1
\$\begingroup\$

I recently took another look at the L298 since some students were having problems (due to the commonly discussed high V_CE-on of the combined high side and low side drivers). One thing that struck me as weird is that the L298 uses NPN transistors as both high side and low-side drivers.

How does the high-side driver actually switch when VB < VE < VC? For example, if Vs is 40V and Vss is only 5V, the base (as the diagram is drawn) is lower than both collector and emitter voltages, so I don't see how the high-side transistors are doing anything (other than maybe pulling the load up to ~4V and getting hot). Is there some base-drive logic that's not drawn in the diagram? I was thinking that it's possible the high side AND gates are actually open-collector, so that normally they would be pulled up to Vs through an internal resistor which is not drawn. That would sort of work - but in that case, the current gain is still limited by the inherent diode drop in the transistor, which also would change depending on the load current...

Is there a good analysis of the actual operation of the L298 h-bridge? It seems to be more complicated than it looks.

I've attached the diagram from the datasheet for convenience. enter image description here

\$\endgroup\$
4
\$\begingroup\$

I think you'll probably find your answer in the L293 data sheet - it's the lower-power version of the 298 (without the heatsink). Here's what the 293 output stage looks like: -

enter image description here

The 293 (and I strongly suspect the same for the 298) has a PNP transistor that is much more conveniently driven from a low side logic signal.

I'm fairly sure the two devices work internally the same way - they have pretty-much exactly the same poor output specification. The diagram shown in the 298 spec "hints" at the limitations inherent in this type of H bridge because of the use of NPN transistors at the "top" and darlington transistors at the "bottom". See also this for alternative devices using MOSFETs.

\$\endgroup\$
  • \$\begingroup\$ The PNP devices are typically pretty horrible, being of "lateral" construction, they have very low \$h_{FE}\$ so they can't realistically be used directly. Typical drops are 1.35/2.0V (source) and 1.2/1.7V (sink) for 1A/2A. \$\endgroup\$ – Spehro Pefhany May 5 '14 at 18:48
  • \$\begingroup\$ Ah, okay, that makes a lot more sense that the "NPN transistors" in the L298 datasheet are actually not-really NPN transistors. I guess it's a Sziklai pair...or something similar. \$\endgroup\$ – Zuofu May 5 '14 at 18:50

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.