FAN2013_08 Datasheet, PDF (7/9 Pages) Fairchild Semiconductor – 2A Low-Voltage, Current-Mode Synchronous PWM Buck Regulator

Applications Information
Setting the Output Voltage
The internal voltage reference is 0.8V. The output is
divided down by a voltage divider, R1 and R2 to the FB
pin. The output voltage is:
V
=V
1 + -R---¹-
O UT REF
R
(2)
2
According to this equation, assuming desired output
voltage of 1.2V, and given R2 = 10KΩ as the
recommended resistance for any output voltage setting,
the calculated value of R1 is 5KΩ.
Inductor Selection
The inductor parameters directly related to device
performance are saturation current and DC resistance.
The FAN2013 operates with a typical inductor value of
2.2µH. The lower the DC resistance, the higher the
efficiency. For saturation current, the inductor should be
rated higher than the maximum load current, plus half of
the inductor ripple current, calculated by:
∆ ^I_L ^{= V}O U T × ^{1----–-----(----V---O}-L---^U-_×---^T--f---^⁄----V---^I--^N---)-
(3)
where:
ΔIL = Inductor Ripple Current
f = Switching Frequency
L = Inductor Value
Recommended inductors are listed in Table1.
Table 1. Recommended Inductors
Inductor
Value
Vendor
Part Number
2.2µH
Coiltronics
SD25 2R2
2.2µH
Murata
LQH66SSN2R2M03
Capacitors Selection
For best performances, a low-ESR input capacitor is
required. A ceramic capacitor of at least 10µF, placed
as close to the VIN and AGND pins as possible is
recommended.
The output capacitor determines the output ripple and
the transient response. A minimum of 20µF output
capacitor is required for the FAN2013 to operate in
stable conditions.
Table 2. Recommended Capacitors
Capacitor
Value
Vendor
Part Number
Taiyo Yuden
JMK212BJ106MG
JMK316BJ106KL
10µF
TDK
C2012X5ROJ106K
C3216X5ROJ106M
Murata
GRM32ER61C106K
PCB Layout Recommendations
The inherently high peak currents and switching
frequency of power supplies require a careful PCB
layout design. For best results, use wide traces for high-
current paths and place the input capacitor, the
inductor, and the output capacitor as close as possible
to the integrated circuit terminals. To minimize voltage
stress to the device resulting from ever-present
switching spikes, use an input bypass capacitor with low
ESR. Note that the peak amplitude of the switching
spikes depends upon the load current; the higher the
load current, the higher the switching spikes.
The resistor divider that sets the output voltage should
be routed away from the inductor to avoid RF coupling.
The ground plane at the bottom side of the PCB acts as
an electromagnetic shield to reduce EMI. The
recommended PCB layout is shown below in Figure 11.
Figure 11. Recommended PCB Layout
© 2006 Fairchild Semiconductor Corporation
FAN2013 Rev. 1.0.3
7
www.fairchildsemi.com