BD9532EKN_08 Datasheet, PDF (11/21 Pages) Rohm – Switching Regulators for DDR-SDRAM Cores

●External Component Selection
1. Inductor (L) selection
VIN
IL
L
ΔIL
VOUT
Co
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=
ΔIL×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=
ΔIL×VIN×f
[H]・・・(7)
(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
VOUT
L
ESR
ESL
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+ESL×ΔIL/TON [V]・・・(8)
(ΔIL: Output ripple current; ESR: CO equivalent series resistance,
ESR:equivalent series inductance)
※ 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
Note: Improper capacitor may cause startup malfunctions.
3. Input Capacitor (Cin) Selection
VIN
Cin
VOUT
L
Co
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 (10) below.
√_{Vout(VIN-VOUT)}
IRMS=IOUT×
VIN
[A]・・・(10)
IOUT
Where VIN=2×VOUT, IRMS= ₂
A low ESR capacitor is recommended to reduce ESR loss and maximize efficiency.
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