Operation description

SRK2001A

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DocID029495 Rev 1

Also at the turn-off, during steady-state load conditions, when the decision depends on the

adaptive control circuitry, there is no need to have a very fast drive with hard pull-down,

because the current has not yet reached zero and the operation is far from the current

inversion occurrence. Moreover, slow transitions also help reducing the perturbation

introduced into the system that arise due to the MOSFET turn-on and turn-off, contributing

to improve the overall behavior of the LLC resonant converter.

The gate-drive circuit is specifically designed to reduce the switching noise at the turn-off,

due to parasitic inductance in the driving current path. In fact, during the adaptive turn-off, it

provides a controlled turn-off time (with the characteristic two-slope falling edge) in order to

limit the current peak during gate drive transition, and does not require any resistor in series

to the SR MOSFET gates.

On the other side, during very fast load transitions or on a sudden occurrence of the above

resonance operation, when the turn-off decision is taken by ZCD_OFF logic, the MOSFET

turn-off needs to be very fast to avoid current inversion: therefore, the two gate drivers are

designed to guarantee for a very short turn-off total delay.

In order to avoid current inversions, the SRK2001A stops driving SR MOSFETs during any

operating condition where the converter enters deeply into the below resonance region

(i.e. switching frequency gets lower than 60% of resonance frequency).

7.6

EN and PROG pins: function and usage

The SRK2001A can enter the low consumption mode in one of the following ways:



By EN pin remote on-off (through a small signal NPN transistor)



By detecting the primary controller burst-mode operation

The PROG pin allows the user to program the conduction duty-cycle of the SR MOSFET

body diode to exit the low consumption mode: the configuration choice is done during the

consumption state is set, among those contained in the internal lookup table (see Table 6).

during the startup the EN pin is kept low, the pinstrap phase will happen only after the EN

pin is pulled high.

Figure 9. EN - PROG pins configuration