LM46000: Not possible to Enable when the temperature is > 50degC

Hi Jose,

Please monitor the voltage between the enable and the chip ground, and perhaps investigate your level shifter. 

If you think this is a thermal issue, please post an image of your PCB layout and well evaluate it.

Thanks

Hi Daniel,

Thank you for your reply. Attached all the screenshots and investigation made so far:

- At ambient temperature (25-30deg), the circuit responses as expected after the disable->enable sequence. Monitoring the TP1306 (Enable pin) vs TP1303 (chip ground) I get a first step up to 2.4V followed by a second ramp up to 4.6V, as shown in Fig. 1. (the later ramp is due to the IC ground ramping from [0V] to [-16V].

- If I heat up the circuit externally (50-60deg approx, measured with a general infrared thermometer), there is a first step up to 2.4V (same as before), but the circuit does not get enabled even though the voltage at the enable pin is the same. See Fig. 2.

- Fig. 3 shows the second voltage ramp up to 4.6V happening only after the circuit has cooled down.

      
                          Fig.1: 30degC                                                                              Fig.2: 50degC 
                                  


          Fig3.After cooling down 50degC -> 35degC


I have checked the driving circuit and the outputs are as expected, but you can also see that looking at the previous figures as the EN pin goes up to 2.4V.

Note that even if I tie the EN pin to Vin, the circuit does not start after the first step - so I discarded a low EN voltage issue.


The circuit is placed on the bottom layer of a 10 layers PCB. Here some information:

   
                            Bottom layer                                                                                 4th layer
            
                              3rd layer                                                                                  2nd layer
            
                             Top layer                                                                               Top layer assembly 


Thanks!

 

Hi Jose,

The device sinks a very small amount of current at the EN pin. Normally this isn't an issue, but when resistor values on the 1MOhm order of magnitude are selected, then it becomes an issue.

My suggestion would be to lower the EN resistors to less than 100KOhm max, and see if you have this issues. I'm also not sure what the reason for the BJT is and probably can be removed. Let me know if you can get the circuit to work after you implement both of those things.

Thanks,

Andrew

Hi Andrew,

Thanks for your reply. I have changed both resistors R1728=62k and R1315=11k, keeping the same ratio. The results are unfortunally the same as in my previous message:

Fig.4 shows the circuit working with "cold" temperature.
Fig. 5 shows the circuit not working with "hot" temperature. The voltage at the enable pin is high, but the circuit does not turn ON.
Fig.6 shows the circuit working around 1 min after I stop applying external heat, and the temperature reaches "cold" values.

       

                          Fig.4: 30degC                                                                              Fig.4: 50degC 


          Fig6.After cooling down 50degC -> 35degC



"I'm also not sure what the reason for the BJT is and probably can be removed." :

- I understand that you refer to the BJT pnp, Q1302. Its function is to enable the LM46000, since the EN pin is connected to the IC ground when the BJT is open. So the two cases are:


1) When -16V_EN is high, Q1303 turns ON -> Q1302 base=13.37V ->Q1302 emitter=16V, so Vbe < 0V -> Q1302 is ON -> Enable = 2.41V respect to chip GND (which goes down to -16V when the chip is enabled).

2) When -16V_EN is low, Q1303 turns OFF-> Q1302 base=16V ->Q1302 emitter=16V, so Vbe = 0V -> Q1302 is OFF-> Enable = 0V respect to chip GND.

Summarizing, I cannot control the enable pin if I remove BJT.

Thanks!

Hi Jose,

Understood on the Level shifting BJT circuit.

a collogue of mine wrote an app note on IBB's (see below), and I'm trying to see what he did differently than you. For one, he did opt to use MOSFETs instead of BJTs, but I'm going to assume that's not the issue.

https://www.ti.com/lit/an/snvaa65/snvaa65.pdf?ts=1689267817548&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FLM63615-Q1

Does removing the soft start capacitor do anything? What does SS pin do in Hot and Cold conditions?

Thanks,
Andrew

Hi Andrew,

Attached the tests with and without SS capacitor (C1327)

Fig.7) SS pin (TP1301) vs IC gnd (TP1303) - COLD reset with SS capacitor - disable->enable - the circuit works

Fig.8) SS pin (TP1301) vs IC gnd (TP1303) - COLD reset with SS capacitor - enable->disable - the circuit works

Fig.9) SS pin (TP1301) vs IC gnd (TP1303) - HOT reset with SS capacitor - disable->enable - the circuit doesn't work

Fig.10) SS pin (TP1301) vs IC gnd (TP1303) - COLD after being HOT reset with SS capacitor - enable-> disable - the circuit works (after cooling down)

Fig.11) SS pin (TP1301) vs IC gnd (TP1303) - COLD reset without SS capacitor - disable-> enable - the circuit works

Fig.12) SS pin (TP1301) vs IC gnd (TP1303) - COLD reset without SS capacitor - enable -> disable - the circuit works

                              Fig7.                                                                                     Fig8.

                         Fig.9                                                                                             Fig.10

                           Fig.11                                                                                         Fig.12

Thanks again for the support!

Hi Jose,

If I had to guess, I would say it's something about the way IBB's are configured messes with the startup of the Bias LDO. And then this affects how the device switches. It seems to work fine at room temp, but it changes at higher temperatures.

VIN/VCC/BIAS are all different voltages. I asked to measure VCC  at hot temp as I suspected it wasn't turning on, but you confirmed that's not the case.

Thanks,
Andrew

Okay. Maybe that's the case.

Anyway, thank you so much. I greatly appreciate your help. You've been very thorough.

Have a great day.

José  

Hi Jose,

You're welcome!

Have a great day!

Thanks,

Andrew