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Audio Amplifiers Forum
LM4780 application question
we slow increase input single in LM4780, but when input single reach large enought then IC output single will dissappear.
Is this phenomenon is normal(Due to LM4780 should be have SPiKe Protection)?
SPiKe is used to protect from SOA failures.
It sounds like maybe you're suffering from thermal? Check out the data sheet, the bottom of page 15 where it discusses "THERMAL PROTECTION."
It sounds to me like this might be your problem?
Audio Applications Engineering Manager
Dallas, TX USA
Many thanks for your reply.
May I verify with you again that just long term over thermal stress will let LM4780 shuts down?
SPiKe PROTECTION(OCP/OVP peak-temperature stressing) enable will just let LM4780 output waveformdistortion,right?
Unfortunately, I'm not super familiar with this family of devices. The apps guys who work on them are currently on vacation, but will be back next week.
The LM4780 and all SPiKe protected devices are protected against both long-term thermal issues (like a degradation in the interface to the heatsink causing thermal shutdown to cycle on and off) as well as short-term instantaneous high power bursts (both in output and IC power dissipation).
When thermal shutdown cycles on and off due to a poor interface between the IC and the heatsink, the output of the IC will turn off when it gets too hot and then turn back on again when it cools down. If music is still trying to be played through the device, the IC will again heat up and then shut down; continuing this cycle. The user will then understand that there is an issue with the unit and get it repaired.
The factors that affect this protection mechanism are a bit harder to quantify, but I will try to explain.
For the shorter-term instantaneous protection, the IC will protect itself when its output power stage senses extreme instantaneous power dissipation. The factors that affect this protection are:
1. overall IC junction temperation, which is dependent upon level of heatsinking as well as ventilation and ambient temperature. (Try to keep IC case temp low.)
2. power supply voltage and the power supply stiffness. (Only go as high as needed on Vcc)
3. load impedance it must drive. depending upon input signal amplitude & frequency and the reactance of the load, the IC's internal power dissipation can reach very high levels, but the IC will still be protected.
What happens when SPiKe is enabled? Please refer to the following application note: SNAA008A http://www.ti.com/lit/an/snaa008a/snaa008a.pdf
Essentially, you don't want to have this protection enabled during normal music playback.
For a proper thermal design this protection mechanism should not come on under normal operating conditions. Testing what the IC can provide in terms of maximum power supply voltage and minimum load impedance before releasing to production is highly recommended. Overall thermal dissipation will be increased when supply voltages are maximized while load impedances are minimized.
If you find in your testing that SPiKe is coming on a lot, then you could either have too poor of a thermal design and/or you are trying to drive the IC too hard. Back off on the supply voltage or beef up the thermal design. Quickly adding a fan in your testing will exemplify this.
Finally, you can use this application note as a general guideline. SNAA021A http://www.ti.com/lit/an/snaa021a/snaa021a.pdf
This IC and its counterparts are extremely robust and are meant to minimize customer returns.
There are two different types of protection reffered to in your email. SPiKe will limit the instantaneous power in the output trasistors but will not turn off the output signal. There is more information avaiable in the appp note: http://www.ti.com/lit/an/snaa008a/snaa008a.pdf
The output will turn off when the device gets too hot. That seems to be what you are describing. As the input signal increases the output power dissipation also increases. If the device gets too hot, the output will turn off, or dissapear. When the device cools the output will turn on, or re-appear.
Hope this helps,
Many thanks for your reply,
But I met condition is strange, I increase input single reach 680mV, the Out A will no signal output but OUT B still have output. when i reduce input signal.
OUT A will recover output. it seems doesn't like thermal thermal shutdown. also doen't SPiKe protection.
Is any event will casue this condition?
This is probably caused by the fact that the thermal shutdown of the two channels is independent. The two channels may be at slightly different temperatures and the thermal shudown of each channel may be slightly different. Once one channel shuts down, the total power dissipation is cut in half. This will allow the remaining active channel to function at a higher power dissipation. The key is to make sure the part has adequate heat sinking to keep both channels on.
Did you get this sorted out?
Mant thanks for your reply.
Your answer is ok for us.
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