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TLC3702: Output Rise Time Dependent on 2nd Stage Comparator

Antonio Fadhel
Genius 9410 points
Part Number: TLC3702
Other Parts Discussed in Thread: LMC6762, TLV1805

Hi Team,

My customer is implementing the modified circuit below from page 18 of the TLC3702 datasheet:

With "control" low, they see the correct output and triangle waveform for the A half (pins 1 green and 2 purple below), with less than 1uS rise time at the output:

With control adjusted to barely cause the output of the second amp (pin 7 yellow) to activate, the output rise time of pin 1 (green) slows to 11uS, which stretches the output triangle wave, and thus also stretches the output pulse:

Further increasing the control, the rise time improves (back under 1uS rise time):

Any ideas why the input of the B comparator is affecting the rise time of the A stage output? They are trying the same configuration with 2 separate comparators (using only the A comparator on 2 separate TLC3702s) and don't seem to have the same issue.

Thanks,

Antonio

  • 0
    TI__Guru 55226 points

    Hello Antonio,

    The TLC3702 has a known issue when both channels transition simultaneously within a 10-20us window of each other. The first channel to transition will cause the other channel to experience an extended prop delay. The rounded-edge "S" shape of the output is a symptom of this effect. The effect is much more pronounced at higher supply voltages (which they are using).

    At their minimum speed setting, the output transitions are within this time window, causing the prolonged delay to the trailing channel.

    We do not recommend the TLC3702/4 for applications with a high rep rate (>10kHz) and concurrent output transitions and high supply voltages. In short, the TLC3702 was designed as a micro power comparator and cannot internally handle fast, simultaneous repetitive transitions from both channels. We are in the process of characterizing the issue, and will update the datasheet when complete.

    Since they are driving a MOSFET, we would recommend the new TLV1805, which has enhanced peak output drive capability specifically to drive MOSFET gate capacitance. If they need a drop-in replacement, they can try the LMC6762, though it is a bit slower.