FAN5091MTC Fairchild Semiconductor, FAN5091MTC Datasheet - Page 18

• Manufacturer Part Number: FAN5091MTC
• Description: IC CTRLR DC-DC SYNC 2PH 24TSSOP
• Manufacturer: Fairchild Semiconductor

Specifications of FAN5091MTC

• Applications: Controller, Intel Pentium® IV
• Voltage - Input: 5V
• Number Of Outputs: 1
• Voltage - Output: 1.1 ~ 1.85 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 24-TSSOP
• Output Voltage: 13.2 V
• Mounting Style: SMD/SMT
• Operating Temperature Range: 0 C to + 70 C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

FAN5091
f = DC/DC converter switching frequency
ESR = Equivalent series resistance of all output capacitors in
parallel
Vripple = Maximum peak to peak output ripple voltage
budget.
One other limitation on the minimum size of the inductor is
caused by the current feedback loop stability criterion. The
inductor must be greater than:
where L is the inductance in Henries, R
resistance of one slice’s low-side MOSFET, R
value of the droop resistor in Ohms, V
and V
mula will not present any limitation on the selection of the
inductor value.
A typical value for the inductor is 1.3 H at an oscillator
frequency of 600KHz (300KHz each slice) and 220nH at an
oscillator frequency of 2MHz (1MHz each slice). For other
frequencies, use the interpolating formula
Schottky Diode Selection
The application circuits of Figures 1-3 show a Schottky
diode, D1 (D2 respectively), one in each slice. They are used
as free-wheeling diodes to ensure that the body-diodes in the
low-side MOSFETs do not conduct when the upper
MOSFET is turning off and the lower MOSFETs are turning
on. It is undesirable for this diode to conduct because its high
forward voltage drop and long reverse recovery time
degrades efficiency, and so the Schottky provides a shunt
path for the current. Since this time duration is extremely
short, being minimized by the adaptive gate delay, the selec-
tion criterion for the diode is that the forward voltage of the
Schottky at the output current should be less than the forward
voltage of the MOSFET’s body diode. Power capability is
not a criterion for this device, as its dissipation is very small.
Output Filter Capacitors
The output bulk capacitors of a converter help determine its
output ripple voltage and its transient response. It has
already been seen in the section on selecting an inductor that
the ESR helps set the minimum inductance. For most con-
verters, the number of capacitors required is determined by
the transient response and the output ripple voltage, and
these are determined by the ESR and not the capacitance
value. That is, in order to achieve the necessary ESR to meet
the transient and ripple requirements, the capacitance value
required is already very large.
18
L 3 10
o
is the output voltage. For most applications, this for-
–
10
L nH
R
DS on
930,000
-------------------- - 240
f KHz
R
Droop
–
in
DS,on
is either 5V or 12V,
V
in
–
is the on-state
Droop
2V
o
is the
The most commonly used choice for output bulk capacitors
is aluminum electrolytics, because of their low cost and low
ESR. The only type of aluminum capacitor used should be
those that have an ESR rated at 100kHz. Consult Application
Bulletin AB-14 for detailed information on output capacitor
selection.
For higher frequency applications, particularly those running
the FAN5091 oscillator at >1MHz, Oscon or ceramic capaci-
tors may be considered. They have much smaller ESR than
comparable electrolytics, but also much smaller capacitance.
The output capacitance should also include a number of
small value ceramic capacitors placed as close as possible to
the processor; 0.1µF and 0.01µF are recommended values.
Input Filter
The DC-DC converter design may include an input inductor
between the system main supply and the converter input as
shown in Figure 6. This inductor serves to isolate the main
supply from the noise in the switching portion of the DC-DC
converter, and to limit the inrush current into the input capac-
itors during power up. A value of 1.3µH is recommended.
It is necessary to have some low ESR capacitors at the input
to the converter. These capacitors deliver current when the
high side MOSFET switches on. Because of the interleaving,
the number of such capacitors required is greatly reduced
from that required for a single-slice buck converter. Figure 6
shows 3 x 1000 F, but the exact number required will vary
with the output voltage and current, according to the formula
for the two slice FAN5091, where DC is the duty cycle,
DC = Vout / Vin. Capacitor ripple current rating is a function
of temperature, and so the manufacturer should be contacted
to find out the ripple current rating at the expected opera-
tional temperature. For details on the design of an input filter,
refer to Applications Bulletin AB-16.
Design Considerations and Component
Selection
Additional information on design and component selection
may be found in Fairchild’s Application Note 59.
+5V
I
rms
Figure 6. Input Filter
1.3 H
=
I
-------- - 2DC 4DC
out
2
PRODUCT SPECIFICATION
–
REV. 1.0.0 5/10/01
2
1000 F, 16V
Electrolytic
Vin