BD9040FV, BD9045FV

Technical Note

10/18

www.rohm.com

2009.05 - Rev.A

© 2009 ROHM Co., Ltd. All rights reserved.

Output ripple current

L

VCC

VOUT

Co

●

Application components selection

(1) Setting of output L constant value

The coil value significantly influences the output ripple current. Thus, as seen

in equation (1), the bigger the coil, and the higher the switching frequency, the

lower the drop in ripple current.

The optimal output ripple current setting is 30% of maximum current.

ΔIL = 0.3×IOUTmax.[A]・・・(2)

(ΔIL：output ripple current, f：switching frequency)

※

Outputting a current in excess of the coil current rating will cause magnetic saturation of the coil and decrease efficiency.

Please establish sufficient margin to ensure that peak current does not exceed the coil current rating.

※

Use low resistance (DCR, ACR) coils to minimize coil loss and increase efficiency.

(2) Setting of output constant Co

(at rapid load change). The following equation is used to determine the output ripple voltage.

Step down

Be sure to keep the output Co setting within the allowable ripple voltage range.

※

Please allow sufficient output voltage margin in establishing the capacitor rating.

Note that low-ESR capacitors enable lower output ripple voltage.

Also, to meet the requirement for setting the output startup time parameter within the soft start time range, please factor in

the conditions described in the capacitance equation (5) for output capacitors, below.

Tss：soft start time

ILimit：over current detection value reference

Note: less than optimal capacitance values may cause problems at startup.

(3) Input capacitor(Cin)selection

The input capacitor serves to lower the output impedance of the power

source connected to the input pin (VCC). Increased power supply

output impedance can cause input voltage (VCC) instability, and may

negatively impact oscillation and ripple rejection characteristics.

Therefore, be certain to establish an input capacitor in close proximity

to the VCC and GND pins. Select a low-ESR capacitor with the required

ripple current capacity and the capability to withstand temperature

changes without wide tolerance fluctuations. The ripple current IRMS is

determined using equation (6).

Also, be certain to ascertain the operating temperature, load range and

MOSFET conditions for the application in which the capacitor will be

used, since capacitor performance is heavily dependent on the

application’s input power characteristics, substrate wiring and MOSFET

gate drain capacity.

(VCC-VOUT)×VOUT

L×VCC×f

Δ

IL =

[A]・・・（1）

(VCC-VOUT)×VOUT

Δ

IL×VCC×f

L =

[H]・・・（3）

Δ

Vo

1

Co

Vcc

f

Δ

×

×

[V]･･･(4)

provided that f： switching frequency

Co ≦

・・・ （

5）

Δ

IL

L

VIN

VOUT

Co

Cin

Input capacitor

VOUT（VCC - VOUT）

VCC

IRMS = IOUT ×

[A]・・・(6)