MP1472 – 2A, 18V SYNCHRONOUS RECTIFIED, STEP-DOWN CONVERTER
APPLICATIONS INFORMATION
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:
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:
R2
VFB VOUT
R1 R2
Where VFB is the feedback voltage and VOUT is
the output voltage.
VOUT
VOUT
VIN
L
1
fS IL
Thus the output voltage is:
Where VOUT is the output voltage, VIN is the input
voltage, fS is the switching frequency, and ΔIL is
the peak-to-peak inductor ripple current.
R1 R2
R2
VOUT 0.923
R2 can be as high as 100kΩ, but a typical value
is 10kΩ. Using the typical value for R2, R1 is
determined by:
Choose an inductor that will not saturate under
the maximum inductor peak current. The peak
inductor current can be calculated by:
R1 10.83 (VOUT 0.923) (kꢀ)
For example, for a 3.3V output voltage, R2 is
10kꢀ, and R1 is 26.1kꢀ.
VOUT
VOUT
VIN
ILP ILOAD
1
2 fS L
Inductor
Where ILOAD is the load current.
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
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.
Table 1—Inductor Selection Guide
Dimensions
Part Number
Inductance (µH) Max DCR (Ω) Current Rating (A)
L x W x H (mm3)
Wurth Electronics
7440650068
6.8
10
15
0.033
0.035
0.050
3.6
3.6
3.2
10x10x2.8
10x10x3.8
10x10x3.8
744066100
744066150
TDK
SLF10165T-6R8N4R33PF
SLF10165T-100M3R83PF
SLF10165T-150M3R13PF
Toko
6.8
10
15
0.014
0.0185
0.027
4.3
3.8
3.1
10x10x4.5
10x10x4.5
10x10x4.5
#B952AS-6R8N
#B892NAS-100M
#B892NAS-150M
6.8
10
15
0.035
0.0225
0.0355
3.1
4.2
3.2
10.4x10.4x4.8
12.3x12.3x4.5
12.3x12.3x4.5
MP1472 Rev. 1.0
9/2/2011
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