Datasheet
SLUS486B − AUGUST 2001 − REVISED JULY 2003
15
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APPLICATION INFORMATION
adaptive vs. predictive waveforms
Figures 9 through 11 illustrate the adaptive (left) vs. predictive (right) switching waveforms. Key comparison
regions are denoted with (A), (B), (C), (D), and (E) for the adaptive control waveforms and (A′), (B′), (C′), (D′),
and (E′) for the predictive control waveforms. Figures 10 and 11 are close-ups of each transition edge.
At (A), the propagation delay from sensing the synchronous rectifier gate going low to the high-side gate going
high results in approximately 60 ns of body-diode conduction shown at (B). With the predictive drive, as soon
as the body-diode conduction of the SR MOSFET (B) is sensed, the high-side turn-on delay is adjusted to
minimize the body-diode conduction time (B′).
At (A′), the high side gate-to-source voltage is increasing while the synchronous rectifier gate-to-source voltage
is decreasing. A natural result of the precise timing of the high-side MOSFET turn-on is shown at (C) and (C′).
The overshoot and ringing for the predictive drive (C′) has much smaller amplitude than the adaptive drive (C)
due a reduction in reverse recovery in the SR MOSFET body diode. This reduction in reverse recovery is only
possible with the extremely precise gate timing used in the predictive drive technique.
At (D), the propagation delay from the synchronous rectifier drain-to-source voltage falling to the gate-to-source
voltage rising causes the body diode of the SR MOSFET to conduct for approximately 60 ns (E). When the
predictive drive is enabled (D′), the inherent delay is eliminated and virtually no body-diode conduction is shown
at (E′).
Complementary
Gate Drive
Waveforms
2 V / div
VDS of SR
MOSFET
Switch
2 V / div
Adaptive Drive
100 ns / div
Predictive Drive
100 ns / div
B
C
A4
D
D4
E
E4
A
B4
C4
Figure 9. Adaptive vs. Predictive Switching Waveforms