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Old 27th April 2013, 12:08 AM   #1
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Default Class D IC running hot with just filter connected?

Any ideas why (no output load, just the two inductors & capacitors connected to pin 6 & 9 as per the datasheet)....

Click the image to open in full size. (NJU8752V)


put the IC into standby (pin 11 low) ....IC cools rapidly, but out of standby the IC heat ups....I thought class D ICs were meant to run cool?

I'm using 33uh inductors & 0.1uf caps.

PS I don't have the 5-10R resistors on the output pins....as I think they are just there to present some resistance at high frequencies as this IC is meant for a piezo speaker.

Last edited by peskywinnets; 27th April 2013 at 12:12 AM.
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Old 28th April 2013, 10:38 PM   #2
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Nobody?

I'm quite new to class d, so could use a little help here! I've scoped the output with no filter connected (and with the 'in' left side of the input cap grounded)...I see a nice clean square wave @200khz 50% duty cycle (i.e. everything as expected)

I only seek bandwidth up to about 10khz.

How can I progress this so that I can have a cool to touch class D IC?!!
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Old 28th April 2013, 10:58 PM   #3
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Class D is efficient, not "no loss". Datasheet says 10 mA idle max with 13.7 volt max. That's 137 mW which would make the IC "warm". If your output supply voltage is lower than that and you have more loss then something is wrong. Did you measure the actual supply current?
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Old 28th April 2013, 11:11 PM   #4
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Class D amplifiers have idle losses, ie a queiscent current they run at when doing nothing multipled by the voltage the device is run at.

I had a quick look at the datasheet for the device you're talking about and I couldn't see any where they listed the quiescent current draw for the analogue portion of the device. This makes trouble shooting hard as we don't know how hot it should actually run. Now most Class D parts are fine running hot, this could just be one of them. The package is very small and cannot dissipate that much heat.

If you've got a scope it'd be worth checking the output waveforms for the possibility of any unwanted oscillations, an unstable amplifier will run hotter than a stable one.
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Old 28th April 2013, 11:14 PM   #5
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Andrew, as far as I could see the current listed is for IDD, which I took to mean the digital portion of the chip only, as IDDO would = the current draw for the analogue part.
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Old 28th April 2013, 11:17 PM   #6
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The control supply probably doesn't use much power at all but must be below 3.6 volts. If the circuit is found to be operating normally, by checking the idle current of the output section, try reducing output supply voltage to 9 volts or so for reduced no signal dissipation, even lower if you don't absolutely need the power. Post supply voltages and currents if more help is needed.
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Old 28th April 2013, 11:22 PM   #7
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You are right 5th! Along with no idle power dissipation spec, makes it difficult to be confident in the chip and circuit condition. Only dissipation max is given at 450mW max for the SSOP14, which probably would make it "hot". Such small packages get hot fast with fairly little power dissipation. Reducing the output supply voltage should make a noticeable difference in switching loss. Below 10kHz with a piezo load, most of the power loss will be switching loss anyway, changing pretty slightly depending on drive signal.
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Old 28th April 2013, 11:41 PM   #8
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Note that this chip is intended for capacitative loads. Piezoes are typical over hundreds of ohms in the audible range, so you shouldn't remove the 5-10 ohm resistor before the output filter. You should replace it with a similar load as the capacitance in the piezo would have been. Typically around 0.10 uF.
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Old 29th April 2013, 01:32 AM   #9
godfrey is online now godfrey  South Africa
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With no load and without the 5-10 Ohm resistors, the output filter forms a high-Q tuned circuit with a very low impedance at some frequency. Maybe that's causing instability or some other problem.
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Old 29th April 2013, 04:18 AM   #10
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Quote:
Originally Posted by peskywinnets View Post
... this IC is meant for a piezo speaker.
What are you using it for?
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