L6699 datasheet(25/38 Pages) STMICROELECTRONICS

25 / 38 page

L6699

Current sensing, OCP and OLP

Doc ID 022835 Rev 2

25/38

The ISEN pin, which is also able to withstand negative voltages, is internally connected to

the input of a first comparator, referenced to VISENx (0.8 V typ. 0.76 V min.), and to that of a

second comparator referenced to 1.5 V. The operation of the second comparator is

described later.

If the voltage externally applied to the pin by either circuit in

Figure 18 and 19 exceeds 0.8 V

the first comparator is tripped and this causes an internal switch to be turned on for 5 µs and

discharges the soft-start capacitor CSS (see Section 6.3: Safe-start procedure). This quickly

increases the oscillator frequency and thereby limits energy transfer. Under output short-

circuit conditions, this operation results in a peak primary current that periodically oscillates

below the maximum value allowed by the sense resistor Rs.

In the circuit shown in

Figure 18, Rs is placed directly in series to the resonant tank. Its

value can be determined using the equation:

Equation 7

where ICrpkx is the maximum expected peak current flowing through the resonant capacitor

and the primary winding of the transformer, which is related to the maximum load and the

minimum input voltage. The power dissipation in Rs can be approximately found with:

Equation 8

The circuit shown in

Figure 19 operates as a capacitive current divider; Cs is typically

selected equal to Cr/100 or less and is a low-loss type, and the sense resistor Rs is selected

as:

Equation 9

and the associated power dissipation is reduced by a factor (1+Cr/Cs). This circuit is then

recommended when the efficiency target is very high.

This 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 may be so high that it endangers converter safety if continuously

flowing. To prevent any damage it is customary to force the converter to work intermittently,

which brings the average output current to values such that the thermal stress for the

transformer and the rectifiers can be easily handled.

If intermittent operation upon overload or short-circuit is desired, the designer can program

externally the maximum time TSH that the converter is allowed to run under these

conditions. Overloads or short-circuits lasting less than TSH do not cause any other action,

Crpkx

Crp k x

≈

)

(

Crpkx

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