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FAN53611AUC11X View Datasheet(PDF) -

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FAN53611AUC11X
 
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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:
I VOUT
VIN

VIN
L
VOUT
fSW

(5)
The maximum average load current, IMAX(LOAD), is related to
the peak current limit, ILIM(PK), by the ripple current, given by:
IMAX(LOAD)
ILIM(PK )
I
2
(6)
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, IDCM, is:
IDCM
I
2
(7)
The FAN53601/11 is optimized for operation with L = 470 nH,
but is stable with inductances up to 1H (nominal). The
inductor should be rated to maintain at least 80% of its value
at ILIM(PK).
Efficiency is affected by the inductor DCR and inductance
value. Decreasing the inductor value for a given physical size
typically decreases the DCR; but because I increases, the
RMS current increases, as do the core and skin effect losses.
IRMS
IOUT(DC ) 2
I2
12
(8)
The increased RMS current produces higher losses through
the RDS(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 H 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 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:
VOUT
IL

fSW
2
COUT
D 1
ESR2
D
1
8 fSW COUT

(9)
Input Capacitor
The 2.2 F 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 the 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.
Table 1. Effects of Changes in Inductor Value (from 470nH Recommended Value) on Regulator Performance
Inductor Value
Increase
Decrease
IMAX(LOAD)
Increase
Decrease
VOUT
Decrease
Increase
Transient Response
Degraded
Improved
Table 2. Recommended Passive Components and their Variation Due to DC Bias
Component Description
Vendor
470 nH,
L1
2012,90 m,
1.1 A
Murata LQM21PNR47MC0
Murata LQM21PNR54MG0
Hitachi Metals HLSI 201210R47
CIN
2.2 F, 6.3 V,
X5R, 0402
Murata or Equivalent GRM155R60J225ME15
GRM188R60J225KE19D
COUT
4.7 F, X5R,
0402
Murata or Equivalent GRM155R60G475M
GRM155R60E475ME760
Min. Typ. Max.
300 nH 470 nH 520 nH
1.0 F 2.2 F
1.6 F 4.7 F
© 2010 Fairchild Semiconductor Corporation
FAN53601 / FAN53611 • Rev. 1.0.1
11
www.fairchildsemi.com
 

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