[ LMG3410 ] Optimal Dead Time

Hi, Ken

Thank you for your questions and interest in LMG3410.

Q1:

Answer: In most cases, we want reduce the deadtime as small as possible to minimize the third-quadrant operation. However, the minimal deadtime is limited by propagation delay of LMG3410 and digital Isolator. LMG3410 has 20-ns Typical Propagation Delay and digital isolator (ISO7831) on the daughter card has 11 ns Typical Propagation Delay. We recommended to have more than 30 ns deadtime to prevent shoot-through.

For your customer’s case, if using our daughter card, 50ns deadtime will be good enough. If using different digital isolator, the deadtime will be limited by the Propagation Delay of digital isolator in your application.

Q2:

Answer: For bootstrap operation, during deadtime or third-quadrant operation, the voltage drop from source to drain of the bottom FET may cause the high side bias voltage to overcharge by several volts. Using a series resistor with the bootstrap supply will create a charging time constant in conjunction with the bypass capacitance on the order of a microsecond. In our daughter card, the charging time constant is around 200 us with 20 Ohm resistor and 10 uF capacitor. When the dead time, or third-quadrant conduction time, is much lower than this time constant, the bootstrap voltage will be well-controlled and the optional zener clamp will not be necessary.

For your cases with 2us deadtime, it may be OK without Zener clamp. But it is still highly recommended to have the Zener clamp for VDD overvoltage protection.

If you have any other questions, please let me know.

Best regards.

Ted