FAN8303 Fairchild Semiconductor, FAN8303 Datasheet - Page 10

• Manufacturer Part Number: FAN8303
• Description: The FAN8303 is a monolithic, non-synchronous, stepdown (buck) regulator with internal power MOSFETs
• Manufacturer: Fairchild Semiconductor

© 2008 Fairchild Semiconductor Corporation
FAN8303 • Rev. 1.0.0
Design example
Assume the V
The maximum load current is 2A and the output voltage
is set to 2.5V at 2A maximum load. Calculate the
inductor value from the following formula:
Substituting V
f
A 15µH inductor is chosen for this application.
If the V
R3 can be calculated from:
Choose R3=5.6kΩ (1%).
In this application, with the desired crossover frequency
at 30kHz, R
If R
If R
C
Table 1. Recommended Compensation Values
(V
1.8V
2.5V
3.3V
S
V
5V
C
L
L
R
R
C
= 370kHz in the formula gives:
IN
O
3
C
C
= 0.965nF, choose 1nF for the design.
=
=
C
C
=12V)
=
=
=22.72kΩ , choose 22kΩ for the design.
=22kΩ , use the following equation to get C
370
=
f
OSC
18
2
π
10µH
15µH
15µH
22µH
2
OUT
π
k
kHz
A
L
⋅
Ω
V
22
⋅
2
/
⋅
⎛
⎜
⎝
22
OUT
5 .
V
Δ
2
(
voltage is 2.5V, choose R2=18kΩ(1%), and
k
C
0
I
5 .
⋅
μ
Ω
L
4 .
380
value is calculated as follows:
2
0
F
,
OUT
MLCC
MAX
−
6 .
⋅
C
22µF
IN
A
30
⋅
0
OUT
)
30
6 .
μ
=2.5V, V
voltage is 12V with a 10% tolerance.
⎛
⎜
⎝
kHz
A
1
kHz
⎞
⎟
⎠
⎛
⎜
⎜
⎝
−
/
1
=
V
2
−
12
. 5
5 .
⋅
⋅
V
0
68
18k Ω
2
V
⎞
⎟
⎠
6 .
IN
5 .
R
OUT
k
=
IN,MAX
V
,
V
2
Ω
MAX
13
μ
H
=12V, Δ I
⎞
⎟
⎟
⎠
2.45k Ω
5.6k Ω
9k Ω
4k Ω
R
3
L,MAX
16k Ω
22k Ω
27k Ω
43k Ω
R
=0.4A, and
C
C
:
820pF
560pF
1.5nF
(14)
(15)
(16)
(17)
(18)
1nF
C
C
10
Layout Consideration
As with all switching power supplies, careful attention to
PCB layout is important to the design. A few design
rules should be implemented to ensure good layout:
Keep the high-current traces and load connections
as short as possible.
Place the input capacitor, the inductor, the
freewheeling diode, and the output capacitor as
close as possible to the IC terminals.
Keep the loop area between the SW node,
freewheeling diode, inductor, and output capacitor
as small as possible. Minimizing ground loops
reduces EMI issues.
Route high-dV/dt signals, such as SW node, away
from the error amplifier input/output pins. Keep
components connected to these pins close to the
pins.
To effectively remove heat from the MOSFETs, use
wide land areas with appropriate thermal vias.
Figure 17. Recommended PCB Layout
www.fairchildsemi.com