BD8305MUV Rohm, BD8305MUV Datasheet - Page 10

• Manufacturer Part Number: BD8305MUV
• Description: High-effciency Step-up/down Switching Regulators
• Manufacturer: Rohm

●Selection of Parts for Applications
○
BD8305MUV
c
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(1) Output inductor
(2) Output capacitor
(3) Setting of oscillation frequency
* Note that the above example of frequency setting is just a design target value, and may differ from the actual equipment.
⊿IL=
⊿IL=
⊿IL=
A shielded inductor that satisfies the current rating (current value, Ipeak as shown in the drawing below) and has a low
DCR (direct current resistance component) is recommended.
Inductor values affect output ripple current greatly.
Ripple current can be reduced as the coil L value becomes larger and the
switching frequency becomes higher as the equations shown below.
Ipeak =Iout ×(Vout/VIN) /η＋ ∆IL/2 [A]
(η: Efficiency, ∆IL: Output ripple current, f: Switching frequency)
As a guide, output ripple current should be set at about 20 to 50% of the maximum output current.
＊Current over the coil rating flowing in the coil brings the coil into magnetic saturation, which may lead to lower efficiency
A ceramic capacitor with low ESR is recommended for output in order to reduce output ripple.
There must be an adequate margin between the maximum rating and output voltage of the capacitor, taking the DC bias
property into consideration.
Output ripple voltage when ceramic capacitor is used is obtained by the following equation.
Setting must be performed so that output ripple is within the allowable ripple voltage.
Oscillation frequency can be set using a resistance value connected to the RT pin (1 pin).
Oscillation frequency is set at 1 MHz when RT = 47 kΩ, and frequency is inversely proportional to RT value.
See Fig. 30 for the relationship between RT and frequency.
Soft-start time changes along with oscillation frequency.
See Fig. 31 for the relationship between RT and soft-start time.
Fig. 30 Oscillation frequency – RT pin resistance
Vpp=⊿IL×
or output oscillation. Select an inductor with an adequate margin so that the peak current does not exceed the rated
current of the coil.
(Vin-Vout)
|(Vin-Vout)|
(Vout-Vin)
10000
1000
L
L
L
100
2π×f×Co
1
×
×
1
RT PIN RESISTANCE [kΩ]
×
Vout
Vout
10
Vin
Vin
Vout×2×0.85
(Vin+Vout)
＋ ⊿IL×R
×
×
100
1
1
f
f
1000
ESR
[A] (in step-down mode)
[A] (in step-up mode)
×
[V] ・・・ (5)
1
f
[A] (in step-up/down mode)
10/15
Fig. 31 Soft-start time – RT pin resistance
0.1
10
1
1
(1)
RT PIN RESISTANCE [kΩ]
10
(4)
(2)
100
Fig. 29 Ripple current
1000
(3)
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Technical Note
2010.05 - Rev.D
Δ I
L