Question from a data-sheet challenged programmer:

So I have a 3.3 volt microcontroller (ARM LPC2129) and the PWM connection to the VNH3SP30 carrier board (older version) seems to have no effect - the H-Bridge output is running at full voltage all the time, even though my multimeter can detect the PWM frequency (1000 hz) and varying duty cycle just fine. The uC can signal the INA and INB pins just fine at 3.3 volts, properly toggling the motor direction. But your web page for the part shows a 5v application. Shouldn’t it work with a 3.3v cmos uC too?

Another question - the V+ input at the control pins - that’s just for pulling up the En/Diag pins, right? Is supplying that pin with 3.3v sufficient?




The datasheet indicates a minimum high level voltage of 3.25 V, so running on the low end of a 3.3 V supply would put you below that. I’m not sure what you mean by the H-bridge output running at full voltage. Do you mean that a motor connected to the output will run at full speed, or are you measuring the full voltage on each output pin relative to ground (i.e. no voltage difference between the two outputs)?

- Jan

I was worried about that 3.25 V minimum, particularly with that resistor in series to the pin. I guess I’ll need to amplify the signal a tiny bit, but I have no clue how to do that.

Yes - a connected motor would run at full speed. I was seeing the full input voltage difference between OutA and OutB - with the polarity reversing appropriately depending on the INA vs. INB signals. Which kind of implies that the chip is seeing a 100% duty cycle?

It is strange that the chip is seeing 100% duty cycle. Have you tried physically tying the PWM input low to see if the motor turns off?

- Jan

I’ll try that tonight. Thanks.

Not looking good.

I’m supplying the VNH3SP30’s VCC with a 9v battery for testing. It reads 9.13v at the input power terminals. When the PWM pin is unconnected or connected to ground the output (OutA to OutB) difference is about 8.85v.

But when I connect the PWM to 3.299v, 5v or to the real PWM signal the output goes to 9.13v

To eliminate the 3.3v source as an issue I repeated the above with 5v as input to the VIN (for DIAG/EN pullups) and the INA pin and got the same result.

Do I have a bad board?


OK I’m baffled. I soldered up the connectors to another carrier board and I’m getting exactly the same results.

It seems that when INA is enabled and INB is not, then OUTB will be either at ground or at about 30millivots depending on whether PWM is set or cleared, respectively.

Could this be because I don’t have a motor connected - I’m just checking the voltage at the output pins?

It’s encouraging that you see the bigger voltage when PWM is high. Your test is not the best since a pin that is not driven still has to be at some voltage, so measuring a voltage across two different pins (e.g. your 8.85 V) doesn’t necessarily mean the chip is in an “on” mode. If using a motor is a pain, try something like a resistor. Make sure it’s physically big enough to be able to dissipate the power if it has a small resistance.

Also, the 9V battery is a bad source for something like the VNH3. The chip does have some shoot-through issues that can cause current spikes beyond what the 9V battery can handle. At least put a decent (few hundred uF) cap across the power input, and try to go to something with lower internal resistance, like a 7.2V radio control car battery pack.

- Jan

:blush: , novice mistake. I thought I was playing it safe by running off of a 9v and just checking the outputs. I guess it was considering the situation an open load fault, because once I went to an SLA and had a motor in the circuit it worked fine. Even with the 3.3v control & pwm levels. I guess that’s why they call it a motor controller…

Anyhow, many thanks for the assistance.