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MP9447 Datasheet(PDF) 15 Page - Monolithic Power Systems

No. de Pieza. MP9447
Descripción  High-Efficiency, Fast-Transient, 5A, 36V Synchronous, Step-Down Converter
Descarga  19 Pages
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Fabricante  MPS [Monolithic Power Systems]
Página de inicio  http://www.monolithicpower.com
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MP9447 Datasheet(HTML) 15 Page - Monolithic Power Systems

 
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MP9447–HIGH-EFFICIENCY, FAST-TRANSIENT, SYNCHRONOUS, STEP-DOWN CONVERTER
MP9447 Rev. 1.2
www.MonolithicPower.com
15
1/5/2016
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2016 MPS. All Rights Reserved.
Output Capacitor
The output capacitor maintains the DC output
voltage. Use ceramic or POSCAP capacitors.
The output voltage ripple can be estimated as:
OUT
OUT
OUT
ESR
SW
IN
SW
OUT
VV
1
V(1
) (R
)
FL
V
8 F
C

 

(16)
Where RESR is the equivalent series resistance of
the output capacitor.
For ceramic capacitors, capacitance dominates
the impedance at the switching frequency, can is
the primary cause of the output-voltage ripple.
For simplification, estimate the output voltage
ripple as:
OUT
OUT
OUT
2
SW
OUT
IN
VV
V(1
)
8F
L C
V

 
 
(17)
The output voltage ripple caused by ESR is very
small and therefore requires an external ramp to
stabilize the system. The voltage ramp is ~30mV.
The external ramp can be generated through R4
and C4 using the following equation:
IN
OUT
ON
RAMP
(V
V
) T
V
R4
C4

(18)
Select C4 to meet the following condition:
SW
R1 R2
11
()
2F
C4
5
R1 R2


(19)
For POSCAP capacitors, the ESR dominates the
impedance at the switching frequency. The ramp
voltage generated from the ESR is high enough
to stabilize the system. Therefore, an external
ramp is not needed. A minimum ESR value of
12mΩ is required to ensure stable operation of
the converter. For simplification, the output ripple
can be approximated as:
OUT
OUT
OUT
ESR
SW
IN
VV
V(1
) R
FL
V

 
(20)
Inductor
The inductor is required to supply constant
current to the output load while being driven by
the switching input voltage. A larger inductance
will result in less ripple current and a lower output
ripple voltage. However, a larger inductance
resultsin a larger inductor, which will physically
larger, and have a higher series resistance
and/or lower saturation current. A good rule for
determining the inductor value is to allow the
peak-to-peak ripple current in the inductor to be
approximately 30% to 40% of the maximum
switch current limit. Ensure that the peak inductor
current is below the maximum switch current limit.
The inductance value can be calculated as:
OUT
OUT
SW
L
IN
VV
L(1
)
FI
V
 

(21)
Where ΔIL 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 as:
OUT
OUT
LP
OUT
SW
IN
VV
II
(1
)
2F
L
V

 
(22)
Typical Design Parameter Tables
The following tables include recommended
component values for typical output voltages
(3.3V, 5V) and switching frequencies (300kHz,
500kHz). Refer to Tables 1 through 2 for design
cases without external ramp compensation, and
Tables 3 through 4 for design cases with external
ramp compensation. An external ramp is not
needed when using high-ESR capacitors, such
as electrolytic or POSCAPs. An external ramp is
needed when using low-ESR capacitors, such as
ceramic capacitors. For cases not listed in this
datasheet, an Excel spreadsheet available
through your local sales representative can
calculate approximate component values.
Table 1—300kHz, 24VIN
VOUT
(V)
L
(μH)
R1
(kΩ)
R2
(kΩ)
RFREQ
(kΩ)
3.3
10
30.1
10
110
5
10
51.1
10
169
Table 2—500kHz, 24VIN
VOUT
(V)
L
(μH)
R1
(kΩ)
R2
(kΩ)
RFREQ
(kΩ)
3.3
10
30.1
10
63.4
5
10
51.1
10
100


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