFVM Datasheet, PDF (8/15 Pages) Rohm – Single-output Step-up,High-efficiency Switching Regulator(Controller Type)

BD9306AFVM, BD9305AFVM
Technical Note
(2) Setting the output L constant (Step Up DC/DC)
The inductance L to use for output is decided by the rated current ILR and input current maximum value IINMAX of the
inductance.
IINMAX+ΔIL should not
VCC
IL
2
reach the rated value level _L
IL
Vo
IINMAX mean
current
Co
t
Fig.24 Coil Current Waveform (Step Up DC/DC)
Fig.25 Output Application Circuit (Step Up DC/DC)
Adjust so that IINMAX+ ΔIL/ 2 does not reach the rated current value ILR.
At this time, ∆I_L can be obtained by the following equation.
ΔI_L=
1
L
Vcc X
Vo-Vcc
Vo
X
1
ｆ
[A] Where, f is the switching frequency
Set with sufficient margin because the inductance L value may have the dispersion of ± 30%.
If the coil current exceeds the rating current I_LR of the coil, it may damage the IC internal element.
(3) Setting the output capacitor
For the capacitor C to use for the output, select the capacitor which has the larger value in the ripple voltage VPP
allowance value and the drop voltage allowance value at the time of sudden load change.
Output ripple voltage is decided by the following equation.
ΔVPP = ΔIL X RESR +
ΔIL
2Co
X
Vo
Vcc
X
1
f
[V]
(Step Down DC/DC)
ΔVPP = ILMAX X RESR +
1
fCo
X
Vcc
Vo
X (ILMAX -
ΔIL
2
) [V]
(Step Up DC/DC)
Perform setting so that the voltage is within the allowable ripple voltage range.
For the drop voltage during sudden load change; VDR, please perform the rough calculation by the following equation.
VDR =
ΔI
Co
X 10μ sec [V]
However, 10 s is the rough calculation value of the DC/DC response speed. Please set the capacitance considering the
sufficient margin so that these two values are within the standard value range.
(4)Setting of feedback resistance constant
For both BD9305AFVM (step down) and BD9306AFVM (step up), please refer to the following formula for setting of
feedback resistance.
We recommend 10kΩ~330kΩ as the setting range. If a resistance below 10kΩ is set, a drop in voltage efficiency will be
caused; if a resistance more than 330kΩ is set, the offset voltage becomes large because of the internal error amplifier’s
input bias current of 0.05µA(Typ). Please set the maximum setting voltage of BD9306AFVM (step up) in such a way that
Duty : (Vo - Vcc) / Vo is less than 70%.
Vo ＝ ^{R1 + R2} x 1.25
R2
Vo
Reference Voltage 1.25V
[V]
R1 _FB
8
－
ERR
R2
＋
Fig.26 Feedback Resistance Setting
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8/14
2009.05 - Rev.A