FAN5090MTC Fairchild Semiconductor, FAN5090MTC Datasheet - Page 14

• Manufacturer Part Number: FAN5090MTC
• Description: IC CTRLR DC-DC SYNC 2PH 24TSSOP
• Manufacturer: Fairchild Semiconductor

Specifications of FAN5090MTC

• Applications: Controller, Intel Pentium® IV
• Voltage - Input: 12V
• 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: 22 V
• Output Current: 50 A
• Input Voltage: 10.8 V to 13.2 V
• Mounting Style: SMD/SMT
• Maximum Operating Temperature: + 70 C
• Minimum Operating Temperature: 0 C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

FAN5090
Component Selection
MOSFET Selection
This application requires N-channel Enhancement Mode Field
Effect Transistors. Desired characteristics are as follows:
• Low Drain-Source On-Resistance,
• R
• Power package with low Thermal Resistance;
• Drain-Source voltage rating > 15V;
• Low gate charge, especially for higher frequency
For the low-side MOSFET, the on-resistance (R
primary parameter for selection. Because of the small duty
cycle of the high-side, the on-resistance determines the
power dissipation in the low-side MOSFET and therefore
significantly affects the efficiency of the DC-DC converter.
For high current applications, it may be necessary to use two
MOSFETs in parallel for the low-side for each phase.
For the high-side MOSFET, the gate charge is as important as
the on-resistance, especially with a 12V input and with higher
switching frequencies. This is because the speed of the transi-
tion greatly affects the power dissipation. It may be a good
trade-off to select a MOSFET with a somewhat higher R
if by so doing a much smaller gate charge is available. For
high current applications, it may be necessary to use two
MOSFETs in parallel for the high-side for each phase.
At the FAN5090’s highest operating frequencies, it may be
necessary to limit the total gate charge of both the high-side
and low-side MOSFETs together, to avert excess power dis-
sipation in the IC.
Gate Resistors
Use of a gate resistor on some MOSFETs may be required.
The gate resistor prevents high-frequency oscillations caused
by the trace inductance ringing with the MOSFET gate
capacitance. The gate resistors should be located physically
as close to the MOSFET gate as possible.
The gate resistor also limits the power dissipation inside the
IC, which could otherwise be a limiting factor on the switch-
ing frequency. It may thus carry significant power, especially
at higher frequencies. As an example: The FDB7045L has a
maximum gate charge of 70nC at 5V, and an input capaci-
tance of 5.4nF. The total energy used in powering the gate
during one cycle is the energy needed to get it up to 5V, plus
the energy to get it up to 12V:
This power is dissipated every cycle, and is divided between
the internal resistance of the FAN5090 gate driver and the
gate resistor. Thus,
14
E
operation.
=
=
DS,ON
QV
482nJ
+
< 10m (lower is better);
1
-- - C
2
V 2
=
70nC 5V
+ 5.4nF
1
-- -
2
DS,ON
12V 5V
–
) is the
DS,on
2
,
and each gate resistor thus requires a 1/4W resistor to ensure
worst case power dissipation.
Inductor Selection
Choosing the value of the inductor is a tradeoff between
allowable ripple voltage and required transient response.
A smaller inductor produces greater ripple while producing
better transient response. In any case, the minimum induc-
tance is determined by the allowable ripple. The first order
equation (close approximation) for minimum inductance for
a two-phase converter is:
where:
Vin = Input Power Supply
Vout = Output Voltage
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.
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 transient response and the output ripple voltage
determines the number of capacitors. Selection is typically
dominated 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.
For higher frequency applications, particularly those running
the FAN5090 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 2. 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.
P
Rgate
L
min
=
=
-------------------------------- -
4.7
------------------------------------------------ -
V
---------------------------------- -
R
in
gate
E f R
4.7
–
+
2 V
0.5
f
+
R
internal
out
gate
=
131mW
V
---------- -
V
out
in
=
PRODUCT SPECIFICATION
482nJ 300KHz
-----------------
V
ESR
ripple
REV. 1.0.1 9/16/03