© 2010 Fairchild Semiconductor Corporation www.fairchildsemi.com
FAN53600 / FAN53610 • Rev. 1.0.0 13
FAN53600 / FAN53610 — 3MHz, 600mA / 1A Synchronous Buck Regulator
Applications Information
Selecting the Inductor
The output inductor must meet both the required inductance
and the energy handling capability of the application. The
inductor value affects average current limit, the PWM-to-
PFM transition point, output voltage ripple, and efficiency.
The ripple current (∆I) of the regulator is:
•
−
•≈Δ
SW
OUTIN
IN
OUT
fL
VV
V
V
I
(6)
The maximum average load current, I
MAX(LOAD),
is related to
the peak current limit, I
LIM(PK)
, by the ripple current, given by:
2
)()(
I
II
PKLIMLOADMAX
Δ
−=
(7)
The transition between PFM and PWM operation is
determined by the point at which the inductor valley current
crosses zero. The regulator DC current when the inductor
current crosses zero, I
DCM
, is:
2
I
I
DCM
Δ
=
(8)
The FAN53600/10 is optimized for operation with L = 1
μH,
but is stable with inductances up to 2.2
μH (nominal). The
inductor should be rated to maintain at least 80% of its value
at I
LIM(PK)
.
Efficiency is affected by inductor DCR and inductance value.
Decreasing the inductor value for a given physical size
typically decreases DCR; but since ∆I increases, the RMS
current increases, as do the core and skin effect losses:
12
I
I I
2
2
)DC(OUTRMS
Δ
+=
(9)
The increased RMS current produces higher losses through
the R
DS(ON)
of the IC MOSFETs, as well as the inductor DCR.
Increasing the inductor value produces lower RMS currents,
but degrades transient response. For a given physical
inductor size, increased inductance usually results in an
inductor with lower saturation current and higher DCR.
Table 1 shows the effects of inductance higher or lower than
the recommended 1
μH on regulator performance.
Output Capacitor
Table 2 suggests 0402 capacitors. 0603 capacitors may
further improve performance in that the effective capacitance
is higher. This improves transient response and output ripple.
Increasing C
OUT
has no effect on loop stability and can
therefore be increased to reduce output voltage ripple or to
improve transient response. Output voltage ripple, ∆V
OUT
, is:
()
⋅⋅
+
−⋅⋅
⋅⋅
Δ=Δ
OUTSW
2
OUTSW
LOUT
Cf8
1
D1D2
ESRCf
IV
(10)
Input Capacitor
The 2.2 μF ceramic input capacitor should be placed as
close as possible between the VIN pin and GND to minimize
parasitic inductance. If a long wire is used to bring power to
the IC, additional “bulk” capacitance (electrolytic or tantalum)
should be placed between C
IN
and the power source lead to
reduce ringing that can occur between the inductance of the
power source leads and C
IN
.
The effective capacitance value decreases as V
IN
increases
due to DC bias effects.
Table 1. Effects of Changes in Inductor Value (470 nH Recommended Value) on Regulator Performance
Inductor Value I
MAX(LOAD)
∆V
OUT
Transient Response
Increase Increase Decrease Degraded
Decrease Decrease Increase Improved
Table 2. Recommended Passive Components and Variation Due to DC Bias
Component Description Vendor Min. Typ. Comment
L1
1
μH, 2012, 190 mΩ,
0.8 A
Murata LQM21PN1R0MC0
1
μH
Not recommended for 1A load
1 μH, 1.4 A, 85 mΩ,
2016
Murata LQM2MPN1R0M
1
μH
Utilized to generate graphs,
Figure 4 — Figure 36
C
IN
2.2
μF, 6.3 V, X5R,
0402
Murata or Equivalent
GRM155R60J225ME15
GRM188R60J225KE19D
1.0
μF 2.2 μF
Decrease primarily due to DC bias
(V
IN
) and elevated temperature
C
OUT
4.7
μF, X5R 0603
Murata or Equivalent
GRM188R60G106ME47D
1.6 μF 4.7 μF
Decrease primarily due to DC bias
(V
OUT
) and elevated temperature