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MP9447 Datasheet(PDF) 14 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) 14 Page - Monolithic Power Systems

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MP9447–HIGH-EFFICIENCY, FAST-TRANSIENT, SYNCHRONOUS, STEP-DOWN CONVERTER
MP9447 Rev. 1.2
www.MonolithicPower.com
14
1/5/2016
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2016 MPS. All Rights Reserved.
APPLICATION INFORMATION
Setting the Output Voltage
A resistor divider from the output voltage to the
FB pin set VOUT.
Without
an
external
ramp
employed,
the
feedback resistors (R1 and R2) set the output
voltage. To determine the values for the resistors,
first, choose R2 (typically 5kΩ-40kΩ). Then R1 is:
OUT
REF
REF
VV
R1
R2
V

(10)
When using a low-ESR ceramic capacitor on the
output, add an external voltage ramp to the FB
pin through R4 and C4. The ramp voltage (VRAMP)
affects output voltage. Calculate VRAMP as per
equation 19. Choose R2 between 5kΩ and 40kΩ.
Determine R1 as:
1
4
2
1
1
2
1
2
1
R
)
V
V
V
(
R
V
V
R
RAMP
REF
OUT
RAMP
REF
(11)
Using equation 11 to calculate the output voltage
can be complicated. Furthermore, as VRAMP
changes due to changes in VOUT and VIN, VFB also
varies. To improve the output voltage accuracy
and simplify the R2 calculation from equation 11,
add a DC-blocking capacitor (CDC). Figure 9
shows a simplified circuit with external ramp
compensation and a DC-blocking capacitor.
Equation 10 can then estimate R1)
Select a CDC value between 1µF and 4.7μF to
improve DC-blocking performance.
Cdc
Figure 9: Simplified Circuit with External Ramp
Compensation and DC Blocking Capacitor
Input Capacitor
The input current to the step-down converter is
discontinuous,
and
Therefore
requires
a
capacitor to supply the AC current to the step-
down converter while maintaining the DC input
voltage. Ceramic capacitors are recommended
for best performance. Be sure to place the input
capacitors as close to the IN pin as possible.
The
capacitance
varies
significantly
with
temperature. Capacitors with X5R and X7R
ceramic dielectrics are are fairly stable over
temperature fluctuations.
The capacitors must also have a ripple-current
rating greater than the converter’s maximum
input-ripple current. The input ripple current can
be estimated as follows:
OUT
OUT
CIN
OUT
IN
IN
VV
II
(1
)
VV

 
(12)
The worst-case condition occurs at VIN = 2VOUT,
where:
OUT
CIN
I
I
2
(13)
For simplification, choose an input capacitor
whose RMS current rating is 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 design, choose an
input capacitor that meets the specification
The input voltage ripple can be estimated as
follows:
OUT
OUT
OUT
IN
SW
IN
IN
IN
IV
V
V(1
)
FC
V
V

 
(14)
The worst-case condition occurs at VIN = 2VOUT,
where:
OUT
IN
SW
IN
I
1
V
4F
C
 
(15)


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