MP4560 – 2A, 2MHz, 55V STEP-DOWN CONVERTER
Inductor
APPLICATION INFORMATION
The inductor is required to supply constant
current to the output load while being driven by
the switched input voltage. A larger value
inductor will result in less ripple current that will
result in lower output ripple voltage. However, the
larger value inductor will have a larger physical
size, higher series resistance, and/or lower
saturation current.
COMPONENT SELECTION
Setting the Output Voltage
The output voltage is set using a resistive voltage
divider from the output voltage to FB pin. The
voltage divider divides the output voltage down to
the feedback voltage by the ratio:
R2
VFB =VOUT
×
A good rule for determining the inductance to use
is to allow the peak-to-peak ripple current in the
inductor to be approximately 30% of the
maximum switch current limit. Also, make sure
that the peak inductor current is below the
maximum switch current limit. The inductance
value can be calculated by:
R1+R2
Thus the output voltage is:
R1+R2
R2
VOUT =VFB
×
For example, the value for R2 can be 10kꢀ. With
this value, R1 can be determined by:
VOUT
VOUT
L1=
×(1-
)
fs × ΔIL
V
R1=12.5×(VOUT -0.8)(Kꢀ)
IN
Where VOUT is the output voltage, VIN is the input
voltage, fS is the switching frequency, and ∆IL is
the peak-to-peak inductor ripple current.
For example, for a 3.3V output voltage, R2 is
10kꢀ, and R1 is 31.6kꢀ.
Choose an inductor that will not saturate under
the maximum inductor peak current. The peak
inductor current can be calculated by:
⎛
⎜
⎝
⎞
⎟
⎟
⎠
VOUT
VOUT
⎜
ILP = ILOAD
+
× 1−
2 × fS × L1
V
IN
Where ILOAD is the load current.
Table 1 lists a number of suitable inductors from
various manufacturers. The choice of which style
inductor to use mainly depends on the price vs.
size requirements and any EMI requirement.
MP4560 Rev. 1.0
11/5/2012
www.MonolithicPower.com
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