PRODUCT SPECIFICATION

FAN5071

REV. 1.0.4 1/29/02

11

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

Ω output short, the voltage is

reduced to 29A * 4m

Ω = 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

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

during its on time, as shown in Figures 1 and 2.

To program the amount of droop, use the formula

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

Ω sense resistor, use R

10m

Ω/(120mV *3) = 12KΩ. 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

Bulletin AB-24.

Remote Sense

The FAN5071 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 FAN5071

may be found in Applications Bulletin AB-24.

2.5

µH

5V

0.1

µF

1000

µF, 10V

Electrolytic

Vin

14.4K

Ω *I