• State TI Thinks Resolved
  • Locked Locked
  • Replies 5 replies
  • Answers 2 answers
  • Subscribers 63 subscribers
  • Views 123 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TPS3808: Supervisor IC selection and DSP reset requirement

Carol Guo
Prodigy 20 points
Part Number: TPS3808

Supervisor selection for DSP TMS320F28377:

The VDDIO and VDD of DSP are powered by 5V/3.3V LDO1( ADP3339AKCZ-3.3) and 3.3V/1.2V LDO2(AMS1117-1.2) respectively. 

The minimum output of LDO1 is Vcc1_min= 3.2373V （-1.9% of 3.3V).

The minimum output of LDO2 is  Vcc2_min=1.152V（-4% of 1.2V）.

According to the output range of the LDOs and the recommended working supply range, Is it OK to choose TPS3808G09 and TPS3808G33 as voltage supervisors?

 

The picture above shows the circuit related to voltage monitoring, the reset delay time Td is 130mS(decided by C606) , 

Can this design meet the reset requirements and power supervisor requirements of the DSP?

  • 0
    TI__Genius 11540 points

    Hi Carol,

    Thank you for your question. Below is the threshold table for the TPS3808.

    For the 1.2V rail, the TPS3808G12 should fit-in well. They can even use the TPS3808G125 which has a threshold voltage of 1.16V to detect if the 1.2V is about to hit the Vmin.

    For the 3.3V rail, the TPS3808G33 might detect the failure too late since it's threshold is at 3.07V while Vmin is 3.2373V. I would suggest using the adjustable variant, TPS3808G01, and setting the threshold closer to the Vmin of the LDO.  

    Jesse 

  • 0 in reply to Jesse Villanueva
    Prodigy 20 points

    Thanks for your reply.

    Based on your answer, Can it be understood that that the threshold of the supervisor should be closer to the Vmin of the LDO?

    For the supervisor, we should not only pay attention to the threshold Vit which has accuracy about +/-2%, but also pay attention to the VHYS.

    VIT_max=VITmax+VHYS=VIT_typ*(1+0.015)+VITmax*0.025= VIT_typ *1.015*1.025= VIT_typ *1.04       （1）

    VIT_max =（ Vcc_min + Vin_min）/2           (2)     

     Vcc_min= 3.135V (the minimum supply recommended for DSP),

    Vin_min=3.2373V the minimun output of LDO.

    Based on equation (1) and (2) , Choose IC TPS3808G33 .

     If the Vit is close to 3.2373 as your commend, after adding the Vhys, I'm afraid for some LDO, the reset will never be triggered.

  • 0 in reply to Carol Guo
    TI__Genius 11540 points

    Hi Carol, 

    The hysteresis is applied on de-assertion of the reset. When asserting Reset it is just the Vit-*(1±accuracy)=Vit-max/min needs to be accounted. If the customer would want to allow their LDO to operate outside of spec or have the DSP below Vcc_min then the TPS3808G33 will work fine.  

    Jesse 

  • 0 in reply to Jesse Villanueva
    Prodigy 20 points

    Hi Jesse

    I agree with the definition of Vit and the hysptersis. VIt if for Reset assertion, while the hysteresis is applied on reset de-assertion.

    Carol Guo said:
     If the Vit is close to 3.2373 as your commend, after adding the Vhys, I'm afraid for some LDO, the reset will never be triggered.

    This sentence should be "If the Vit is colse to the minimum output of LDO, after adding the Vtys, the reset will never be deassertion."

  • 0 in reply to Carol Guo
    TI__Genius 11540 points

    Hi Carol,

    My recommendation was to set the Vit closer to Vmin than the 3.07V threshold of the TPS3808G33. I did not necessarily intended for Vit to equal Vmin. I did not give the exact value Vit because the customer will have to consider their systems overall tolerance and make the balance there. But yes, if the customer is okay with the threshold of the 3.3V rail to be 3.07V then that is fine as well.

    Jesse