DS12787 Rev 1

13/27

SRK1000 / SRK1000A / SRK1000B

Operation description

27

Equation 1

(with RTON expressed in ohm and TON_MIN in seconds, starting when the DVS signal goes

below VTH_A). This blanking time of course sets a minimum turn-on time of the SR MOSFET

as well: hence, when by reducing the load the SR MOSFET conduction time would become

shorter than the programmed blanking time, the IC must stop driving the SR MOSFET to

avoid current inversion (see Section 5.7: Low consumption mode operation: sleep mode

and burst mode below).

5.4

Adaptive turn-off and TIMER

The SR MOSFET can be turned off through two coexisting mechanisms (whichever triggers

first); the first based on an adaptive algorithm, the second on the internal timer.

The adaptive turn-off consists of a ZCD_OFF comparator, where the DVS signal is

compared to an adapting threshold. This threshold is adapted in such a way that, at steady

state, the measured residual conduction time of the SR MOSFET body diode after turn-off

meets the target value Tdiode_off (as shown in Figure 8). The residual conduction time of the

body diode is measured between the falling edge of the driving signal and the rising edge of

the DVS signal (first time surpassing the threshold VTH_A).

Figure 8. ZCD_OFF threshold adapting for body diode target conduction

The internal timer basically turns off the SR MOSFET with a fixed anticipation time Tant_timer

with respect to the first rising edge of the DVS signal. The IC has two different timer

operating modes, selectable by the user, optimized for fixed frequency mixed CCM/DCM

converters or for QR/DCM applications. The selection of the proper timer mode is done

TON_MIN = 12 10

12

–

RTON