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MP8720 Datasheet(PDF) 18 Page - Monolithic Power Systems

Part No. MP8720
Description  26V,10A, High Efficiency, Fast Transient, Synchronous, Buck Converter with adjustable CLM
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Manufacturer  MPS [Monolithic Power Systems]
Direct Link  http://www.monolithicpower.com
Logo MPS - Monolithic Power Systems

MP8720 Datasheet(HTML) 18 Page - Monolithic Power Systems

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MP8720 – 26V, 10A, SYNCHRONOUS BUCK CONVERTER WITH ADJUSTABLE CLM
MP8720 Rev. 1.01
www.MonolithicPower.com
18
8/1/2017
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2017 MPS. All Rights Reserved.
The worst-case condition occurs at VIN = 2VOUT,
shown in Equation (7):
OUT
CIN
I
I
2
(7)
For simplification, choose an input capacitor
with an RMS current rating greater than half of
the maximum load current.
The input capacitance value determines the
input voltage ripple of the converter. If there is
an input voltage ripple requirement in the
system, choose an input capacitor that meets
the specification.
The input voltage ripple can be estimated with
Equation (8):
OUT
OUT
OUT
IN
SW
IN
IN
IN
IV
V
V(1
)
FC
V
V

 
(8)
The worst-case condition occurs at VIN = 2VOUT,
shown in Equation (9):
OUT
IN
SW
IN
I
1
V
4F
C
 
(9)
Selecting the Output Capacitor
The output capacitor is required to maintain the
DC output voltage. Ceramic or POSCAP
capacitors are recommended. The output
voltage ripple can be estimated using Equation
(10):
OUT
OUT
OUT
ESR
SW
IN
SW
OUT
VV
1
V(1
) (R
)
FL
V
8 F
C

 

(10)
When using ceramic capacitors, the impedance
at the switching frequency is dominated by the
capacitance, which mainly causes the output
voltage ripple. For simplification, the output
voltage ripple can be estimated with Equation
(11):
OUT
OUT
OUT
2
SW
OUT
IN
VV
V(1
)
8F
L C
V

 
 
(11)
The output voltage ripple caused by the ESR is
very small. Therefore, an external ramp is
needed to stabilize the system. The external
ramp can be generated through resistor R4 and
capacitor C4.
When using POSCAP capacitors, the ESR
dominates the impedance at the switching
frequency. The ramp voltage generated from
the ESR dominates the output ripple. The
output ripple can be approximated with
Equation (12):
OUT
OUT
OUT
ESR
SW
IN
VV
V(1
) R
FL
V

 
(12)
The maximum output capacitor limitation should
be considered in the design application. The
MP8720 has a soft-start time period around
1.6ms. If the output capacitor value is too high,
the output voltage cannot reach the design
value during the soft-start time, causing it to fail
to regulate. The maximum output capacitor
value (Co_max) can be limited approximately with
Equation (13):
O_ MAX
LIM _ AVG
OUT
ss
OUT
C(I
I
) T / V

(13)
Where ILIM_AVG is the average start-up current
during the soft-start period (which can be
equivalent to the current limit), and Tss is the
soft-start time.
Selecting the Inductor
An inductor is necessary for supplying constant
current to the output load while being driven by
the switched input voltage. A larger-value
inductor results in less ripple current, resulting
in a lower output ripple voltage, but also has a
larger physical footprint, a higher series
resistance, and a lower saturation current. A
good rule for determining the inductance value
is to design the peak-to-peak ripple current in
the inductor to be in the range of 30% to 50% of
the maximum output current, and the peak
inductor current below the maximum switch
current limit. The inductance value can be
calculated with Equation (14):
OUT
OUT
SW
L
IN
VV
L(1
)
FI
V
 

(14)
Where ∆IL is the peak-to-peak inductor ripple
current.
The inductor should not saturate under the
maximum inductor peak current (including a
short-current), so ISAT is recommended to
be >13A.


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