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ISO721: Missing pulses on the output of ISO721 - noise related issue?

Part Number: ISO721

Dear all,

I am using the ISO721 to galv. isolate the PWM signal of my microcontroller to the gate of a MOSFET. A schematic of the isolation as well as the gate driver is shown below:

In particular, the PWM signal from the uC is 0V to 3.3V, whereas the supply of the isolator is +5V. That is why I am using the ISO721 which has a TTL logic input.

The waveforms below show the measured isolator input signal in yellow (0V-3.3V) and the MOSFET gate signal in magenta.

The waveforms clearly show that, although the input is a logic high level, the output is low.

I tracked down the issue and measured directly at the isolator output as well as the gate.

I am not 100% clear about what exactly is happening in the isolator, and what could be the cause of such behavior. The layout of the isolator is shown below, to my best knowledge on following your design guideline according to your datasheet:

My question to you experts:

Have you seen this kind of behavior (or similar) with the dig. isolators and have an idea of the cause of such? The decoupling caps (100nF) are roughly <2.5mm away from the pins of the isolator.

If you need any further information, feel free to let me know.

Thank you.



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