BD8961NV-E2 View Datasheet(PDF) - ROHM Semiconductor

Part Name

Description

MFG CO.

BD8961NV-E2

Low Noise High Efficiency Step-down Switching Regulator with Built-in Power MOSFET

ROHM Semiconductor 

BD8961NV

Technical Note

●Selection of components externally connected

1. Selection of inductor (L)

IL

ΔIL

VCC

IL

VOUT

L

Co

Fig.26 Output ripple current

The inductance significantly depends on output ripple current.

As seen in the equation (1), the ripple current decreases as the

inductor and/or switching frequency increases.

ΔIL=

(VCC-VOUT)×VOUT

L×VCC×f

[A]・・・(1)

Appropriate ripple current at output should be 20% more or less of the

maximum output current.

ΔIL=0.3×IOUTmax. [A]・・・(2)

(VCC-VOUT)×VOUT

L=

ΔIL×VCC×f

[H]・・・(3)

(ΔIL: Output ripple current, and f: Switching frequency)

* Current exceeding the current rating of the inductor results in magnetic saturation of the inductor, which decreases efficiency.

The inductor must be selected allowing sufficient margin with which the peak current may not exceed its current rating.

If VCC=5V, VOUT=3.3V, f=1MHz, ΔIL=0.3×2A=0.6A, for example,(BD8961NV)

L=

(5.0-3.3)×3.3

0.6×5.0×1M

=1.87μ → 2.2[μH]

*Select the inductor of low resistance component (such as DCR and ACR) to minimize dissipation in the inductor for

better efficiency.

2. Selection of output capacitor (CO)

VCC

VOUT

L

ESR

Co

Fig.27 Output capacitor

Output capacitor should be selected with the consideration on the stability region

and the equivalent series resistance required to smooth ripple voltage.

Output ripple voltage is determined by the equation (4)：

ΔVOUT=ΔIL×ESR [V]・・・(4)

(ΔIL: Output ripple current, ESR: Equivalent series resistance of output capacitor)

*Rating of the capacitor should be determined allowing sufficient margin

against output voltage. Less ESR allows reduction in output ripple voltage.

22µF to 100µF ceramic capacitor is recommended.

3. Selection of input capacitor (Cin)

VCC

Cin

VOUT

L

Co

Fig.28 Input capacitor

Input capacitor to select must be a low ESR capacitor of the capacitance

sufficient to cope with high ripple current to prevent high transient voltage. The

ripple current IRMS is given by the equation (5):

√VOUT(VCC-VOUT)

IRMS=IOUT×

VCC

[A]・・・(5)

< Worst case > IRMS(max.)

IOUT

When Vcc is twice the VOUT, IRMS= 2

If VCC=5.0V, VOUT=3.3V, and IOUTmax.=2A, (BD8961NV)

IRMS=2×

√3.3(5.0-3.3)

5.0

=0.947[ARMS]

A low ESR 22μF/10V ceramic capacitor is recommended to reduce ESR dissipation of input capacitor for better efficiency.

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9/13

2010.04 - Rev.B

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