BD9540EFV Datasheet, PDF (10/18 Pages) Rohm – 2ch Switching Regulators for Desktop PC

BD9540EFV
Technical Note
●External Component Selection
1. Inductor (L) selection
VIN
HG
IL
SW
L
LG
ΔIL
VOUT
Co
Output Ripple Current
The inductance value has a major influence on output ripple current.
As formula (3) below indicates, the greater the inductance or
switching frequency, the lower the ripple current.
(VIN-VOUT)×VOUT
ΔIL=
L×VIN×f
[A]・・・(3)
The proper output ripple current setting is about 30% of maximum
output current.
ΔIL=0.3×IOUTmax. [A]・・・(4)
(VIN -VOUT)×VOUT
L=
ΔIL×VIN×f
[H]・・・(5)
(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. When selecting an inductor, be sure to allow enough margin to assure that peak current does not
exceed the inductor’s 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
VOUT
ESR
ESL
Co
Output Capacitor
When determining a proper output capacitor, be sure to factor in the equivalent
series resistance and equivalent series inductance required to set the output ripple
voltage to 20mV or more. Also, make sure the capacitor’s voltage rating is high
enough for the set output voltage (including ripple).
Output ripple voltage is determined as in formula (6) below.
ΔVOUT=ΔIL×ESR+ESL×ΔIL/TON・・・(6)
(ΔIL: Output ripple current; ESR: CO equivalent series resistance,
ESL: equivalent series inductance)
Also, give due consideration to the conditions in formula (7) below for output capacitance, bearing in mind that output rise
time must be established within the soft start time frame:
Co≦ ^{1ms×(Limit-IOUT)}
VOUT
・・・(7)
Tss: Soft start time
Limit: Over current detection
IOUT : Output current
Note: an improper output capacitor may cause startup malfunctions.
3. Input Capacitor (Cin) Selection
VIN
Cin
HG
SW
VOUT
L
Co
LG
Input Capacitor
In order to prevent transient spikes in voltage, the input capacitor selected must
have a low enough ESR resistance to fully support a large ripple current on the
output. The formula for ripple current IRMS is given in equation (8) below:
IRMS=IOUT×
√_{VOUT (VIN-VOUT)}
VIN
[A]・・・(8)
Where VIN=2×VOUT, IRMS=
IOUT
2
A low-ESR capacitor is recommended to reduce ESR loss and maximize efficiency.
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10/17
2009.04 - Rev.B