L6599 datasheet(25/36 Pages) STMICROELECTRONICS

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L6599

Application information

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The circuit shown in Figure 30 can be operated in two different ways. If the resistor RA in

series to CA is small (not above some hundred Ω, just to limit current spiking) the circuit

operates like a capacitive current divider; CA will be typically selected equal to CR/100 or

less and will be a low-loss type, the sense resistor RB will be selected as:

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

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

Ω ), 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 will be typically

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

(provided it is << RA) according to:

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

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

Whichever circuit one is going to use, the calculated values of RS or RB should be

considered just a first cut value that needs 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 to endanger converter's safety if continuously

flowing. To prevent any damage during these conditions it is customary to force converter's

intermittent operation, 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 L6599 the designer can program externally 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 will 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 device and, in case of

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

defined duty cycle.

0.8

Crpkx

--------------- 1

-------

⎝⎠

⎛⎞

0.8

Crpkx

---------------

----------------------------

⋅

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