BD9528AMUV Datasheet, PDF (18/30 Pages) Rohm – High Performance Regulators for PCs Main Power Supply for Notebook PCs(With Built-in Linear Regulator)

BD9528AMUV
Technical Note
●External Component Selection
1. Inductor (L) selection
⊿IL
VIN
The inductor value is a major influence on the output ripple current.
As formula (4) below indicates, the greater the inductor or the switching
frequency, the lower the ripple current.
(VIN-VOUT)×VOUT
⊿IL=
L×VIN×f
[A]・・・(4)
IL
VOUT
L
Co
The proper output ripple current setting is about 30% of maximum
output current.
⊿IL=0.3×IOUTmax. [A]・・・(5)
(VIN-VOUT)×VOUT
L=
⊿IL×VIN×f
[H]・・・(6)
Output ripple current
(⊿IL: output ripple current; f: switch frequency)
※Passing a current larger than the inductor’s rated current will cause magnetic saturation in the inductor and decrease
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.
※To minimize possible inductor damage and maximize efficiency, choose a inductor with a low (DCR, ACR) resistance.
2.Output Capacitor (CO) Selection
VIN
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 (7) below.
VOUT
L
ESR
Load
ESL ^CEXT
⊿VOUT=⊿IL×ESR+ESL×⊿IL/TON・・・(7)
Co
(⊿IL: Output ripple current、ESR: CO equivalent series resistance、
ESL: CO equivalent series inductance)
Output Capacitor
※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.
Please give due consideration to the conditions in formula (8) below for output capacity, bear in mind that output rise time
must be established within the soft start time frame. Capacitor for bypass capacitor is connected to Load side which
connect to output in output capacitor capacity (C_EXT, figure above). Please set the soft start time or over current detecting
value, regarding these capacities.
Co+CEXT ≦ ^{TSS×(Limit-IOUT)}
VOUT
・・・(8)
Tss: Soft start time
Limit: Over current detection
Note: Improper capacitor may cause startup malfunctions.
3. Input Capacitor (Cin) Selection
VIN
Cin
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 (9) below.
VOUT
L
Co
VIN (VIN-VOUT)
IRMS=IOUT×
VIN
IOUT
Where VIN=2×VOUT, IRMS =
2
[A]・・・(9)
Input Capacitor
A low ESR capacitor is recommended to reduce ESR loss and maximize efficiency.
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18/29
2011.11 - Rev.A