BD9560MUV_10 Datasheet, PDF (10/22 Pages) Rohm – Switching Regulator Controller for Graphic Chip Cores

BD9560MUV
Technical Note
●External Component Selection
1. Inductor (L) selection
VIN
IL
L
ΔIL
VOUT
Co
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.
(VIN-VOUT)×VOUT
ΔIL=
L×VIN×f
[A]・・・(5)
The proper output ripple current setting is about 30% of
maximum output current.
ΔIL=0.3×IOUTmax. [A]・・・(6)
(VIN-VOUT)×VOUT
L=
L×VIN×f
[H]・・・(7)
(ΔIL: output ripple current; f: switch frequency)
Output ripple current
※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
VOUT
L
ESR
Co
Output Capacitor
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.
ΔVOUT=ΔIL×ESR [V]・・・(8)
(ΔIL: Output ripple current; ESR: CO equivalent series resistance)
※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 (9) below for output capacity, bearing in mind that output rise
time must be established within the soft start time frame.
Co≦ ^{Tss×(Limit-IOUT)}
VOUT
・・・(9)
Tss: Soft start time
Limit: Over current detection 2A(Typ)
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 (10) below.
VOUT
L
Co
Input Capacitor
√_{VCC(VCC-VOUT)}
IRMS=IOUT×
VCC
Where VCC=2×VOUT, IRMS=
IOUT
2
[A]・・・(10)
A low ESR capacitor is recommended to reduce ESR loss and maximize efficiency.
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10/21
2010.04 - Rev.C