The driver voltage level VHIGH is limited by the supply voltage on VCC pin and, in any case, when VCC voltage

supply is detected lower than a threshold, the driver high-level modulation is disabled. A comparator with

hysteresis enables adaptive drive when VCC supply increases above VCC_AGD_en and disables it as VCC falls

below VCC_AGD_dis. This means that if the VCC supply is low (but higher than VCC_AGD_dis), the driver high-level

VHIGH will be the minimum between the value of above formula and a value equal to:

•

VCC supply if VAUX function is used (and VAUX pin voltage is larger than VCC - 1.2V), or

•

VCC - 1.2 V if VAUX function is not used.

The adaptive gate drive is also disabled when the circuit enters burst-mode operation. On resuming operation

from burst-mode, the gate drive always starts from the highest level and then it begins adapting the level

according to the sensed demagnetization period.

In sleep-mode, the behavior is different. When the load (after entering sleep-mode) increases and the SRK1001

controller resumes switching, the gate drive starts from the lower level and then progressively adapts with step

VGD_ad_step according to the sensed demagnetization period. The same also happens when the SRK1001 device

exits the low-consumption state by DIS-SYNC pin. Also in this case, the gate drive starts from the lower level and

then begins adapting with step VGD_ad_step according to the sensed demagnetization period.

Generally speaking, an SR MOSFET is always switched on after current starts flowing through its body diode

when the drain-source voltage is already low (equal to VF). Therefore, there is no Miller effect or switching losses

at MOSFET turn-on. This is true also at turn-off, since rectifier current after the SR MOSFET is switched off

continues flowing into the body diode. Consequently, the required gate charge the driver has to provide each

cycle for ON/OFF switching is rather lower than in case of hard switching and can be easily determined from

MOSFET datasheets in order to calculate the driver power dissipation.

SRK1001

Adaptive gate drive

DS13155 - Rev 1

page 19/27