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LM5020: LM5020MM-1 won't start up near -40ºC

Jung-hui Cheng
Prodigy 150 points
Part Number: LM5020

We have two power supplies using LMM5020MM-1 which could not be powered up near or under -40ºC. If the PWM IC are powered up successfully above -30ºC, then the PWM IC can continuously to work even under -40ºC.  Pin SS (soft start) is held low voltage  (0.74V across external 0.01uF cap) all the time during  -40ºC start-up, indicating "Softstart Current Source" (10uA) may be disabled.  Could you check if "Softstart Current Source" is sensitive to -40ºC or lower? 

  • 0
    TI__Guru 68595 points

    Hi,

    Thank you for using the LM5020.  SS can only be activated after the device is enabled.  Can you check your UVLO pin voltage?  Would you please show us your schematic, and also the operating conditions (Vin, Vout, Iout, etc)? 

    Thanks,

    Youhao Xi, Applications Engineering

  • 0 in reply to Youhao Xi
    Prodigy 150 points

    Schematics is similar to Figure 1 of LM5020MM-1 datasheet.  We checked all the voltages on the IC's pinouts.  VCC=7.56V and  UVLO= 2.98V

  • 0 in reply to Jung-hui Cheng
    TI__Guru 68595 points

    Do you check the IC pinouts when the circuit stops working at -40C, or you measured these at room temperature?  When is the Vin, and your Vout setting?

    Also the datasheet figure 1 is just a simplified schematic without any value labeled.  I need to review your actual schematic.

    Thanks,

    Youhao 

  • 0 in reply to Youhao Xi
    Prodigy 150 points

    We have two designs - one is below and the other is Pin VCC not tied to Pin VIN (measure VCC = 7.5V at cold but voltage on pin SS was not charged up) . VIN was 10.5VDC during startup. we measured at cold when they did not not start up by low voltage on Pin SS. 

  • 0 in reply to Jung-hui Cheng
    TI__Guru 68595 points

    Hi,

    According to the datahseet, EC table on page 3, the VCC UVLO rising edge typical is VccReg-100mV.  VccReg typical is 7.78, and max can be 8.0, this means your Vcc=7.5V may not exceeding the VCC rising edge threshold, explaining why the circuit stays off. 

    I also I noticed you do have a diode to feed into VIN pin, and at cold the diode has more voltage drops, and the min VIN condition of 13V is not met. 

    Can you disconnect VCC from VIN, and add a VCC capacitor, and remove D64 (or D84, which I cannot read clearly from your posted schematic?  I mean the diode from +15V_BA to VIN path).

    If you want to supply VCC externally to work with VIN<13V,  you should supply VCC>8V but <15V (leave at least 1V margin from VCC abs max rating for safety considerations).

    Hope this explains.


    Regards,

    Youhao Xi, Applications Engineering