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MAX232: MAX232 overheat

Part Number: MAX232

Hello people,

How are you?

I'm using the MAX232DWR part number in my project to  connect in a smart card reader, according attached file.

Sometimes during card reading operation, occurs an overheat in MAX232 IC.

Why is it occuring? Can you help me to resolve this issue?

Best Regards

  • Hi Sergio,

    The datasheet for this part recommends using 1-uF capacitors for the charge pump circuit. Can you try using those instead and see if it resolves the heating issue? With the smaller capacitance I am concerned that maybe the charge pump needs to switch too frequently, resulting in increased power dissipation in the device.

    If that does not resolve the issue, then it would be good to understand what application conditions exist when the issue occurs. For example, does it occur at power-up or later on during normal operation? If during normal operation, does it correlate to a certain DOUT condition like driving high, driving low, or switching? It would also be useful to probe some relevant signals on an oscilloscope like the logic IOs, the RS-232 signals, and the power rails (V+/V- as well as VCC).

    Regards,
    Max
  • Hi Max,

    Thank you for your help.

    I followed your recommendation to change the capacitors for the  charge pump circuit from 100nF t0 1uF. I did 6 tests to read the smart card . In the 5 try, occured the overheat and the circuit stoped to work. I powered off the circuit and I did the 6 try. In this case, didn,t occur the overheat and the circuit worked as expected.

    Below is the schematic of the supply circuit to MAX232. It is supplied trough a microcontroller enable signal in the read smart card operation. So, the issue occurs after the power supply circuit is enabled.

  • Hi Max.

    I have another question about this issue. You told me that the capacitors for the charge pump circuit must be of 1uF. My question is if the capacitors between C1+ and C1- and C2+ and C2- also must be of 1uF.

    Best Regards

    Sergio Fink
  • Hello Max.

    Bellow are the wave form of pins 2 and 6 of MAX232 in NO OVERHEAT state and in OVERHEAT state:

    NO OVERHEAT STATE: PIN 2 = 9,2V

    PIN 6 = -8,2V

    OVERHEAT STATE: PIN 2 = 3,4V

    PIN 6 =1,8V

    Best Regards

  • Sergio,

    Thanks for your information. If changing the capacitors doesn't help but power cycle does, we may need to take another look at your power design. When overheat happens, is it possible to measure the voltage and current output of Q401? Or you could capture the waveform of MCU enable and power ramping up. BTW, when you enable the read smart card operation, what's the states of MAX232, like is there signal on RS-232 bus?

    Regards,

    Hao

  • Hello Hao,

    How are you?

    Answering your question, in the overheat state the voltage is 5V and the current is 530mA. In no everheat  state  (normal operation)  the voltage is 5V and the current is 75mA.

    Best Regards

  • Sergio,

    Thanks for your data. Have you ever been able to check the ramp up voltage/current plots? I suspect sometimes somehow during power up, the device draws too much current from the supply and locks the system in a high current state.

    Regards,
    Hao
  • Hao.

    Below is the voltage wave form during power up. Unfortunately we can't provide the current wave form because we don't have a current probe point for osciloscope.

    Regards

    Sergio

  • Sergio,

    Thanks. Several more questions: if you power cycle the overheat devices (maybe several times), are the devices always able to work normally? Have you ever tried with L400 shorted (connecting Vcc_smart and 5V directly)? Is MAX232 powered by Vcc_smart for 5V? What's MAX232 T1OUT pin's load during power up? Is TXD high or low? Can you test with the load removed or setting TXD in other state?

    Regards,
    Hao
  • Hao,

    Answering your questions:

    1) if you power cycle the overheat devices (maybe several times), are the devices always able to work normally?
    R: Yes

    2) Have you ever tried with L400 shorted (connecting Vcc_smart and 5V directly)? Is MAX232 powered by Vcc_smart for 5V?
    R: Yes. But the result is the same.

    3) What's MAX232 T1OUT pin's load during power up? Is TXD high or low?
    R: TXD is in 240mV

    4) Can you test with the load removed or setting TXD in other state?
    R: When I removed the load of TXD pin, the overheat occurs and the voltage is the same (240mV)

    Regards

    Sergio

  • Sergio,

    Do you mean TXD (T1IN) pin is 240mV during power up? If nothing is connected to T1IN, does it stay at 240mV? What's the voltage of T1OUT? Could you monitor the voltage of these two pins during power up? Do you still see any difference between normal and overheat?

    Regards,
    Hao
  • Hello Hao.

    Correcting the last informations, bellow follows the voltage in no overheat and in overheat states:

    1) TI1INI NO OVERHEAT:

    2) R1OUT NO OVERHEAT

    3) T1IN WITH OVERHEAT

    4) R1OUT WITH OVERHEAT

    The signals R1OUT (RXD) and TI1INI (TXD) are conected in the IC AP6502SP-13.

    Best Regards

  • Sergio,

    Is AP6502SP-13 powered up the same time as MAX232? Does MAX232's supply have anything to do with AP6502SP-13? After you disconnect R1OUT from AP6502SP-13, do you still see overheat?

    Regards,
    Hao
  • Hello Hao.

    1) Is AP6502SP-13 powered up the same time as MAX232?
    R: Yes, because the supply of AP6502SP-13 is 3.3V from TPS76933. The TPS76933 is supplied from 5V, according schematic bellow.


    2) Does MAX232's supply have anything to do with AP6502SP-13?
    R: No. According last reply, the MAX232 supply is 5V, not 3.3V.

    3) After you disconnect R1OUT from AP6502SP-13, do you still see overheat?
    R: No. The R1OUT voltage when it pin is disconnected is 5V and the overheat does not occur.

    Best Regards

    Sergio Fink

  • Hello Hao.

    Sometimes even with the R1OUT pin disconnected, the R1OUT  signal occurs as the picture bellow:

    Best Regards

    Sergio Fink

  • Sergio,

    One possible explanation is that RXD of AP6502SP-13 is configured as an output somehow during power up. Therefore it could draw high current from MAX232, like a short current. I could not find the data sheet of AP6502SP-13, except this link (https://www.diodes.com/assets/Datasheets/AP6502.pdf). Can you try to flop the logic of R1IN of MAX232 to see if it makes any difference?

    Regards,
    Hao

  • Hao,

    Good morning.

    I apologize, because I sent for you the wrong schematic of RXD pin in uC. The correct schematic is shown below:

    Best Regards

    Sergio

  • Hao,

    Analyzing the STM32F030K6T datasheet, it's possible to configure as a RXD pin. Do you mean that is it possible that after reset the pin can became as output?

    Best Regards

    Sergio

  • Sergio,

    Thanks for your update. In general GPIO pins should not be set as output after reset. There is no 'default' column in the datasheet either. I'm not sure if it's possible the pins are output during power up. Sometimes as the supply ramps up, the chip is not ready and the output state could be undetermined. There is one more thing you can try. In stead of powering up together, you could power up MAX232 and MCU separately then connect RXD between them.

    Regards,

    Hao

  • Hao,

    good morning.

    Analyzing your reply below:

    "In stead of powering up together, you could power up MAX232 and MCU separately then connect RXD between them."

    I have some considerations:

    1) This suggestion can even resolve the issue, but in reality, this is not a usual pratic in projects develpopment, because in a real development
    situation, I never saw any project that the ICs power supply are done step by step.

    2) I believe that the better next step is to test other MAX232 samples to verify if this problem also occurs.

    Do you agree?

    I have a question for you. Is there another RS232 IC Drivers/Receivers that it can be used instead MAX232?

    Best Regards

    Sergio

  • Sergio,

    I get your point. The experiment was used to identify the root cause. If we're sure where the problem is, we could start thinking of a solution. There are some other ways other than powering up ICs in steps. For example, we could control the state of PA3 in power up, or choose another RS-232 so you can disable R1OUT. TI has many other RS-232 transceivers you can take a look. The easiest way to go to www.ti.com/RS232 and browse based on your need.

    Regards,
    Hao
  • Hao,

    I received MAX232 samples from other manufacturers and the next step will be to test them. 

    I thank for your support.

    Regards

    Sergio

  • Sergio,

    Good luck with your test. Please let me know if you have more questions.

    Regards,

    Hao