FAN5232 View Datasheet(PDF) - Fairchild Semiconductor
Part Name
Description
Manufacturer
FAN5232 Datasheet PDF : 9 Pages
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PRODUCT SPECIFICATION
FAN5232
The purpose of these two supplies -whose combined current
is specified to never exceed 50mA- is to provide power to the
system micro-controller (8051 class) as well as a few other
ICs needing a stand-by power. The micro-controller as well
as the other IC’s discussed here are migrating from 5V to
3.3V power at different times and we expect that some
“legacy” devices will continue to need 5V indefinitely.
5V/3.3V-ALWAYS Protections
The two internal linear regulators are current limit and
undervoltage protected. Once protection is triggered all out-
puts go off until power is recycled.
ALWAYS mode of Operation
If it is desired that the ALWAYS voltages are always ON
then the SDWN pin must be connected to VCC permanently.
This way the ALWAYS regulator comes up as soon as there
is power while the state of the switcher can be controlled via
the SDNADJ pin.
Component Selection
Switcher MOSFET Selection
The application circuit shown in Figure 1 is designed to run
with an input voltage operating range of 16–22V. This input
range helps determine the selection of the MOSFETs for the
switcher, since the high-side MOSFET can be on as much as
(Vout / Vin) = 12V / 16V = 75% of the time, and the low-side
MOSFET as much as 1 – (Vout / Vin) = 1 – (12V / 22V) =
45% of the time.
The MOSFETs have maximum duty cycles greater than
45%. Thus, it is necessary to size both approximately the
same.
Switcher Schottky Selection
In the application shown in Figure 1, the use of a SynchFET
eliminates the need of a Schottky diode for the synchronous
buck. If SynchFETs are not used, selection of a schottky is
determined by the maximum current at which the converter
operates. Select a diode whose instantaneous Vf is less than
0.75V at the maximum output current. The schottky dissi-
pates no power, because it is on for only a very small portion
of the switching cycle.
Input Capacitor Selection
Input capacitor selection is determined by ripple current
rating by the formula:
Irms = Iout DC – DC2
where Iout is the output current of the converter, and DC is
the duty cycle, DC = Vin / Vout. Capacitor ripple current
rating is a function of temperature and switching frequency,
and so the manufacturer should be contacted to find out the
ripple current rating at the expected operational temperature
and frequency.
Soft Start Capacitor selection
The recommended value of the soft start capacitor is 100nF.
This will result in roughly 20msec turn on time. The general
formula is:
CSS
=
(ISS • TSS)
---------------------------
1.125V
Where ISS is the soft start current (5µA), TSS is the soft start
delay (i.e. 20msec).
Control and Signal Circuitry
Power Good
Power Good is an open-collector signal, and is asserted when
the outputs are greater than 88% of nominal for more than
2µsec. When PWRGD goes low it will stay low for at least
2µsec.
Fault Handling
The FAN5232 has a full suite of protection against faults.
Consult Table 2 for an overview, and the individual sections
for details.
Table 2. Fault Handling
Fault
Condition
Switcher
OC Switcher Latch off
OC Always No Change
UV Switcher
UV Always
Latch off after
2µsec
No Change
UV VCC
Off
3V- and
5V-Always
No Change
Ramp Down
till UV
No Change
Latch off after
2µsec
Off
REV. 1.1.1 10/7/02
7
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