FAN2013 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, V_REF, is 0.8V. The output
is divided down by a voltage divider, R1 and R2 to the FB
pin. The output voltage is:
V_OUT
=
V_REF
⎛
⎝
1
+
RR-----_{12-⎠⎞}
EQ 2
According to this equation, assuming desired output volt-
age of 1.2V, and given R2 = 10KΩ, the calculated value
of R1 is 5KΩ.
Inductor Selection
The inductor parameters directly related to device perfor-
mance 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:
ΔI_L
=
V_OUT
×
1_{-----–-----}(_--V_{-----O---U----T-----}⁄_---V_----I--N----)
L×f
where:
ΔI_L = Inductor Ripple Current
f = Switching Frequency
L = Inductor Value
EQ 3
Recommended inductors are listed in Table 1.
Inductor Value
2.2µH
2.2µH
Vendor
Coiltronics
Murata
Part Number
SD25 2R2
LQH66SSN2R2M03
PCB Layout Recommendations
The inherently high peak currents and switching fre-
quency of power supplies require 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 recom-
mended PCB layout is shown in Figure 11.
Table 1: Recommended Inductors
Capacitors Selection
For best performance, a low-ESR input capacitor is
required. A ceramic capacitor of at least 10µF, placed as
close to the V_IN and AGND pins as possible, is recom-
mended.
The output capacitor determines the output voltage rip-
ple and the transient response. A minimum 20µF output
capacitor is required for the FAN2013 to operate in sta-
ble conditions.
Capacitor
Value
Vendor
10µF Taiyo Yuden
TDK
Murata
Part Number
JMK212BJ106MG
JMK316BJ106KL
C2012X5ROJ106K
C3216X5ROJ106M
GRM32ER61C106K
Table 2: Recommended Capacitors
Figure 11. Recommended PCB Layout
© 2006 Fairchild Semiconductor Corporation
FAN2013 Rev. 1.0.0
7
www.fairchildsemi.com