FAN5063 Datasheet, PDF (11/14 Pages) Fairchild Semiconductor – ACPI Dual Switch Controller

PRODUCT SPECIFICATION
FAN5063
Softstart
Pin 10 of the FAN5063 functions as a softstart. When power
is first applied to the chip, a constant current is applied from
the pin into an external capacitor, linearly ramping up the
voltage. This ramp in turn controls the internal reference of
the FAN5063. providing a softstart for the linear regulators.
The actual state of the FAN5063 on power up will be deter-
mined by the state of its control lines.
The switches in the system must be either on or off, and so
softstart has no effect on their characteristics: if the appropri-
ate control signals are asserted, they will turn on at once.
The softstart is effective only during power on. During a
transition between states, such as from S5 → S0, the linear
regulators are not softstarted.
It is important to note that the softstart pin is not an enable;
pulling it low will not necessarily turn off all outputs.
Charge Pump
In main power operation, the FAN5063 is run from the +12V
main supply. This supply also provides voltage to the various
MOSFET gates. However, during standby, this supply is off.
To provide power to the chip and the appropriate gates, the
FAN5063 incorporates a free-running charge pump. As
shown in Figure 4, and in the block diagram on the front
page, a capacitor attached between pins 1 and 2 of the
FAN5063 acts as a charge pump with internal diodes. The
charge pump output is internally diode or’red with the 12V
input. The 12V input must have a series diode to prevent
back-feeding the charge pump to the + 12V main when in
standby. The 12V input line needs a bypass capacitor for
high-frequency noise rejection.
Overcurrent
The FAN5063 does not directly detect current through the
devices that power its outputs. Instead, it monitors the output
voltages. In the event of a hard short, the voltage drops
below 80% of nominal, and all outputs are latched off, and
remain off until 5V standby power is recycled. The overcur-
rent latch off is delayed by 150µsec to prevent nuisance trips.
During softstart, the overcurrent voltage monitors are kept
proportional to the reference, to avoid tripping overcurrent
during startup.
In the S5 state, when the memory outputs are off, the voltage
monitors on the memory lines are disabled, to prevent trip-
ping the overcurrent. When turning these lines back on from
the S5 state, overcurrent is prevented from tripping because
the S3 state is blocked. See Table 2.
If the adjustable dual is not used, its feedback line, pin 12,
must be connected to 5V STBY, to prevent an overcurrent
trip.
UVLO
If the +5V standby is below approximately 4.5V, the
FAN5063 will leave off or turn off all outputs. Similar com-
ments apply to the +12V main at 7.5V. The +5V standby
UVLO has approximately 0.5V hysteresis, the +12V main
UVLO 1V.
Over Temperature
The FAN5063 is capable of sourcing substantial current,
200mA minimum to the adjustable voltage transistor’s base dur-
ing S0 and 144mA to the line during S3. As a result, there can
be heavy power dissipation in the IC. While the FAN5063 is
designed to accept this power dissipation, any overloading of
outputs can cause excessive heating. If the FAN5063 die
temperature exceeds about 150°, all outputs are shut off.
Outputs remain off until the die temperature returns to its
safe area.
Transistor Selection
External transistor selection depends on usage, differing for
the linear regulators and the switches.
The MOSFET switches, should be sized based on regulation
requirements and power dissipation. Since the ATX outputs
are ±5%, the outputs driven from them must be wider. As an
example, if we want to hold 3.3V PCI to -10%, we can drop
only 5% = 165mV across Q1. At 2.4A, this means Ql must
have a maximum R_DS,on of 165mV/2.7A = 68mΩ, including
tolerance and self-heating effects. We thus choose a Fairchild
FDC633N, which has 72mΩ maximum R_{DS, on} at 4.5V V_GS
at 25°C. We can estimate power dissipation as (2.4A)² *
42mΩ = 270mW, which should be acceptable for this pack-
age.
Q2 is a MOSFET functioning as a linear regulator. Since it
delivers only 500mA, it is easy to select a MOSFET, it need
only be able to handle 500mA * (5V + 5% – 3.3V) = 1W.
We select the Fairchild FDS6630A in an SO-8 package.
Q3 is an NPN bipolar functioning as a linear regulator. As
already discussed, it must have a V_CE,sat lower than 1.45V at
I_E = 2A and I_B = 200mA. Its power dissipation can be as
high as (5V + 5%–3.3V) * 2A = 3.9W.
REV. 1.0.0 12/4/00
11