FAN5361 Fairchild Semiconductor, FAN5361 Datasheet - Page 14

• Manufacturer Part Number: FAN5361
• Description: The FAN5361 is a 600mA or 750mA, step-down, switching voltage regulator that delivers a fixed output from an input voltage supply of 2
• Manufacturer: Fairchild Semiconductor

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:
The maximum average load current, I
the peak current limit, I
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 FAN5361 is optimized for operation with L = 470nH, but is
stable with inductances up to 1.2H (nominal). Up to
2.2H(nominal) may be used; however, in that case, V
be greater than or equal to 2.7V. 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 2. Effects of Changes in Inductor Value (from 470nH Recommended Value) on Regulator Performance
Table 3. Recommended Passive Components and their Variation Due to DC Bias
Component Description
Note:
6.
© 2008 Fairchild Semiconductor Corporation
FAN5361 • Rev. 1.1.2
I
I

MAX
DCM
I

C
Higher inductance values are also acceptable. See “Selecting the Inductor” instructions in Applications Information.
(
V
LOAD
C
L1

V
OUT
OUT
IN
IN
Inductor Value

2
I
)

Decrease

Increase




I
V
LIM
IN
470nH, 2012,
L
2.2F, 6.3V,
4.7F, X5R,
90m,1.1A
(
X5R, 0402

PK

f
V
SW
0402
)
OUT

DCM
LIM(PK)

2
I




, is:
by the ripple current, given by:
Murata or Equivalent GRM155R60G475M
Hitachi Metals HSLI-201210AG-R47
LIM(PK)
Murata LQM21PNR54MG0
I
Murata LQM21PNR47MC0
MAX(LOAD),
Decrease
MAX(LOAD)
Increase
GRM155R60E475ME760
GRM188R60J225KE19D
GRM155R60J225ME15
Murata or Equivalent
.
Vendor
is related to
IN
must
(5)
(6)
(7)
14
Decrease
Increase
∆V
The increased RMS current produces higher losses through
the R
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 2 shows the effects of inductance higher or lower than
the recommended 470nH on regulator performance.
Output Capacitor
Table 3 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
therefore be increased to reduce output voltage ripple or to
improve transient response. Output voltage ripple, ∆V
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 C
reduce ringing that can occur between the inductance of the
power source leads and C
The effective capacitance value decreases as V
due to DC bias effects.
I

RMS
V
OUT
OUT
DS(ON)



I
300nH 470nH
OUT
1.0F 2.2F
1.6F 4.7F
of the IC MOSFETs, as well as the inductor DCR.
I
Min.

OUT




(
DC
8

)
has no effect on loop stability and can
C
2
Typ.
OUT

1

12
I

2
f
SW
IN
Max.
Transient Response
520nH
.
2.4F
5.2F
IN

and the power source lead to
ESR
(6)
Degraded
Improved




Decrease primarily due
Decrease primarily due
Minimum value occurs
elevated temperature
to DC bias (V
at maximum current
to DC bias (V
Comment
www.fairchildsemi.com
IN
increases
OUT
IN
OUT
) and
, is:
)
(8)
(9)