L6986I datasheet(14/60 Pages) STMICROELECTRONICS

14 / 60 page

Figure 10. Enable the device with external voltage step

More details about the component selection can be found in Section 6.5 Design of the external components.

Secondary side soft-start

As soon as the soft-start begins at the primary circuit, the device already operates in forced PWM and the

energy transfer to the secondary coil during the off-time can take place. Consequently, a tracked soft-start at the

secondary side takes place too. Therefore, the secondary output voltage reaches its steady state value in tracking

with the primary output voltage, as shown in figure below.

Figure 11. Tracked soft-start at secondary side

4.6

Minimum On-Time

The current sense in the high-side MOSFET is not active (masked) for a certain time at the beginning to the

on-time (masking time, from which the parameter TON MIN derives).

This current sense blanking time is implemented to prevent any primary side pulsed current at the high side

turn-on (resonating with transformer drive leakage inductance and secondary side capacitance) overcomes the

EA programmed switch current for the conversion and make to conversion unstable (depicted in Figure 12 and

Figure 13. Simulation with appropriate masking time (330 ns)).

L6986I

Minimum On-Time

DS13647 - Rev 1

page 14/60

L6986I Datasheet(PDF) 14 Page - STMicroelectronics