bd9575fv-e2 ROHM Co. Ltd., bd9575fv-e2 Datasheet - Page 11

• Manufacturer Part Number: bd9575fv-e2
• Description: Switching Regulators Ddr-sdram Cores
• Manufacturer: ROHM Co. Ltd.

●External Component Selection
© 2009 ROHM Co., Ltd. All rights reserved.
BD9575FV
www.rohm.com
1. Inductor (L) selection
2.Output Capacitor (C
3. Input Capacitor (Cin) Selection
※Passing a current larger than the inductor’s rated current will cause magnetic saturation in the inductor and decrease
※To minimize possible inductor damage and maximize efficiency, choose a inductor with a low (DCR, ACR) resistance.
Please give due consideration to the conditions in formula (9) below for output capacity, bearing in mind that output rise
time must be established within the soft start time frame.
Note: Improper capacitor may cause startup malfunctions.
A low ESR capacitor is recommended to reduce ESR loss and maximize efficiency.
system efficiency. In selecting the inductor, be sure to allow enough margin to assure that peak current does not exceed
the inductor rated current value.
Input Capacitor
Co≦
Output ripple current
Output capacitor
V
V
V
IN
IN
IN
L
L
I
L
L
Tss×(Limit-I
Cin
Co
V
Co
OUT
V
O
OUT
ΔI
) Selection
ESR
ESL
Co
L
OUT
V
V
OUT
OUT
)
・・・(9)
When determining the proper output capacitor, be sure to factor in the equivalent
series resistance required to smooth out ripple volume and maintain a stable
output voltage range.
Output ripple voltage is determined as in formula (8) below.
ΔV
(ΔI
※ In selecting a capacitor, make sure the capacitor rating allows sufficient
margin relative to output voltage. Note that a lower ESR can minimize output
ripple voltage.
The input capacitor selected must have low enough ESR resistance to fully
support large ripple output, in order to prevent extreme over current. The formula
for ripple current IRMS is given in (10) below.
L
I
Where V
OUT
RMS
: Output ripple current; ESR: C
=ΔI
=I
OUT
L
CC
×ESR+ESL×ΔI
The proper output ripple current setting is about 30% of maximum
output current.
The inductor value is a major influence on the output ripple
current. As formula (5) below indicates, the greater the inductor or
the switching frequency, the lower the ripple current.
×
=2×V
Tss: Soft start time
Limit: Over current detection
11/20
√
V
OUT
OUT
ΔI
ΔI
(ΔI
ESR:equivalent series inductance)
(V
V
, I
L
L
L=
L
=
=0.3×I
IN
IN
: output ripple current; f: switch frequency)
RMS
-V
(V
(V
L
OUT
=
/T
IN
IN
ΔI
ΔI
ON
OUT
I
-V
-V
)
OUT
O
2
L
L
OUT
OUT
×V
×V
max. [A]・・・(6)
equivalent series resistance,
[V]・・・(8)
[A]・・・(10)
)×V
)×V
IN
IN
×f
×f
OUT
OUT
[A]・・・(5)
[H]・・・(7)
Technical Note
2009.04 - Rev.B