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參數資料
| 型號： | FAN5070 |
| 廠商： | Fairchild Semiconductor Corporation |
| 英文描述： | High Performance Programmable Synchronous DC-DC Controller |
| 中文描述： | 高性能可編程同步DC - DC控制器 |
| 文件頁數： | 10/14頁 |
| 文件大小： | 147K |
| 代理商： | FAN5070 |
FAN5070
10
REV. 1.0.1 8/7/01
controller to reduce the converter’s duty cycle to approxi-
mately 20%. This causes a drastic reduction in the output
voltage as the load regulation collapses into the short circuit
control mode. With a 4m
output short, the voltage is
reduced to 29A * 4m
= 116mV. The output voltage does
not return to its nominal value until the output current is
reduced to a value within the safe operating range for the
DC-DC converter.
Schottky Diode Selection
The application circuits 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 MOS-
FET is turning off and the lower MOSFET is turning on. It is
undesirable for this diode to conduct because its high for-
ward 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 for-
ward 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 capaci-
tance 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 4 shows 3 x 1000μF, but the exact number required
will vary with the speed and type of the processor. Capacitor
ripple current rating is a function of temperature, and so the
manufacturer should be contacted to find out the ripple cur-
rent rating at the expected operational temperature. For
details on the design of an input filter, refer to Applications
Bulletin AB-15.
Figure 4. Input Filter
Programmable Active Droop
The FAN5070 includes Programmable Active Droop
: as
the output current increases, the output voltage drops, and
the amount of this drop is user adjustable. This is done in
order to allow maximum headroom for transient response of
the converter. The current is typically sensed by measuring
the voltage across the R
DS,on
of the high-side MOSFET
during its on time, as shown in Figures 1 and 2.
To program the amount of droop, use the formula
where I
max
is the current at which the droop occurs, and
R
sense
is the resistance of the current sensor, either the source
resistor or the high-side MOSFET’s on-resistance. For exam-
ple, to get 120mV of droop with a maximum output current
of 30A and a 10m
sense resistor, use R
D
= 14.4K
* 30A *
10m
/(120mV * 9) = 4K
. The value of the product
Imax*Rsense must be < 600mV for proper functioning of
the droop circuit. If this product is exceeded, a lower resis-
tance MOSFET must be used. Further details on use of the
Programmable Active Droop
may be found in Applications
Bulletin AB-24.
Remote Sense
The FAN5070 offers remote sense of the output voltage to
minimize the output capacitor requirements of the converter.
It is highly recommended that the remote sense pin, Pin 20,
be tied directly to the processor power pins, so that the
effects of power plane impedance are eliminated. Further
details on use of the remote sense feature of the FAN5070
may be found in Applications Bulletin AB-24.
2.5
μ
H
5V
0.1
μ
F
1000
μ
F, 10V
Electrolytic
Vin
R
D
14.4K
*I
max
*R
sense
V
Droop
*9
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