2STR1215 datasheet(13/56 Pages) STMICROELECTRONICS

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5.1.4

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 leakage inductance, parasitic capacitances, reverse junction capacitance of

the Schottky diode of the secondary side) from overcoming the EA programmed switch current for the conversion,

which would make the conversion unstable (depicted in Figure 9). For the L6983I, the TON MIN has a typical value

of 390 ns.

Figure 9. Simulation with different masking times: insufficient (< 100 ns, left) and appropriate (390 ns,

right)

-5

-4

-3

-2

-1

Time [µs]

-5

Time [µs]

-5

-4

-3

-2

-1

Time [µs]

-5

Time [µs]

Despite the masking time, an RC snubber (for example, across the Schottky diode of the secondary side) is

normally recommended to damp oscillations. Figure 10 shows how the RC snubber helps in smothering the

remaining oscillations.

Figure 10. Remaining oscillations (left) and their filtering with RC snubber (right) VIN = 12 V, N = 6, IOUTiso

= 200 mA

L6983I

Primary side

DS14040 - Rev 1

page 13/56

2STR1215 Datasheet(PDF) 13 Page - STMicroelectronics