BD95500MUV Datasheet, PDF (12/21 Pages) Rohm – Switching Regulator with MOSFET for DDR-SDRAM Cores

BD95500MUV
Technical Note
●External Component Selection
1. Inductor (L) selection
ΔIL
VIN
HG
IL
SW
L
LG
VOUT
Co
PGND
Output Ripple 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.
(VIN-VOUT)×VOUT
ΔIL=
L×VIN×f
[A]・・・(4)
The proper output ripple current setting is about 30% of maximum
output current.
ΔIL=0.3×IOUTmax. [A]・・・(5)
L=
(VIN-VOUT)×VOUT _{[H]・・・(6)}
L×VIN×f
(ΔIL: 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
HG
SW
L
LG
PGND
VOUT
ESR
ESL
Co
Output Capacitor
When determining the proper output capacitor, be sure to factor in the equivalent
series resistance and equivalent series inductance required to set the output ripple
voltage 20mV or more.
In selecting the limit of inductor, be sure to allow enough margin for output voltage.
Output ripple voltage is determined as in formula (7) below.
ΔVOUT=ΔIL×ESR + ESL×ΔIL / TON・・・(7)
(ΔIL: Output ripple current; ESR: CO equivalent series resistance,
ESL: equivalent series inductance)
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.
Co≦ ^{TSS×(Limit-IOUT)}
VOUT
・・・(8)
Tss: Soft start time (See formula (2) in P10)
Limit: Over current detection (See formula (10)(11) in P13)
Note: Improper capacitor may cause startup malfunctions
3. Input Capacitor (Cin) Selection
VIN
Cin
HG
SW
L
Co
LG
VOUT
PGND
Input Capacitor
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.
√_{VIN (VIN-VOUT)}
IRMS=IOUT×
VIN
[A]・・・(9)
IOUT
Where VIN=2×VOUT, IRMS= ₂
A low ESR capacitor is recommended to reduce ESR loss and maximize efficiency.
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12/20
2010.05 - Rev.C