FAN5350_08 FAIRCHILD [Fairchild Semiconductor], FAN5350_08 Datasheet - Page 7

• Manufacturer Part Number: FAN5350_08
• Description: 3MHz, 600mA Step-Down DC-DC Converter in Chip-Scale and MLP Packaging
• Manufacturer: FAIRCHILD [Fairchild Semiconductor]

© 2007 Fairchild Semiconductor Corporation
FAN5350 Rev. 1.0.3
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:
The maximum average load current, I
to the peak current limit, I
ripple current:
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
The FAN5350 is optimized for operation with L=1μH,
but is stable with inductances ranging from 700nH to
3.0μH. The inductor should be rated to maintain at least
80% of its value at I
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.
Table 1. Effects of changes in inductor value (from 1µH recommended value) on regulator performance
I
I
I
Δ
MAX
RMS
Inductor Value
DCM
I
≈
(
Decrease
=
Increase
V
LOAD
=
V
OUT
IN
Δ
2
I
OUT
I
)
•
=
(
DC
⎛
⎜
⎜
⎝
I
LIM
V
)
2
IN
L
(
+
PK
•
−
Δ
F
12
)
LIM(PK)
V
I
SW
2
−
OUT
Δ
I
2
MAX(LOAD)
I
.
⎞
⎟
⎟
⎠
Decrease
LIM(PK)
Increase
(see figure 17) by the
EQ. 2
DCM
MAX(LOAD)
, is:
Decrease
Increase
is related
I
LIM(PK)
(1)
(2)
(3)
(4)
7
The increased RMS current produces higher losses
through the R
inductor ESR.
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.
Table 1 shows the effects of inductance higher or lower
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 lower than 0603. This
improves the transient response and output ripple.
Increasing C
therefore be increased to reduce output voltage ripple
or to improve transient response. Output voltage ripple,
∆V
Input Capacitor
The 4.7μF 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 C
and the power source lead to reduce ringing that can
occur between the inductance of the power source
leads and C
Δ
V
OUT
OUT
∆V
, is:
Decrease
Increase
=
OUT
Δ
I
•
IN
EQ. 5
OUT
⎛
⎜
⎜
⎝
.
DS(ON)
8
•
has no effect on loop stability and can
C
OUT
of the IC MOSFETs as well as the
1
•
F
SW
Transient Response
+
ESR
Degraded
⎞
⎟
⎟
⎠
Improved
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(
5)
IN