FAN5059MX Datasheet, PDF (14/18 Pages) Fairchild Semiconductor – High Performance Programmable Synchronous DC-DC Controller for Multi-Voltage Platforms

FAN5059
PRODUCT SPECIFICATION
with I_Detect ≈ 50µA, I_SC is the desired current limit, and
R_DS,on the high-side MOSFET’s on resistance. Remember to
make the R_S large enough to include the effects of initial tol-
erance and temperature variation on the MOSFET’s R_DS,on.
Alternately, use of a sense resistor in series with the source
of the MOSFET eliminates this source of inaccuracy in the
current limit.
As an example, Figure 4 shows the typical characteristic of
the DC-DC converter circuit with an FDB6030L high-side
MOSFET (R_DS = 20mΩ maximum at 25°C * 1.25 at 75°C =
25mΩ) and a 8.2KΩ R_S.
CPU Output Voltage vs. Output Current
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0
5
10
15
20
25
Figure 4. FAN5059 Short Circuit Characteristic
The converter exhibits a normal load regulation characteristic
until the voltage across the MOSFET exceeds the internal
short circuit threshold of 50µA * 8.2KΩ = 410mV, which
occurs at 410mV/25mΩ = 16.4A. (Note that this current limit
level can be as high as 410mV/15mΩ = 27A, if the MOSFET
has typical R_DS,on rather than maximum, and is at 25°C).
At this point, the internal comparator trips and signals the con-
troller to discharge the softstart capacitor. This causes a drastic
reduction in the output voltage as the load regulation collapses
into the short circuit control mode. With a 40mΩ output short,
the voltage is reduced to 16.4A * 40mΩ = 650mV. The output
voltage does not return to its nominal value until the output
current is reduced to a value within the safe operating ranges
for the DC-DC converter.
If any of the linear regulator outputs are loaded heavily
enough that their output voltage drops below 80% of nominal
for >30µsec, all FAN5059 outputs, including the switcher, are
shut off and remain off until power is recycled.
Schottky Diode Selection
The application circuit of Figure 1 shows a Schottky diode,
D1, which is used as a free-wheeling diode to assure that the
body-diode in Q2 does not conduct when the upper MOSFET
is turning off and the lower MOSFET is 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 very short, the selection 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.
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, and the capacitance value
helps set the maximum inductance. For most converters,
however, 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.
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.
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 +5V supply and the converter input as
shown in Figure 5. This inductor serves to isolate the +5V
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 2.5µH is recommended.
It is necessary to have some low ESR aluminum electrolytic
capacitors at the input to the converter. These capacitors
deliver current when the high side MOSFET switches on.
Figure 5 shows 3 x 1000µF, but the exact number required
will vary with the speed and type of the processor. For the
top speed Katmai and Coppermine, the capacitors should be
rated to take 9A and 6A of ripple current respectively.
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 operational temperature.
For details on the design of an input filter, refer to Applica-
tions Bulletin AB-15.
5V
0.1µF
2.5µH
Vin
1000µF, 10V
Electrolytic
Figure 5. Input Filter
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REV. 1.0.4 8/14/03