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MPQ8634B Datasheet(HTML) 18 Page - Monolithic Power Systems

Part No. MPQ8634B
Description  16V, 20A, Sync, Step-Down Converter with Adjustable Current Limit, Programmable Frequency, and Voltage Tracking
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Maker  MPS [Monolithic Power Systems]
Homepage  http://www.monolithicpower.com
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MPQ8634B Datasheet(HTML) 18 Page - Monolithic Power Systems

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MPQ8634B – 16V, 20A, SYNC, STEP-DOWN CONVERTER W/ ADJUSTABLE CURRENT LIMIT,
PROGRAMMBALE FREQUENCY, AND VOLTAGE TRACKING
MPQ8634B Rev. 1.02
www.MonolithicPower.com
18
11/16/2018
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2018 MPS. All Rights Reserved.
APPLICATION INFORMATION
Selecting the Input Capacitor
The input current to the step-down converter is
discontinuous
and
therefore
requires
a
capacitor to supply AC current to the step-down
converter while maintaining the DC input
voltage. Use ceramic capacitors for the best
performance. During layout, place the input
capacitors as close to VIN as possible.
The capacitance can vary significantly with the
temperature. Capacitors with X5R and X7R
ceramic dielectrics are recommended because
they are fairly stable over a wide temperature
range and offer very low ESR.
The capacitors must have a ripple current rating
that exceeds the converter’s maximum input
ripple current. Estimate the input ripple current
with Equation (9):
)
V
V
1
(
V
V
I
I
IN
OUT
IN
OUT
OUT
CIN
(9)
The worst-case condition occurs at VIN = 2VOUT,
shown in Equation (10):
2
I
I
OUT
CIN
(10)
For simplification, choose an input capacitor
with an RMS current rating that exceeds half
the maximum load current. The input capacitor
value determines the converter input voltage
ripple. If there is an input voltage ripple
requirement in the system, select an input
capacitor that meets the specification.
Estimate the input voltage ripple with Equation
(11):
)
V
V
1
(
V
V
C
F
I
V
IN
OUT
IN
OUT
IN
SW
OUT
IN
(11)
The worst-case condition occurs at VIN = 2VOUT,
shown in Equation (12):
IN
SW
OUT
IN
C
F
I
4
1
V
(12)
Selecting the Output Capacitor
The output capacitor maintains the DC output
voltage. Use POSCAP or ceramic capacitors.
Estimate the output voltage ripple with Equation
(13):
)
C
F
8
1
R
(
)
V
V
1
(
L
F
V
V
OUT
SW
ESR
IN
OUT
SW
OUT
OUT
(13)
When
using
ceramic
capacitors,
the
capacitance dominates the impedance at the
switching frequency. The capacitance also
dominates the output voltage ripple. For
simplification, estimate the output voltage ripple
with Equation (14):
)
V
V
1
(
C
L
F
8
V
V
IN
OUT
OUT
2
SW
OUT
OUT
(14)
The ESR dominates the switching frequency
impedance for the POSCAP capacitors. For
simplification,
the
output
ripple
can
be
approximated with Equation (15):
ESR
IN
OUT
SW
OUT
OUT
R
)
V
V
1
(
L
F
V
V
(15)
Selecting the 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 a lower output ripple
voltage, but also has a larger physical size,
higher series resistance, and lower saturation
current. Select an inductor value that allows the
inductor peak-to-peak ripple current to be 30 -
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 using Equation (16):
)
V
V
1
(
I
F
V
L
IN
OUT
L
SW
OUT
(16)
Where ∆IL is the peak-to-peak inductor ripple
current.
Choose an inductor that will not saturate under
the maximum inductor peak current. The peak
inductor
current
can
be
calculated
with
Equation (17):
)
V
V
1
(
L
F
2
V
I
I
IN
OUT
SW
OUT
OUT
LP
(17)


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