FAN5235 Datasheet, PDF (12/15 Pages) Fairchild Semiconductor – System Electronics Regulator for Mobile PCs

FAN5235
MOSFET Selection
The notebook application circuit shown in Figure 1 is designed
to run with an input voltage operating range of 5.4-24V.
This wide input range helps determine the selection of the
MOSFETs for the 3.3V and 5V converters, since the high-side
MOSFET is on (V_out / V_in) of the time, and the low-side
MOSFET 1 – (V_out / V_in) of the time. The maxima and minima
are tabulated in Table 2:
Table 2. MOSFET Duty Cycles
High-side FET
V_out
3.3V
V_in
5.4V
24V
.61
.14
5V
.43
.21
Low-side FET
V_out
3.3V
V_in
5.4V
24V
.39
.86
5V
.07
.79
All four MOSFETs have maximum duty cycles greater than
50%. Thus, it is necessary to size all four approximately the
same.
3.3V and 5V Schottky Selection
The maximum current at which the converters operate in PFM
mode determines selection of a Schottky. In the application
shown in Figure 8, since the transition can occur at a current as
high as 28mV * (17.5KΩ / 10KΩ) / 35mΩ = 1.4A, the diode
(with 24V input) will be conducting 86% of the period (from
Table 2). It thus has an average current of 1.4A * 0.86 = 1.2A,
which requires a Schottky current rating >1A.
3.3V and 5V Inductor Selection
See Table 1.
3.3V and 5V Output Cap Selection
See Table 1.
12V Component Selection
Calculation of the inductor, diode and output capacitor for the
+12V output fly-back is complex, depending on output power
and efficiency. See Applications Bulletin AB-19 for an Excel
spreadsheet calculation tool. See Table 1 also.
Input Capacitor Selection
Input capacitor selection is determined by ripple current rating.
With two converters operating in parallel at differing duty
cycles, calculation of input ripple current is complex; see
Applications Bulletin AB-19 for an Excel spreadsheet
calculation tool.
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REV. 1.3.3 1/3/02