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TPS61021A: How to set a proper UVLO threshold for single cell Ni-MH application?

Alper Yazar
Prodigy 250 points
Other Parts Discussed in Thread: TPS61200, TPS61021A, TPS61030, TPS61020, TPS61024

Hi,

I am planning to use TPS61021A to generate 3V3 from a single Ni-MH (classical AA battery let's say) cell. Light load efficiency is important for this application. Efficiency of TPS61200 seems to be worse than TPS61021A in my case. However unlike TPS61200, TPS61021A doesn't offer an adjustable UVLO threshold values. According to the datasheet of TPS61021A, it operates until the input is low as 0.5V. However to protect Ni-MH battery, I want to cut-off the operation around 0.9V or 1V. How can I achieve this operation? Should I use a comperator with hysteresis to disable TPS61021A via EN pin? In that case, I couldn't find a proper comperator due to low operating voltage limit (0.9V let's say). Could you give some suggestions? The operation temperature will be between -20C and 50C.

Thank you.

Alper.

  • 0
    TI__Mastermind 40080 points

    Yes, the conversion efficiency of TPS61021A is much higher than TPS61200.

    Why do you must need UVLO function? Can you just use the EN pin to realize the UVLO fucntion?

  • 0 in reply to Helen chen
    Prodigy 250 points
    Hi Helen,

    Thank you for clarification about efficiency.

    Need for UVLO function arises from battery protection. Ni-MH batteries should be protected from over discharge. The recommended cut-off voltage during discharge is around 0.9V. Therefore I would like to implemenet a cut-off circuit to protect the battery. TPS61021A couldn't provide the protection itself since its V_IN_UVLO during Vin falling which corresponds to discharge is given as 0.5V (max) in section 6.5 Electrical Characteristics at SLVSDM0. This is a very low threshold value for proper protection.

    I think utilization of EN is not trivial. Here is the my first approach:

    Let's divide Vin with a resistor divider and feed to EN pin. In this case, Ven = R1 / (R1 + R2) * Vin. Let's consider V_EN_L (EN Logic Low threshold) value given in 6.5 Electrical Characteristics section. Let's say that the desired UVLO threshold value is 0.9V. Since 0.9 V< 1.2V, V_EN_L = 0.3 x VIN. It is ratio of the input too. Since resistor based divider also gives a ratiometric output, it is not possible to find a R1 and R2 values for this set point just for Vin = 0.9V.

    My second approach was designing a non-linear voltage divider which is resistor divider + series diode basically. Then, Ven = R1 / (R1 + R2) * (Vin - Vf). Let's take Vf = 0.7V.

    0.3*Vin = R1 / (R1 + R2) * (Vin - 0.7)

    Vin = 0.9V

    0.27 = R1 / (R1 + R2) * 0.2

    A solution exists now. But this is not very good solution due to two reasons. One is Vf value of diode. In order to use a diode as a voltage regulator (i.e. to keep Vf at almost a fixed value) IFshould pass through it. I want to keep current passing through the diode as low as possible to provide long battery life. However value of IF is in mA order in general. Once the diode current is below IF, its drop voltage fluctuates between 0 - 0.7V. The second problem is the operating temperature. As I stated, the operation temperature will be between -20C and 50C and this span of temperature may change behaviour of the diode based design.

    Even if this basic approach works, proper UVLO circuit needs hysteresis. When the converter is disabled using EN pin, battery voltage raises due to no load condition. This raise reenables the converter again. This will cause on/off oscillation.

    I don't know whether I am missing something or not but TPS61021A seems to be missing to me. According to 7.2 Functional Block Diagram circuit the IC contains UVLO block but its internal unlike TPS61200. However this decision (having no UVLO pin externally) makes TPS61021A less useful. In applications section it is stated as suitable for Battery Powered IoT Devices but I think it is not so trivial to provide a proper UVLO mechanism without using a bunch of discrete components. I really don't get the design philosophy behind TPS61021A.

    Best,

    Alper
  • 0 in reply to Alper Yazar
    TI__Mastermind 40080 points

    Thanks for the update.

    The reason we don’t integrate this feature in new products is that the battery voltage is very easily monitored by MCU in the system. So could you use the MCU’s ADC to monitor the battery voltage?
    Our device TPS61030 and TPS61020, TPS61024,25,26,27,28,29   support Low Battery Comparator function.


    Could you please share the detailed application information for us? Like in what kind of equipment? Volume? This is also one of our work :)  You can send the information to me by email, my email address is helen-chen@ti.com.

  • 0 in reply to Helen chen
    Prodigy 250 points

    Hi Helen,

    I see your design philosophy and I agree that it is completely fine with most of the systems. However in may case, system won't include a MCU. It will be a small power converter boarad only. Of course I can include a low cost MCU but I don't want to bother with it. Pure analog solution would be much better.

    Your suggested devices look good actually. Although TPS61030 and TPS61020 have built-in UVLO, for example, they are not listed in the parametric search. This is why I missed them. Also they have LBO and LBI pins. If I am able to connect LBO to EN pin with pull-up to imitate UVLO function, that will be fine for my need, I think. However, I am not sure whether it causes a chicken-egg situation or not. Additionally, they have also built-in UVLO and it may be sufficient standalone. But being able to control EN pin using LBO output makes changing UVLO threshold possible.

    This board will be a part battery powered "IoTish" device requiring long sleep duration. However it should work as a seperate module. Unfortunately, this is not a commercial project and production volume is near zero. :) So, I don't want to take your valuable time more by sending details of the project, requirements etc. This level of help is more than sufficient for my project.

    Best,
    Alper

  • 0 in reply to Alper Yazar
    TI__Mastermind 40080 points
    Got it, good luck to you :)