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Original question:

TPS54020: question about Ratiometric and Simultaneous Startup Sequence

TPS54020: Tracking design questions

Barry Weinberger69
Prodigy 20 points
Part Number: TPS54020

Dear TI Support,

 

Attached is a schematic drawing showing three (3) power supplies, each designed around the TPS54020 switching regulator.

The design team no longer exists and I’m trying to reverse engineer this design to see that it satisfies the requirements of the TPS54020.

A timing diagram is shown on the drawing which indicates Turn ON voltage and Coincidental Tracking.

 

I have a few questions about this design and was hoping TI could provide some recommendations and guidance.

My first question relates to the ON/OFF Threshold specified by the resistor divider of R3 & R5. Each supply has identical thresholds.

According to the datasheet, “To achieve clean transitions between the OFF and ON states, TI recommends that the turn OFF threshold is no less than 4.2 V, and the turn ON threshold is no less than 4.4 V on the VIN pin.”

Since the OFF & ON trip points are 4.2V and 4.4V (rounded), there is no margin. Additionally, the Main +5.0V input can range from +4.634V to +5.234V.

 

Secondly, I remain confused about how to calculate the Tracking resistors.

Based on the attached design, I cannot figure out how ΔV was calculated and if the selected resistors meet the design requirements.

Additionally, can you explain what the SS input threshold is that turns ON the part.

I clearly don’t understand this specification;

 

From a new design perspective, what would be the correct design steps for coincidental tracking. How does one arbitrarily choose values for VOUT1 and VOUT2, then guarantee that value of RS1 is satisfied.

From the datasheet “The ΔV variable is zero volts for simultaneous sequencing.” Well, if ΔV = 0V, then RS1 computes to 0Ω, and RS1 fails to meet the requirements of “RS1 > 2800 ´ VOUT1 -180 x ΔV”

 

So, based on the information provided, any assistance, explanation, and guidance you can provide is GREATLY appreciation.

Thanks so much!

 

Best regards,

 

Barry Weinberger

 

Barry Weinberger
 Sr. Hardware Engineer

Curtiss-Wright
20130 Lakeview Center Plaza, Suite 200, Ashburn, VA 20147
T: 703-840-6727
bweinberger@curtisswright.com | www.curtisswrightds.com

TPS54020_startup.pdf

  • 0
    TI__Mastermind 35115 points

    Hi Barry,

    For the EN pin, I do not see any problem with using the resistors you have now. However something  you may want to consider doing is to tie all EN pins together since you want them to turn on at the same time. This way if one TPS54020 turns on, the EN hysteresis will pull up the EN voltage help to make sure the other TPS54020's turn on very soon after.

    For the SS pin, I think you might have posted an image that didn't come through. Are you asking about SS/TR to VSENSE matching specification?

    Also I checked the datasheet and there is an issue with Equation 5 that might be causing some of the confusion. In the numerator in of the first part of the equation, it should say "VOUT2+ΔV". I have double checked the resistor values that come from the equations with this change and the results make sense. Excel sheet is I used for checking this is attached.

    Simultaneous Startup Calcs.xlsx

    Do you need to use the PWRGD pins in this application? The SS pin needs to come up to 1.8 V worst case for PWRGD to transition high. The 1.8 V output SS pin may not get to a high enough for its PWRGD to go high.

    Best Regards,
    Anthony

  • 0 in reply to Anthony F
    TI__Mastermind 35115 points
    A correction to my last post. SS needs to get to 1.4V for PWRGD to transition high. This is given in the POWER GOOD section of the electrical specifications table.

    I'm also going to mark this as resolved for now. Discussion on this has moved to e-mail through the field apps engineer. Barry, feel free to post again here if you have any further question.