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FR9809CPGTR Datasheet(PDF) 11 Page - Fitipower Integrated Technology Inc.

Part No. FR9809CPGTR
Description  21V, 5A, 500KHz Synchronous PWM-Buck DC/DC Converter
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Maker  FITIPOWER [Fitipower Integrated Technology Inc.]
Homepage  http://www.fitipower.com/en_US/index.asp
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 11 page
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11
FR9809-Preliminary 0.4-OCT-2012
FR9809
85T
fitipower integrated technology lnc.
Application Information (Continued)
+
VNOISE (t)
=
VRIPPLE(t)
R PPL (
R, p p =
T
F
L
1
T
R
R PPL (
L, p p =
L
L+
L
R PPL ( , p p =
T
8 F
2
L
T
1
T
Where FOSC is the switching frequency, L is the
inductance value, VIN is the input voltage, ESR is the
equivalent series resistance value of the output
capacitor, ESL is the equivalent series inductance
value of the output capacitor and the COUT is the
output capacitor.
Low ESR capacitors are preferred to use.
Ceramic,
tantalum or low ESR electrolytic capacitors can be
used depending on the output ripple requirements.
When using the ceramic capacitors,
the ESL
component is usually negligible.
It is important to use the proper method to eliminate
high frequency noise when measuring the output
ripple.
The figure shows how to locate the probe
across the capacitor when measuring output ripple.
Remove the scope probe plastic jacket in order to
expose the ground at the tip of the probe.
It gives a
very short connection from the probe ground to the
capacitor and eliminates noise.
Inductor Selection
The output inductor is used for storing energy and
filtering output ripple current.
But the trade-off
condition often happens between maximum energy
storage and the physical size of the inductor.
The
first consideration for selecting the output inductor
is to make sure that the inductance is large enough
to keep the converter in the continuous current
mode.
That will lower ripple current and result in
lower output ripple voltage.
The
Δ
L is inductor
peak-to-peak ripple current:
L=
T
F
L
1
T
The following diagram is an example to graphical
represent
Δ
L equation.
VIN=12V, FOSC=500KHz
A good compromise value between size and
efficiency is to set the peak-to-peak inductor ripple
current Δ
L
equal to 30% of the maximum load
current.
But setting the peak-to-peak inductor
ripple current Δ
L
between 20%~50% of
the
maximum load current is also acceptable.
Then
the inductance can be calculated with the following
equation:
L=0.
T(MA
L=
T
T
F
L
To guarantee sufficient output current, peak inductor
current must be lower than the FR9809 high-side
MOSFET current limit.
The peak inductor current
is shown as below:
P AK=
T(MA +
L
2
Ceramic Capacitor
VOUT
GND
Probe Ground
L=1.8
μ
L=2.2
μ
L=4.7
μ
(t)




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