BD9859EFJ Datasheet, PDF (10/16 Pages) Rohm – Simple Step-down Switching Regulators with Built-in Power MOSFET

BD9859EFJ
Technical Note
●Application Components Selection Method
(1)Inductor
Something of the shield Type that Fulfills the Current Rating (Current value Ipecac below), with low DCR (Direct
Current Resistance element) is recommended.
Value of Inductor influences Inductor Ripple Current and becomes the cause of Output Ripple.
In the same way as the formula below, this Ripple Current can be made small for as big as the L value of Coil or as
high as the Switching Frequency.
Ipeak =Iout ＋ ⊿IL/2 [A]
⊿IL= ^Vin-Vout × ^Vout × ¹ [A]
L
Vin
f
(1)
Δ IL
(2)
Fig.27 Inductor Current
(η：Efficiency、⊿IL：Output Ripple Current、f：Switching Frequency )
For design value of Inductor Ripple Current, please carry out design tentatively with about 20％～50% of Maximum
Input Current.
※When current that exceeds Coil rating flows to the coil, the Coil causes a Magnetic Saturation, and there are
cases wherein a decline in efficiency, oscillation of output happens. Please have sufficient margin and select so that
Peak Current does not exceed Rating Current of Coil.
(2)Output Capacitor
In order for Capacitor to be used in Output to reduce Output Ripple, Low Ceramic Capacitor of ESR is recommended.
Also, for Capacitor Rating, on top of putting into consideration DC Bias Characteristics, please use something whose
Maximum Rating has sufficient margin with respect to the Output Voltage.
Output Ripple Voltage is looked for using the following formula.
1
Vpp=⊿IL×
＋ ⊿IL×R_ESR
2π×f×Co
[V] ・・・ (3)
Please design in a way that it is held within Capacity Ripple Voltage.
(3)Output Voltage Setting
ERROR AMP internal Standard Voltage is 1.0V.Output Voltage is determined as seen in (4) formula.
VOUT
R1
ERROR AMP
FB
R2
(R1+R2)
Vo=
×1.0 [V] ・・・ (4)
R2
VREF
1.0 V
Fig.28 Voltage Return Resistance Setting Method
(4)Boost Capacitor
Please connect CBST=0.022uF(Laminate Ceramic Capacitor) between BST Pin－Lx Pins as Output capacitors of Gate Drive
Voltage Generator REG(5V).
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10/15
2010.02 - Rev.A