L6599ATD datasheet(21/31 Pages) STMICROELECTRONICS

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L6599AT

Application information

Doc ID 15534 Rev 3

21/31

The circuit shown in Figure 11b can be operated in two different ways. If the resistor RA in

series to CA is small (not above a few hundred ohms, just to limit current spiking) the circuit

operates like a capacitive current divider; CA is typically selected equal to Cr/100 or less and

is a low-loss type, and the sense resistor RB is selected as:

Equation 7

and CB is such that RB·CB is in the range of 10 /fmin.

If the resistor RA in series with CA is not small (in this case it is typically selected in the ten

Ω range), the circuit operates like a divider of the ripple voltage across the resonant

capacitor Cr, which, in turn, is related to its current through the reactance of Cr. Again, CA is

typically selected equal to Cr/100 or less, this time not necessarily a low-loss type, while RB

(provided it is << RA) according to:

Equation 8

where the reactance of CA (XCA) and Cr (XCr) should be calculated at the frequency where

ICrpk = ICrpkx. Again, CB is such that RB·CB is in the range of 10 /fmin.

Whichever circuit is used, the calculated values of Rs or RB should be considered

preliminary values that may need to be adjusted after experimental verification.

OCP is effective in limiting primary-to-secondary energy flow in case of an overload or an

output short-circuit, but the output current through the secondary winding and rectifiers

under these conditions might be so high that they endanger the converter’s safety if

continuously flowing. To prevent any damage during these conditions it is customary to force

the intermittent operation of the converter, in order to bring the average output current to

values such that the thermal stress for the transformer and the rectifiers can be easily

handled.

With the L6599AT the designer can externally program the maximum time TSH that the

converter is allowed to run overloaded or under short-circuit conditions. Overloads or short-

circuits lasting less than TSH do not cause any other action, hence providing the system with

immunity to short duration phenomena. If, instead, TSH is exceeded an overload protection

(OLP) procedure is activated that shuts down the L6599AT and, in cases of continuous

overload/short-circuit, results in continuous intermittent operation with a user-defined duty

cycle.

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Crpkx

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