Sunday, January 17, 2016

Non-inverting Push-Pull driver characteristics

Now let's take a look at the non-inverting push-pull driver circuit. There are two significant differences. First, the inverter formed by Q3/R3 is now in the path of the low-side driver rather than the high-side driver. Second, since it's the high-side driver that needs the higher voltage provided by the bootstrap load, R2 is now 2.2K and R3 is 4.7K.
Let's look first at the 'scope trace when using a BSS83 as Q4. This configuration is found in the other four tri-state push-pull drivers.

The low-to-high transient has gotten worse, now at 2.5mA. Still, this is tolerable.

How about with the FDV301? If you've been reading my blog posts for long, you have to recognize there's a reason I left it for last.

This doesn't look too bad until you realize that I had to change the BLUE trace scale from 200mV to 1.0V per division. That -1.8V transient on the high-to-low transition represents 18mA. I wasn't expecting that.

Why does this occur? It can't be an overlap with Q5 turning on, as the RED trace shows Q5's gate is still grounded until after the peak of the transient. Also note the knee in the output voltage about about 1V. What could cause these?

Again I'm thinking it's the static protection diode between the gate and source of the FDV301. It's a single Zener, rather than back-to-back Zeners like in the DMN26, so as soon as the gate goes about 0.7V (one diode drop) below the source that diode is going to start conducting. And it will continue to conduct until the Vgs again drops below 0.7V, which is about where that knee in the output voltage is.

Is this a problem in my already-assembled IP board? Maybe not. The transient is still less than 100ns in duration, and the FDV301 is rated for 220mA continuous. However, this transient is likely going through the gate lead of the FDV301 which may not be as robust as the drain and source leads, and some of the transistors pulling the gate to ground are less-capable BSS83 types.

What can be done about this? There are only two of these push-pull circuits on the IP board. I laid out space for some bypass capacitors (Vdd to Vss) on the underside of the board directly behind these circuits, but populating them might aggravate the problem. I could replace the FDV301s with DMN26 types; its leads will just barely overlap the pads for an FDV301, and the shoot-through currents appear tolerable (perhaps the subject of a future post). Or I could leave these as they are and address the issue on other boards.

I'll have to give this some thought.

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