DS12787 Rev 1

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SRK1000 / SRK1000A / SRK1000B

Operation description

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Figure 15. VAUX pin operation

5.9

Operation in CC regulation and short-circuit

During CC regulation operation in QR applications, the demagnetization time progressively

increases while reducing the load impedance and the output voltage consequently reduces.

Therefore, the conduction duty cycle of SR MOSFET driving increases: the SRK1000 fixes

the maximum driving pulse width level to TON_MAX and after this time interval has elapsed, it

turns off the SR MOSFET. This means that, for the rest of the demagnetization time after

TON_MAX, the rectified current continues to flow through the body diode.

If CC regulation is extended down to short-circuit condition, (i.e. the output current is

regulated also during short-circuit and the primary controller does not enter hiccup

protection), some care must be taken to avoid temperature increase of the SR MOSFET

(i.e. a proper thermal design or the usage of an external Schottky diode).

5.10

Adaptive gate drive

The IC is provided with a low-noise, high-current gate-drive output, capable of directly

driving N-channel Power MOSFETs.

The high-level voltage provided by the driver is in fact clamped at VGDclamp (11.6 V typ.)

through an accurate circuitry; this avoids excessive voltage levels on the gate in case the

device is supplied with a high VCC, thus minimizing the gate charge provided in each

switching cycle.

Furthermore, the gate driver has a pull-down capability that ensures the SR MOSFET

cannot be spuriously turned on even at low VCC: in fact, the driver has a 1 V (typ.) saturation

level at VCC below the turn-on threshold.

In order to optimize efficiency at low load levels (where driving losses may be relevant with

respect to conduction losses), the high-level of driver output is adapted, decreasing with

decreasing demagnetization time. The adaptive gate drive changes the driving high-level

VHIGH in 16 steps of 400 mV, corresponding to 16 steps of detected demagnetization time

TD of the transformer, as described by the following relationships: