MPQ8632 HIGH EFFICIENCY 18V SYNCHRONOUS STEP-DOWN CONVERTER FAMILY FOR 4A TO 20A
The ESR dominates the switching-frequency
to 40% of the maximum switch current limit. Also,
design for a peak inductor current that is below
the maximum switch current limit. Calculate the
inductance value as:
impedance for POSCAPs,. The ESR ramp
voltage is high enough to stabilize the system.
thus eliminating the need for an external ramp.
Select a minimum ESR value around 12mΩ to
ensure stable operation. For simplification, the
output ripple can be approximated as:
VOUT
VOUT
(27)
L
(1
)
FSW IL
V
IN
Where ΔIL is the peak-to-peak inductor ripple
current.
VOUT
V
(1 OUT )RESR
(26)
VOUT
FSW L
V
IN
Choose an inductor that will not saturate under
the maximum inductor peak current. The peak
inductor current can be calculated as:
Inductor
The inductor supplies constant current to the
output load while being driven by the switching
input voltage. A larger value inductor results in
less ripple current and lower output ripple voltage,
but is larger physical size, has a higher series
resistance, and/or lower saturation current.
Generally, select an inductor value that allows
the inductor peak-to-peak ripple current to 30%
VOUT
VOUT
(28)
ILP IOUT
(1
)
2FSW L
V
IN
Table 2 lists a few highly-recommended high-
efficiency inductors.
Table 2—Inductor Selection Guide
Switching
Frequency
(kHz)
Inductance DCR
Current
Dimensions
Part Number
Manufacturer
(µH)
(mΩ) Rating (A) L x W x H (mm3)
744325072
FDU1250C-1R0M
FDA1055-1R5M
744325180
FDA1055-2R2M
FDA1055-3R3M
HC7-3R9-R
Wurth
TOKO
TOKO
Wurth
TOKO
TOKO
Cooper
0.72
1
1.5
1.8
2.2
3.3
3.9
1.35
1.72
2.8
35
31.3
24
10.2 x 10.5 x 4.7
13.3 x 12.1 x 5
11.6 x 10.8 x 5.5
10.2 x 10.5 x 4.7
11.6 x 10.8 x 5.5
11.6 x 10.8 x 5.5
13.8 x 13 x 5.5
500
500
500
500
500
500
500
3.5
18
3.94
5.92
7.9
20.6
15.6
10.6
Table 3—MPQ8632-4, FSW=500kHz, VIN=12V
Typical Design Parameter Tables
VOUT
(V)
L
(μH)
R1
(kΩ)
R2
(kΩ)
R7
(kΩ)
The following tables include recommended
component values for typical output voltages (1V,
2.5V, 3.3V) and switching frequency (500kHz).
Refer to Tables 3-9 for design cases without
external ramp compensation. And Tables 10-16
are for design cases with external ramp
compensation. An external ramp is not needed
when using high-ESR output capacitors, such as
electrolytic or POSCAPs. Use an external ramp
when using low-ESR capacitors, such as ceramic
capacitors. For cases not listed in this datasheet,
an excel spreadsheet provided by local sales
representatives can assist with the calculations.
1
1.8
3.3
3.9
13.3
63.4
91
20
20
20
357
887
2.5
3.3
1200
Table 4—MPQ8632-6, FSW=500kHz, VIN=12V
VOUT
(V)
L
(μH)
R1
(kΩ)
R2
(kΩ)
R7
(kΩ)
1
1
13.3
63.4
91
20
20
20
357
887
2.5
3.3
2.2
3.3
1200
MPQ8632 Rev.1.24
8/28/2013
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27
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