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TPS65150: 7.3.3.1 Positive Charge Pump Output Voltage

Ray Vincent Aranzaso
Genius 13515 points
Part Number: TPS65150

Hi,

Good Day. Please help to check the customer's query below. I also try to check what he said and I think he has a point. I need your confirmation if his query is correct.

The TPS65150 datasheet I have  (SLVS576B –SEPTEMBER 2005–REVISED JANUARY 2016) has calculations in section 7.3.3.1 Positive Charge Pump Output Voltage. There is a note "TI recommends choosing a value for R2 not greater than 1 MΩ".Using this value for a Vo of 20V gives R1=16Meg Ohms. This is way too big for reliable operation. I think the note should say that R1 should not be more than 1 Meg Ohms.

My computation to check the customer's query/

Vo = (1 + R1/R2) * Vref; Vo=30V (maximum output voltage for positive charge pump), R1=1Mohm, Vref=1.214V

R2 = 42.17 kohm

Vo = (1 + R1/R2) * Vref; Vo=30V (maximum output voltage for positive charge pump), R2=1Mohm, Vref=1.214V

R1 = 23.71 Mohm (too big for reliable operation)

*******************************************************************************************************************************************************************************************************************************************

My computation to check if R2 should not be greater than 1Mohm,

Vo = (1 + R1/R2) * Vref; Vref = 1.214V

--------------------------------------------------------------------------------------------------------------------------------------------

if R1 = 1Mohm, R2 = 1kohm

Vo = (1 + 1Mohm/1kohm) * 1.214V = 1214.214V (it exceeds the maximum output voltage of TPS65150)

 

if R1 = 1Mohm, R2 = 100kohm

Vo = (1 + 1Mohm/100kohm) * 1.214V = 13.354V

--------------------------------------------------------------------------------------------------------------------------------------------

if R1 = 1kohm, R2 = 1Mohm

Vo = (1 + 1kohm/1Mohm) * 1.214V = 1.215214V

 

if R1 = 100kohm, R2 = 1Mohm

Vo = (1 + 100kohm/1Mohm) * 1.214V = 1.3354V

I just confuse when I check this query on also my computation that's why I ask the forum for clarification and double checking. Thank you very much.

Best Regards,
Ray Vincent Aranzaso
Texas Instruments Customer Support

  • 0
    TI__Mastermind 18550 points

    Hi Ray,

    I believe the statement in the datasheet is correct. Think of it this way: feedback voltage (FBP) is maintained nominally at at 1.214V. Because input impedance of the feedback pin is quite high, a small current flows into the feedback pin and one would normally want the overall current through the feedback resistor divider to be much bigger than the current into FBP pin to swap out any error. If value of R2 is bigger than 1M ohm, current through it (1.214V/1M ohm) would be quite small which is not desirable for swamping out the error. Hence the recommendation for resistor value of R2 to be lower than 1M ohm. For example a value of 100K or 150K would be quite acceptable.

    Kind Regards,

    Liaqat

  • 0 in reply to Liaqat Khan
    TI__Genius 13515 points

    Hi Liaqat,

    Good Day. Please see below the response of our customer to your reply. Thank you very much.

    Although your proposal to keep things “as is” is not “wrong”, I believe it is sub-optimal for several reasons.

    1. Historically, the older document for the 65130 mentions “In this example, with R2 = 130 kΩ, choose R1 = 1 MΩ to set VPOS = 10.5 V”, selecting R1 at 1 MΩ.  Had this document chosen R2 for 1 MΩ, R1 would have been 7.6 MΩ.  I’m assuming the R1 choice was selecting a maximum suggested value, although this was not stated.
    2. If somebody does use 1 MΩ for R2, the impedance at that node will be a little less than 1 MΩ, which will be susceptible to noise pickup and measurement errors with the typical 10 MΩ input resistance of voltmeters.
    3. Some of my previous designs (learning experiences) that used resistors greater than 1 MΩ have been susceptible to noise pickup, troubleshooting measurement errors, and PCB leakages.  I’ve stopped doing that.

    I don’t see that suggesting the maximum values of both resistors be kept below 1 MΩ, or that the Thevenin equivalent value be kept below 1 MΩ would cause problems with proper operation of the circuit.  I do believe that if a novice designer uses R2 of 1 MΩ, there will be problems with the resultant circuit.  I gave this equation to an associate (without any of my apprehensions in the request), asking him to calculate the value of R1 for a 10V output.  After he performed the requested calculation, he gave me his returned result and the comments that this was probably a poor choice of values, suggesting that the resistance be dropped down by at least a factor of 10.  Please ask a couple of your general design engineers to comment on my whining complaints.

    Best Regards,
    Ray Vincent

  • 0 in reply to Ray Vincent Aranzaso
    TI__Mastermind 18550 points

    Ray,

    The description in the datasheet of R2 less than 1M ohm is technically not wrong but I can see where customer is coming from. It would have been better if a lower limit, like 200K, was stated instead of 1M ohm. I would make a note and possibly change it if/when datasheet comes up for revision in the future.

    Kind Regards,

    Liaqat

  • 0 in reply to Liaqat Khan
    TI__Genius 13515 points

    Hi Liaqat,

    Good Day. It is ok that I will share your response to our customer so that he will inform about the possible changes in the datasheet? Please advise. Thanks.

    Best Regards,

    Ray Vincent

  • 0 in reply to Ray Vincent Aranzaso
    TI__Mastermind 18550 points

    Ray,

    I have replied to your query using internal mail.

    Kind regards,

    Liaqat