FAN5235QSC Fairchild Semiconductor, FAN5235QSC Datasheet - Page 12

• Manufacturer Part Number: FAN5235QSC
• Description: IC CTRLR DC/DC MOBILE PC 24QSOP
• Manufacturer: Fairchild Semiconductor

Specifications of FAN5235QSC

• Applications: Converter, Mobile PCs
• Voltage - Input: 5.4 ~ 24 V
• Number Of Outputs: 5
• Voltage - Output: 3.3V, 5V, 12V
• Operating Temperature: -20°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 24-QSOP
• Output Voltage: 3.3 V, 5 V
• Output Current: 50 mA
• Input Voltage: 5.4 V to 24 V
• Mounting Style: SMD/SMT
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 20 C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: FAN5235QSC_NL; FAN5235QSC_NL

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
MOSFET 1 – (V
are tabulated in Table 2:
Table 2. MOSFET Duty Cycles
High-side FET
Low-side FET
All four MOSFETs have maximum duty cycles greater than
50%. Thus, it is necessary to size all four approximately the
same.
12
3.3V
3.3V
V
V
5V
5V
out
out
out
out
/ V
/ V
in
5.4V
5.4V
.61
.43
.39
.07
) of the time. The maxima and minima
in
) of the time, and the low-side
V
V
in
in
24V
24V
.14
.21
.86
.79
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 * (17.5K / 10K ) / 35m = 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.
REV. 1.3.3 1/3/02