FAN5350 データシートの表示（PDF） - ON Semiconductor
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FAN5350 Datasheet PDF : 15 Pages
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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 the average current limit, the
PWM-to-PFM transition point, the output voltage ripple, and
the efficiency.
The ripple current (∆I) of the regulator is:
∆I ≈
VOUT
VIN
•
VIN
L
− VOUT
• FSW
(1)
The maximum average load current, IMAX(LOAD) is related to
the peak current limit, ILIM(PK) (see figure 17) by the ripple
current:
IMAX(LOAD)
= ILIM(PK)
−
∆I
2
(2)
The transition betw een PFM and PWM operation is
determined by the point at w hich the inductor valley
current crosses zero. The regulator DC current w hen the
inductor current crosses zero, IDCM, is:
IDCM
=
∆I
2
(3)
The FAN5350 is optimized for operation w ith L=1µH, but is
stable w ith inductances ranging from 700nH to 3.0µH. 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 since ∆I increases,
the RMS current increases, as do the core and skin effect
losses.
IRMS =
IOUT(DC) 2
+
∆I2
12
(4)
The increased RMS current produces higher losses
through the RDS(ON) of the IC MOSFETs as w ell as the
inductor ESR.
Increasing the inductor value produces low er RMS
currents, but degrades transient response. For a given
physical inductor size, increased inductance usually
results in an inductor w ith low er saturation current.
Table 1 show s the effects of inductance higher or low er
than the recommended 1µH on regulator performance.
Output Capacitor
Table 2 suggests 0603 capacitors. 0805 capacitors may
further improve performance in that the effective
capacitance is higher and ESL is low er than 0603. This
improves the 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
=
∆I
•
8
•
1
COUT
•
FSW
+ ESR
(5)
Input Capacitor
The 4.7µF ceramic input capacitor should be placed as
close as possible betw een the VIN pin and GND to
minimize the parasitic inductance. If a long w ire is used to
bring pow er to the IC, additional “bulk” capacitance
(electrolytic or tantalum) should be placed betw een CIN
and the pow er source lead to reduce ringing that can
occur betw een the inductance of the pow er source leads
and CIN.
Table 1. Effects of changes in inductor value (from 1µH recom m ended value) on regulator perform ance
Inductor Value
Increase
Decrease
IMAX(LOAD) EQ. 2
Increase
Decrease
ILIM(PK)
Decrease
Increase
∆VOUT EQ. 5
Decrease
Increase
Transient Response
Degraded
Improved
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