EL7571

FN7298 Rev 1.00

Page 9 of 19

October 25, 2004

where:

Since inductance value tends to decrease with current, ripple

current.

Once the minimum output inductance is determined, an off the

shelf inductor with current rating greater than the maximum DC

output required can be selected. Pulse Engineering and Coil

Craft

are two manufactures of high current inductors. For

converter designers who want to design their own high current

inductors, for experimental purposes or to further reduce costs,

we recommend the Micrometals Powered Iron Cores data

sheet and applications note as a good reference and starting

point.

Inductor current is monitored indirectly via a low value resistor

used to set the maximum operating current, the current

reversal threshold and the system load regulation. To ensure

reliable system operation it is important to sense the actual

voltage drop across the resistor. Accordingly a four wire Kelvin

connection should be made to the controller current sense

inputs. There are two criteria for selecting the resistor value

and type. Firstly, the minimum value is limited by the maximum

output current. The EL7571 current limit capacitor has a typical

threshold of 154mV, 125mV minimum. When the voltage

across the sense resistor exceeds this threshold, the

conduction cycle of the top switch terminates immediately,

providing pulse by pulse current limiting. A resistor value must

be selected which guarantees operation under maximum load.

That is:

where:

Secondly, since the load current passes directly through the

sense resistor, its power rating must be sufficient to handle the

power dissipated during maximum load (current limit)

conditions. Thus:

where:

resistor. High current applications may require parallel sense

resistors to dissipate sufficient power. Current Sense Resistor

Table below lists some popular current sense resistors: the

WLS-2512 series of Power Metal Strip Resistors from Dale

Electronics, OARS series Iron Alloy resistor from IRC, and

Copper Magnanin (CuNi) wire resistor from Mills Resistors.

Mother board copper trace is not recommended because of its

high temperature coefficient and low power dissipation. The

trade-off between the different types of resistors are cost,

space, packaging and performance. Although Power Metal

Strip Resistors are relatively expensive, they are available in

surface mount packaging with tighter tolerances.

Consequently, less board space is used to achieve a more

accurate current sense. Alternatively, Magnanin copper wire

has looser tolerance and higher parasitic inductance. This

results in a less current sense but at a much lower cost. Metal

track on the PCB can also be used as current sense resistor.

The trade-offs are ±30% tolerance and ±4000 ppm

temperature coefficient. Ultimately, the selection of the type of

current sense element must be made on an application by

application basis.

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2

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=

Bill of Materials

MANUFACTURER

PART NO.

TOLERANCE

TEMPERATURE

COEFFICIENT

POWER RATING

PHONE NO.

FAX NO.

Dale

WSL 2512

±1%

±75ppm

1W

402-563-6506

402-563-6418

IRC

OARS Series

±5%

±20ppm

1W - 5W

800-472-6467

800-472-3282

Mills Resistor

MRS1367-TBA

±10%

±20ppm

1.2W

916-422-5461

906-422-1409

PCB Trace Resistor

±30%

±4000ppm

50A/in (1oz Cu)