Trying to design a proper UVLO circuit for single Ni-MH cell discharge. Ideas?

[alper.y]

Hi,

I am trying to design a boost converter that steps up a standard single Ni-MH battery (AA) to 3V3. My initial choice was  TPS61021A. However it doesn't have a proper UVLO mechanism to protect battery from over discharge. Its cut-off voltage is 0.5V and this is too low for safe cut-off. Other option is TPS61200. This chip has a suitable UVLO but its efficiency is much less than TPS61021A. Therefore I am trying to add UVLO mechanism TPS61021A using its EN pin.

The circuit should behave like that:

• Its output should be Vin when Vin > 1V
• Its output should be less than 0.3Vin when Vin < 0.9V. Notice that the circuit has hysteresis to prevent on/off oscillation.
• It should operate between -20C and +50C.
• It should have low Iq to prolong battery life.

Although it seems to be a trivial circuit, I haven't found a satisfactory solution yet. Up to now, I have asked on Stackexchange (http://electronics.stackexchange.com/questions/253479/how-to-protect-single-cell-ni-mh-battery-overcurrent-undervoltage-overtempera) and E2E of TI (https://e2e.ti.com/support/power_management/non-isolated_dcdc/f/196/t/537222).  I will be happy if you suggest any idea.

Thanks.
Alper

[Marco]

Easier said than done, a simple diode voltage reference lacks the precision necessary so you can't just play around with a couple of BJTs. You would need a proper voltage supervisor, they exist. On semiconductor has a few 0.9v ones for instance.

What's wrong with the TPS61025 though?

[tggzzz]

The Tektronix 1502 has an effective under voltage detector - but for a battery of 9 NiCd cells. Essentially it is a voltage detector that fires a two transistor "SCR" crowbar. The service manuals are available on the web.

[David Hess]

I do not quite understand the problem.  The TPS61021A datasheet says that it will not start until the input voltage reaches 0.8 volts typical so drive the enable pin with a comparator powered from the output to compare the output voltage and battery's voltage to raise the cutoff voltage.  Disabling the regulator should then latch it off unless the input voltage rises to 0.8 volts typical.

Isn't it safe to discharge NiMH cells like NiCD cells to zero volts and leave them that way?  At least I thought it was.  Reverse charging is of course another matter entirely but not a consideration where only one cell is used.

Leave it to Texas Instruments to include a useless schematic; can these guys ever get datasheets (and ADCs, and IC sockets) right?  Which side of the PMOS transistor is connected to the output?  Is there a body diode?  If the regulator is shut off, does the output follow the input minus one Vbe?

Assuming that there is a body diode and the source of the PMOS transistor is connected the output, you will probably need to add a common base bipolar PNP transistor in series with the output to shut off the regulator if you want the output to fall below 0.3 volts; the base connects through a resistor to the regulator's input.  The efficiency loss is minimal because the transistors collector to emitter saturation voltage is in series with the output.  Check out Linear Technology design note 109 and 110 for an example.  There are other micropower boost regulators with built in output disconnects if that is a requirement.