FAN5362 Fairchild Semiconductor, FAN5362 Datasheet - Page 10

• Manufacturer Part Number: FAN5362
• Description: The FAN5362 is a 500mA or 750mA, step-down, switching voltage regulator that delivers a fixed output voltage from an input voltage supply of 2
• Manufacturer: Fairchild Semiconductor

© 2009 Fairchild Semiconductor Corporation
FAN5362 • Rev. 1.0.4
Thermal Shutdown
When the die temperature increases, due to a high load
condition and/or a high ambient temperature, the output
switching is disabled until the temperature on the die has
fallen sufficiently. The junction temperature at which the
thermal shutdown activates is nominally 150°C with a 20°C
hysteresis.
Minimum Off-Time Effect on Switching
Frequency
t
output voltage it can provide at low V
fixed switching frequency in PWM mode.
When V
The switching frequency drops when the regulator cannot
provide sufficient duty cycle at 3MHz to maintain regulation.
This occurs when V
The calculation for switching frequency is given by:
where:
where:
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
the peak current limit, I
R
R
I
V
V
f
t

OFF(MIN)
MAX
SW
SW
V
V
OUT
OUT
I
OFF
ON
IN
IN

(
MAX

(
=
V
LOAD
min
=
V
OUT

)
R
IN
R
is 35ns. This imposes constraints on the maximum
that the FAN5362 can provide, or the maximum
IN
1

DSON




DSON

35
)
t
is high, fixed switching is maintained as long as
t

SW

OFF
ns




I
LIM
(
1
_

V
MAX
_
P




(
IN
MIN
1
N
L
(

PK

)



DCR
3 ,
V
f
)
IN
DCR
)
V
SW
IN

MHz

V
OUT
is below 3.3V at nominal load currents.
f
OUT

SW
LIM(PK)

2
I
L
OUT
I
L










I
OUT

0
R
by the ripple current:
7 .
ON

.
R

OFF
V
OUT
OUT
MAX(LOAD)




while maintaining a
is related to
(6)
(7)
(4)
(5)
10
The FAN5362 is optimized for operation with L=1H, but is
stable with inductances up to 1.5H (nominal) and down to
470nH. The inductor should be rated to maintain at least
80% of its value at I
amount of DC current that the IC can deliver.
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.
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.
Inductor Current Rating
The FAN5362’s current limit circuit can allow a peak current
of 1.25A to flow through L1 under worst-case conditions. If it
is possible for the load to draw that much continuous current,
the inductor should be capable of sustaining that current or
failing in a safe manner.
Output Capacitor
While 4.7F capacitors are available in 0402 package size,
0603 capacitors are recommended due to the severe DC
voltage bias degradation in capacitance value that the
0402 exhibits.
Increasing C
therefore be increased to reduce output voltage ripple or to
improve transient response. Output voltage ripple, ∆V
If values greater than 24F of C
may fail to start. See the sections on Enable and Soft Start
for more information.
Input Capacitor
The 2.2F ceramic input capacitor should be placed as close
as possible to 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
ringing that can occur between the inductance of the power
source leads and C
I

RMS
V
OUT

DS(ON)

I
OUT

of the IC MOSFETs as well as the inductor ESR.
I
(

DC
OUT




)
8
2


has no effect on loop stability and can
C

12
IN
I
OUT
2
.
IN
1
LIM(PK)
and the power source lead to reduce

f
SW
. Failure to do so lowers the

ESR
OUT
are used, the regulator




www.fairchildsemi.com
OUT
, is:
(8)
(9)