FAN5358 Datasheet, PDF (11/13 Pages) Fairchild Semiconductor – 2MHz, 500mA, SC70 Synchronous Buck Regulator

PCB Layout Considerations
There are only three external components: the inductor and
the input and output capacitors. For any buck regulator IC,
including the FAN5358, it is important to place a low-ESR
input capacitor very close to the IC, as shown in Figure 21.
The input capacitor ensures good input decoupling, which
helps reduce noise appearing at the output terminals and
ensures that the control sections of the IC do not behave
erratically due to excessive noise. This reduces switching
cycle jitter and ensures good overall performance. It is
important to place the common GND of CIN and COUT as close
as possible to any of the FAN5358 GND terminals. There is
some flexibility in moving the inductor further away from the
IC; in that case, VOUT should be considered at the COUT
terminal.
Output Capacitor
Table 4 suggests 0402 capacitors. 0603 capacitors may
further improve performance in that the effective capacitance
is higher. This improves the transient response and output
ripple as shown in Table 3.
Increasing COUT has no effect on loop stability and can
therefore be increased to reduce output voltage ripple or to
improve transient response. Output voltage ripple, ∆VOUT, is:
ΔV_OUT
=
ΔI •
⎜⎜⎝⎛
8
•
1
C_OUT
•
f_SW
+ ESR _{⎟⎟⎠⎞}
(5)
Figure 21. PCB Layout Guidance
Input Capacitor
The 2.2μF ceramic input capacitor should be placed as close
as possible between the VIN pin and GND to minimize the
parasitic inductance. If a long wire is used to bring power to
the IC, additional “bulk” capacitance (electrolytic or tantalum)
should be placed between CIN and the power source lead to
reduce ringing that can occur between the inductance of the
power source leads and CIN.
The effective capacitance value decreases as VIN increases
due to DC Bias effects. This has no significant impact on
regulator performance.
Table 3. Effects of Changes in Inductor Value (from Recommended Value) on Regulator Performance
Inductor Value
Increase
I_MAX(LOAD)
Increase
I_LIM(PK)
Decrease
∆V_OUT(5⁾
Decrease
Transient Response
Degraded
Decrease
Decrease
Increase
Increase
Improved
Table 4. Recommended Passive Components and Their Variation Due to DC Bias
Component
Description
Vendor
Min. Typ. Max.
Comment
L1
COUT
CIN
2.2μH, 2520, 100mΩ,1.3A
FDK MIPF2520D
2.2μH, 2520, 80mΩ,1.3A
4.7μF, X5R, 0402
2.2μF, X5R, 0402
Hitachi Metal:KSLI -
252010AG-2R2
Murata: LQM31PN2RM00L
TOKO: MDT2520CN2R2M
Murata or Equivalent
GRM155R60G475M
GRM155R60E475ME760
Murata or Equivalent
GRM155R60J225ME15
GRM188R60J225KE19D
1.5μH
1.6μF
1.0μF
2.2μH
4.7μF
2.2μF
5.2μF
2.4μF
Minimum value occurs
at maximum current
Decrease primarily due
to DC bias (VOUT)
Decrease primarily due
to DC bias (VIN) and
elevated temperature
© 2009 Fairchild Semiconductor Corporation
FAN5358 • Rev. 1.0.1
11
www.fairchildsemi.com