BD95513MUV_10 Datasheet, PDF (12/18 Pages) Rohm – Switching Regulator with MOS FET for DDR-SDRAM Cores

BD95513MUV
Technical Note
●External Component Selection
1. Inductor (L) Selection
VIN
IL
L
ΔIL
VOUT
Co
The inductor’s value directly influences the output ripple current.
As formula (4) indicates below, the greater the inductance or switching
frequency, the lower the ripple current:
ΔIL= ^{(VIN-VOUT)×VOUT} [A]・・・(4)
L×VIN×f
The proper output ripple current setting is about 30% of maximum output
current.
ΔIL=0.3×IOUTmax. [A]・・・(5)
(VIN-VOUT)×VOUT
L=
ΔIL×VIN×f
[H]・・・(6)
(ΔIL: output ripple current, f: switching 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’s rated current value.
* To minimize possible inductor damage and maximize efficiency, choose an inductor with a low DCR and ACR resistance.
2. Output Capacitor Selection (C_O)
VIN
When determining the proper output capacitor, be sure to factor in the
equivalent series resistance (ESR) and equivalent series inductance (ESL)
required to set the output ripple voltage at 20 mV or more.
VOUT
L
ESL ^ESR
Co
Output Capacitor
When selecting the limit of the inductor, be sure to allow enough margin for
the output voltage. Output ripple voltage is determined by formula (7)
below:
ΔVOUT=ΔIL×ESR+ESL×ΔIL / TON・・・(7)
(ΔIL: Ouput ripple current, ESR: equivalent series resistance,
ESL: equivalent series inductance)
Give special consideration to the conditions of formula (7) for output capacitance. Also, keep in mind that the output rise
time must be established within the soft start timeframe.
Co≦
tss×(Ilimit-IOUT)
VOUT
・・・(8)
tss: Soft start timeframe (see p. 10, equation (2))
I_limit: Maximum output current
Choosing a capacitance that is too large can cause startup malfunctions, or in some cases, may engage the short circuit
protection.
3. Input Capacitor Selection (C_IN)
VIN
CIN
In order to prevent extreme over-current conditions, the input capacitor
must have a low enough ESR to fully support a large ripple in the output.
The formula for RMS ripple current (I_RMS) is given by equation (9) below:
VOUT
L
Co
Input Capacitor
IRMS=IOUT×
VIN(VIN-VOUT)
VIN
[A]・・・(9)
IOUT
When VIN=2×VOUT, IRMS= ₂
A low-ESR capacitor is recommended to reduce ESR loss and maximize efficiency.
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12/17
2010.10- Rev.A