ADum4221 Truth-Table Problem

Hello!

I'm using the ADuM4221 as a gate-drive for a synchronous buck converter. It's implemented according to the bootsrapped Half Bridge Operation Circuit found in Figure 38 in its datasheet (ADuM4221/ADuM4221-1/ADuM4221-2 (Rev. B) (analog.com)), with a few minor tweaks. 

Fig. 1 - Ref. from Datasheet

Fig.2 - My implemented circuit

It works fine when operating the converter. However, when I try to set both VIA input = LOW and VIB INPUT= LOW, the VOB output will still be kept HIGH. This keeps the low-side MOSFET on, which is unsuitable for my application. 

The truth table for this device (Table 11 in datasheet) says it should keep both VOA and VOB outputs LOW when both VIA and VIB inputs are being kept LOW, which is the case. The disable pin still works as intended and sets both outputs to LOW.

What am I doing wrong? Thank you in advance

  • Hello,

    When VIA and VIB are both low, the outputs should definitely both be low. When you are measuring the VOB output, are you measuring from GNDB to VOB? If you are measuring two outputs at once, are you using differential probes? If you use single ended probes, the oscilloscope can ground pins you might not want shorted together.

    Thanks,

    RSchnell

  • Hello again,

    I just had a thought. If you are using the PWM input part (ADuM4221-1), this might explain the action you are seeing. What is the top marking on the part? The PWM version of the ADuM4221 only has one input pin for the PWM, and the outputs are complimentary to each other.

    Thanks,

    RSchnell

  • I have just checked and you are exactly right!

    I've accidently bought both the ADuM4221 and the ADuM4221-1 unknowingly and mixed them up while soldering. I will change the IC and try again, but I believe this will solve my problem. 

    Thank you so much for the help, 

    João