FAN4855 Fairchild Semiconductor, FAN4855 Datasheet - Page 10

• Manufacturer Part Number: FAN4855
• Description: The FAN4855 is a low power boost regulator designed for low voltage DC to DC conversion in 2-cell battery powered systems such as digital cameras, cell phones and PDAs
• Manufacturer: Fairchild Semiconductor

FAN4855 Rev. 1.0.1
Layout and Grounding Considerations
Careful design of printed circuit board is recommended
since high frequency switching and high peak currents
are present in DC/DC converters applications. A general
rule is to place the converter circuitry well away from any
sensitive analog components. The printed circuit board
layout should be based on some simple rules to minimize
EMI and to ensure good regulation performances:
1.
2.
3.
4.
5.
6.
Place the IC, inductor, input and output capacitor as
close together as possible.
Keep the output capacitor as close to the FAN4855
as possible with very short traces to V
pins. Typically it should be within 0.25 inches or 6
mm.
Keep the traces for the power components wide,
typically > 50 mils or 1.25 mm.
Place the external networks for LBI and FB close to
FAN4855, but away from the power components as
far as possible to prevent voltage transient from cou-
pling into sensitive nodes.
On multilayer boards use component side copper for
grounding around the IC and connect back to a quiet
ground plane using vias. The ground planes act as
electromagnetic shields for some of the RF energy
radiated.
The connection of the GND pin of the IC (pin 8) to
the overall grounding system should be directly to
the bottom of the output filter capacitor. A star
grounding system radiating from where the power
enters the PCB, is a recommended practice.
OUT
and GND
10
Application Example
The FAN4855 can be used as a constant current source
to drive white LEDs like QTLP670C-IW. As shown in the
diagram below, the current is maintained constant over a
wide range of input voltages.
19.8
19.6
19.4
19.2
18.8
18.6
18.4
18.2
+
20
19
18
Cin
10µF
0
+
1
2
3
4
1
FAN4855
L = 10µH
+
Cout
10µF
Input Voltage (V)
2
8
7
6
5
3
D1
D2
D3
D4
62Ω
62Ω
62Ω
62Ω
www.fairchildsemi.com
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