Sunday, January 17, 2016

Inverting Push-Pull driver characteristics

I haven't had time or motivation to dig into the shoot-through analysis described in the previous few posts, but I thought I'd look into another aspect of the same problem: the push-pull drivers.

These come in two basic categories: inverting and non-inverting. I then have two implementations of each, depending on whether the high-side driver is an FDV301 or a BSS83 FET. Each of these requires separate evaluation. In this post we'll look at only the inverting flavors, leaving the non-inverting flavors for the next post.

As usual I'm driving this circuit with my PIC, which has a rail-to-rail push-pull output. To provide an input more characteristic of the 4004 circuit I've cascaded two inverters (Q1/R1, Q2/R2). Q3/R3 drives the high-side FET, Q4, 180° out of phase with the low-side driver, Q5.

Just like the IP board I've already assembled, load resistors R1 and R2 are 4.7K; R3 is 2.2K, replacing a "bootstrap load". R4 is a precision 100 ohm resistor; since there is no load on the output, any voltage drop across R4 represents current passing from Vdd through both Q4 and Q5 to Vss as they switch states [not entirely true; see the later post FDV301 turn-off characteristics].

Here's the 'scope trace using an FDV301 as Q4. Since the BLUE trace sits at Vdd (+5V) when the circuit isn't switching I've selected AC coupling to make selecting a useful scale easier. Here it's 200mV per division.

As expected, there are momentary transients when the driver switches states in either direction. The largest transient appears in the low-to-high transition, with a voltage drop of about 180mV or a current of about 1.8mA. That's less current than drawn through R3, and it persists for less than 100nS.

What about when there is a BSS83 as the high-side FET? This configuration shows up in half of the tri-state push-pull drivers. The BSS83 has a greater threshold voltage than the FDV301, so I'd expect it to be slower turning on and faster turning off.

Here we see the high-to-low transient has all but vanished, and the low-to-high transient is slightly diminished.

These results show the transients in the inverting push-pull driver are quite tolerable. Next we'll look at the non-inverting push-pull driver.

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