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PWM MOSFET driver question

Hi all,

I'm currently designing a full-bridge PWM controlled DC motor driver with a

24V/10A supply voltage. For driving each MOSFET half-bridge, I selected the LM5106 driver IC.

As the '5106 is only available in a 20mil lead spacing MSOP, I can't do any bench tests without designing a PCB right away, so I'd like to check up on things to prevent costly mistakes (this is my first PWM motor driver). Also, the datasheet

formatting link
is rather concise.

Apparently, the high side driver derives its positive gate drive voltage via the bootstrap capacitor. This doesn't mean that the PWM signal *must* have a duty-cycle of less than 100% (full positive DC) for the driver to function properly, or does it? Or does the chip contain some sort of internal oscillator-plus-charge-pump, with the bootstrap capacitor only necessary for storing the energy?

This is important, because I'm using a rather simple way of generating a PWM signal with a triangle wave and an LM339 comparator(*). I don't need precise speed control; it's only important that the motors don't start or stop abruptly. This also means that for most of the time, the "PWM" signal is actually a logic-level DC voltage.

Any information on this (including examples) is very welcome.

*: I'd rather use a microcrontroller-generated PWM signal -- but I'd need six PWM outputs at least, two (both directions) for each motor. The PIC16Fxx controllers I can program don't have this many PWM outputs, and I calculated that it's impossible to generate even a 4-bit resolution 20kHz PWM output in software: the program would have to check/change outputs at 20x16=320kHz; with a PIC running at 20MHz (5MHz internal clock), this would require an interrupt rate of once per 16 clock cycles, which is not possible. The PIC30F2010 does have enough PWM outputs -- but my development environmnent (both the assembler and programmer) can't handle this device.

Thanks in advance, best regards,

Richard Rasker

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http://www.linetec.nl/
Reply to
Richard Rasker
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Bootstrapping designs will require PWM to maintain the charge. They cannot be used for a DC signal to the load. In the case of the 5106, the HB UVLO function will kill the upper driver if the bootstrap voltage falls. This could happen on very high duty cycle (~100%), and it could also happen if the gate capacitance discharges the bootstrap cap too far. Make sure the BS cap is big enough for your application if you stick with this part.

Having HB UVLO disable the upper driver may not be fatal in your application. Its OK to supply a DC input to the chip if your oscillator stops. But if you need a DC output from the driver in those conditions, you are in trouble with this part.

BTW - for prototyping, check Digikey for "Surfboard" products that let you mount a single SMD part on a small FR4 board which can then be attached to a perfboard and hand-wired. They sell for about a buck and are great for sample evaluations. All kinds of SMD patterns are available.

Steve

Reply to
Steve

...

[snip]

Hm, that's a bit of a bummer. I was hoping that the device would feature an internal charge pump or the likes. Then again, the way the capacitor is connected does indeed suggest a mandatory non-100% duty cycle (which leads to complications in the analog PWM generating circuit ...).

I've been searching quite thoroughly, but virtually all combined low+high-side drivers use a bootstrap capacitor. But perhaps it's an idea to generate a voltage of +12 volts above VDD, and hook that up to HB? Or could this cause trouble with the HS line? The datasheet isn't clear at all on this point.

I know, I have created several "standard" mini-PCB's myself for easily hooking up individual SOT23-5, SSOP and other SMD devices. I just don't have an MSOP-10 board -- and the only one from DigiKey I could find (551012876-001-ND) is a bit on the pricey side, at $600 ;-) So I guess I'll have to fire up the 'ole etch tank, and create something myself.

Anyway, thank you for your reply, it helped me avoid at least one mistake.

Best regards,

Richard Rasker

--
http://www.linetec.nl/
Reply to
Richard Rasker

If you must have 100% duty cycle to the motor, then make a bootstrap supply for yourself -- you could either do this with a little flyback circuit, or a charge pump.

You can arrange for a guaranteed maximum duty cycle of less than 100% using analog components if you really want to.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
Reply to
Tim Wescott

Hi Richard,

This doesn't answers your question directly, but can you use a HIP4080A/4081A? This one has a built-in charge pump and is a full bridge driver.

Best regards,

Wim

Reply to
Wimpie

Hm, somehow I completely overlooked these in my search ... it seems that the '4080 is ideal for my application, as it even features a comparator for direct use with a triangle wave.

Thanks for the tip-off, I'll order a handful of these first thing tomorrow morning!

Richard Rasker

--
http://www.linetec.nl/
Reply to
Richard Rasker

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