FAN5018B View Datasheet(PDF) - Fairchild Semiconductor

Part Name

Description

Manufacturer

FAN5018B

6-Bit VID Controller 2-4 Phase VR10.X Controller

Fairchild Semiconductor 

PRODUCT SPECIFICATION

FAN5018B

CDLY

= ⎨⎧20μA −

⎩

VVID

2 × RDLY

⎫

⎬

⎭

×

tSS

VVID

(2)

where tSS is the desired soft-start time. Assuming an RDLY of

301kΩ and a desired a soft-start time of 3ms, CDLY is 35nF.

A close standard value for CDLY is 47nF. Once CDLY has

been chosen, RDLY can be calculated for the current limit

latch-off time using:

RDLY

=

1.96 × tDELAY

CDLY

(3)

If the result for RDLY is less than 200kΩ, then a smaller soft-

start time should be considered by recalculating the equation

for CDLY or a longer latch-off time should be used. RDLY

should never be less than 200kΩ. In this example, a delay

time of 8ms gives RDLY = 334kΩ. A close

standard 1% value is 301kΩ.

Inductor Selection

The choice of inductance value for the inductor determines

the ripple current in the inductor. Less inductance leads to

more ripple current, which increases the output ripple volt-

age and conduction losses in the MOSFETs, but allows the

use of smaller-size inductors and, for a specified peak-to-

peak transient deviation, less total output capacitance. Con-

versely, a higher inductance means lower ripple current and

reduced conduction losses, but requires larger inductors and

more output capacitance for the same peak-to-peak transient

deviation. In any multi-phase converter, a practical value for

the peak-to-peak inductor ripple current is less than 50% of

the maximum DC current in the same inductor. Equation 4

shows the relationship between the inductance, oscillator

frequency, and peak-to-peak ripple current in the inductor.

Equation 5 can be used to determine the minimum induc-

tance based on a given output ripple voltage:

IR

=

VO × (1 − D)

fSW × L

(4)

L ≥ VVID × RO × (1− (n × D))

(5)

f SW ×VRIPPLE

Solving Equation 5 for a 10 mVp-p output ripple voltage

yields:

L ≥ 1.5V ×1.3m Ω × (1− 0.375) = 534nH

228 kHz ×10mV

If the ripple voltage ends up less than that designed for, the

inductor can be made smaller until the ripple value is met.

This will allow optimal transient response and minimum

output decoupling.

The smallest possible inductor should be used to minimize

the number of output capacitors. Choosing a 650nH inductor

is a good choice for a starting point and gives a calculated

ripple current of 8.86A. The inductor should not saturate at

the peak current of 26.1A and should be able to handle the

sum of the power dissipation caused by the average current

of 21.7A in the winding and core loss.

Another important factor in the inductor design is the DC

Resistance (DCR), which is used for measuring the phase

currents. A large DCR will cause excessive power losses,

while too small a value will lead to increased measurement

error. A good rule of thumb is to have the DCR be about

1 to 1 1/2 times the droop resistance (RO). For our example,

we are using an inductor with a DCR of 1.6 mΩ.

Designing an Inductor

Once the inductance and Direct-Current resistance (DCR)

are known, the next step is either to design an inductor or

find a standard inductor that comes as close as possible to

meeting the overall design goals. It is also important to have

the inductance and DCR tolerance specified to keep the

accuracy of the system controlled. Using 15% for the induc-

tance and 8% for the DCR (at room temperature) are reason-

able tolerances that most manufacturers can meet.

The first decision in designing the inductor is to choose the

core material. There are several possibilities for providing

low core loss at high frequencies. Two examples are the

powder cores (e.g., Kool-Mm® from Magnetics, Inc. or

Micrometals) and the gapped soft ferrite cores (e.g., 3F3

or 3F4 from Philips). Low-frequency powdered iron cores

should be avoided due to their high core loss, especially

when the inductor value is relatively low and the ripple

current is high.

The best choice for a core geometry is a closed-loop type,

such as pot cores, PQ, U, and E cores, or toroids. A good

compromise between price and performance are cores with a

toroidal shape.

There are many useful references for quickly designing a

power inductor, such as:

Magnetics Design References

1. Magnetic Designer Software: Intusoft

(www.intusoft.com)

2. Designing Magnetic Components for High-Frequency

DC-DC Converters, by William T. McLyman,

Kg Magnetics, Inc. ISBN 1883107008

REV. 1.0.0 Jul/15/05

19

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