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FR9809CPGTR Datasheet(PDF) 12 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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 12 page
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12
FR9809-Preliminary 0.4-OCT-2012
FR9809
85T
fitipower integrated technology lnc.
Application Information (Continued)
Feedforward Capacitor Selection
Internal compensation function allows users saving
time in design and saving cost by reducing the
number of external components.
The use of a
feedforward capacitor C6 in the feedback network is
recommended to improve the transient response or
higher phase margin.
FR9809
VOUT
FB
R1
R2
C6
For optimizing the feedforward capacitor, knowing the
cross frequency is the first thing.
The cross
frequency (or the converter bandwidth) can be
determined by using a network analyzer.
When
getting the cross frequency with no feedforward
capacitor
identified,
the
value
of
feedforward
capacitor C6 can be calculated with the following
equation:
=
1
2
F R
1
R1
1
R1
+
1
R2
Where FCROSS is the cross frequency.
To reduce transient ripple, the feedforward capacitor
value can be increased to push the cross frequency
to higher region.
Although this can improve
transient response, it also decreases phase margin
and causes more ringing.
In the other hand, if more
phase margin is desired, the feedforward capacitor
value can be decreased to push the cross frequency
to lower region.
In general, the feedforward
capacitor range is between 10pF to 1nF.
External Diode Selection
For 5V input applications, it is recommended to add
an external bootstrap diode.
This helps improving
efficiency.
The bootstrap diode can be a low cost
one such as 1N4148.
FR9809
VIN
BST
VIN
LX
D1
1N4148
5V
C4
PCB Layout Recommendation
The device’s performance and stability
are
dramatically
affected
by
PCB
layout.
It
is
recommended to follow these general guidelines
shown as below:
1.
Place the input capacitors and output capacitors
as close to the device as possible.
The traces
which connect to these capacitors should be as
short and wide as possible to minimize parasitic
inductance and resistance.
2.
Place feedback resistors close to the FB pin.
3.
Keep the sensitive signal (FB) away from the
switching signal (LX).
4. The exposed pad of the package should be
soldered to an equivalent area of metal on the
PCB.
This area should connect to the GND
plane and have multiple via connections to the
back of the PCB as well as connections to
intermediate PCB layers.
The GND plane area
connecting to the exposed pad should be
maximized to improve thermal performance.
5. Multi-layer PCB design is recommended.
1
2
3
4
5
6
7
8
L1
LX
GND
VOUT
VIN
R2
R1
C3
Exposed
Pad
GND
C5
+
C1
C2
Figure 23. Recommended PCB Layout Diagram




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