FAN5071

PRODUCT SPECIFICATION

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Design Considerations and Component

Selection

Additional information on design and component selection

may be found in Fairchild’s Application Note 57.

MOSFET Selection

This application requires N-channel Logic Level Enhance-

ment Mode Field Effect Transistors. Desired characteristics

are as follows:

(lower is better)

• Power package with low Thermal Resistance

• Drain-Source voltage rating > 15V.

MOSFET selection. The on-resistance determines the power

dissipation within the MOSFET and therefore significantly

affects the efficiency of the DC-DC Converter. For details

and a spreadsheet on MOSFET selection, refer to Applica-

tions Bulletin AB-8.

Inductor Selection

Choosing the value of the inductor is a trade-off between

allowable ripple voltage and required transient response. The

system designer can choose any value within the allowed

minimum to maximum range in order to either minimize rip-

ple or maximize transient performance. The first order equa-

tion (close approximation) for minimum inductance is:

where:

f = DC/DC converter switching frequency

ESR = Equivalent series resistance of all output capacitors in

parallel

The first order equation for maximum allowed inductance is:

where:

transient

ally 95%).

expense since all the variables are predetermined by system

increase L. Adding margin by decreasing L can be done by

purchasing capacitors with lower ESR. The FAN5071 pro-

vides significant cost savings for the newer CPU systems

that typically run at high supply current.

FAN5071 Short Circuit Current Characteristics

The FAN5071 protects against output short circuit on the

core supply by latching off both the high-side and low-side

MOSFETs. The FAN5071 short circuit current characteristic

includes a hysteresis function that prevents the DC-DC con-

verter from oscillating in the event of a short circuit. The

formula

the high-side MOSFET’s on resistance. Remember to make

nately, use of a sense resistor in series with the source of the

MOSFET, as shown in Figure 6, eliminates this source of

inaccuracy in the current limit.

As an example, Figure 3 shows the typical characteristic of

the DC-DC converter circuit with two FDD6690A high-side

= 21.25m

Ω each for a total of 10.6mΩ) and a 6.19KΩ R

Figure 3. FAN5071 Short Circuit Characteristic

The converter exhibits a normal load regulation characteris-

tic until the voltage across the MOSFET exceeds the internal

short circuit threshold of 50µA * 6.2K

Ω = 310mV, which

occurs at 310mV/10.6m

Ω = 29A. [Note that this current

limit level can be as high as 310mV/6.5m

Ω = 48A, if the

25°C. This is the reason for using the external sense resistor.]

At this point, the internal comparator trips and signals the

f

x

x

ESR

=

=

0

10

20

30

40

50

Output Current (A)