FAN5063 Datasheet, PDF (12/14 Pages) Fairchild Semiconductor – ACPI Dual Switch Controller

FAN5063
PRODUCT SPECIFICATION
Alternate for Adjustable Dual
Instead of the bipolar transistor shown in Figure 4 for Q3, the
linear pass element for the adjustable dual, a MOSFET and
schottky diode can be used as shown in Figure 7.
5V Main
FAN5063 ¹⁴
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Adjustable Dual
Figure 7. Adjustable Dual with MOSFET
The schottky should be chosen to have a low V_f at the speci-
fied adjustable voltage and current. The MOSFET’s R_DS,on
must then be lower than (5V -5% -VADJ -V_f)/I_Dual including
temperature. An additional constraint is that the MOSFET
must have a gate threshold voltage lower than 1.5V. For exam-
ple, for 2.8A @3.3V, choose the diode to be an MBR835, and
the MOSFET a Fairchild FDC653M. This same technique
can then also be used for adjustable currents higher than can
be achieved with the bipolar transistor.
Output Capacitor Selection
Output capacitor selection depends on whether the line has
overlap time or not.
For both the adjustable dual, there is guaranteed overlap time
between when one source is turned on and the other source
turned off. For this output, the output capacitor is not needed
to hold up the supply, but only for noise filtering and to
respond to transient loading.
The 3.3V dual output has deadtime between when one
source is turned off and the other source turned on. During
the time when both are off, the output current must be sup-
plied by the output capacitor. Mitigating this, it must be real-
ized that the system will be designed in such a way that the
current has gone to its sleep value before the transition
occurs. For example, the 3.3V dual has a sleep current of
500mA maximum. Maximum deadtime is 6µsec, and so
charge depletion is 500mA * 6µsec = 3µC. Suppose that we
have a total of 8% drop due to the source tolerance and the
MOSFET drop, and we are trying to hold 10% regulation.
The remaining 2% = 66mV implies a minimum capacitance
of 3µC/66mV = 45µF.
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