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Is it safe to use a MOSFET with gate connected to the low battery comparator output of this TPS61027 switching regulator in order to disconnect the load once batteries become low? The LBO is open-drain, and recommends a \$1 M \Omega\$ pullup to the 5V boost output line (I'm also wondering whether that resistor is a bit too big to effectively turn on the MOSFET when the open-drain is not active [no low battery condition]).

Schematic of idea

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It'll work with 1Mohm and you could use a 10k or 100k with out problems BUT consider this scenario: -

The battery gets low and switches off the load. The dumping of the load will inevitably cause the battery voltage to rise again because it no longer is under a load condition. This rise then re-enables the load FET and you are into a possible problematic situation. You really should be "latching" the battery low condition with a flip-flop BUT now we're into trying to work out how this situation remedies itself - what mechanism is acceptable for resetting the power?

The low-battery comparator does have hysterisis but it's only 10mV - it maybe enough but i wouldn't count on it. I'm sure the battery could rise (under load to no-load conditions more than 5% and this would be 25mV).

Another option is to have a resistor from the LB output to the LBI - this will increase hysterisis but it's a suck it and see approach. Having said that it'll probably work. You'll need more like a 10k resistor on the LBO and possible no smaller that 100k as feedback to LBI. Your normal LBI resistors will need to be about the 10k area to prevent too much hysterisis too.

EDIT following disclosure of circuit Q1 won't work how you imagine it to - if the source was connected to 0V and the drain to the negative end of the load I believe this would work. The 1Mohm pull-up would keep the load connected via the FET to 0V. As you now have it this won't work. If you are intent on a high-side switch (maybe because the load HAS to be always connected to ground) then there is a way but it involves two fets somewhat along these lines: -

enter image description here BUT.....

Replace the BC547 with an n-channel FET and get rid of the 10kohm. Ignore the 5.5V - read it as 5V - it's getting late over here and i didn't have the perseverence to fully correct the diagram in paint!!! The MCU line is your LBO. The high-side FET is P-channel - choose one with very low Rds(on) and one that has a "logic-level" gate drive voltage. The n-channel is less stringent - it can't be a bioplar given the high impedance pull-up on the LBO pin BUT, given that you may use a 10kohm (previouse pre-edit comments) it could be a bipolar (just about). Stick with an n-channel logic gate driven device to be sure!

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  • \$\begingroup\$ This is pretty spot on. I'd like to add that since the LBO is active low, you would want to use an N-Channel MOSFET to disconnect the circuit from ground when LBO becomes active. Of course, a MOSFET with a very low gate threshold voltage would need to be used to make sure it is saturated when the LBO is inactive and the gate is pulled high. I only mention this since the OP didn't provide any clue as to how he would be actually using the MOSFET to disconnect the load. Of course, this could change if some latching circuit were implemented. \$\endgroup\$ – Kurt E. Clothier Apr 27 '13 at 20:27
  • \$\begingroup\$ I'd definitely be using logic level MOSFET. In regards to the latching circuit, what if there was also a LiFePO4 charging circuit on board (effectively what this is, a lithium battery charging and boost circuit with auto shutoff). Could the plugging in of USB power to charge possibly trigger this reset of the latch? I'll look up the internal resistance of the battery I'll be using and see what the drop would be like. \$\endgroup\$ – James Apr 27 '13 at 21:46
  • \$\begingroup\$ See if you can put a circuit up or link one somewhere and someone will paste it into your question \$\endgroup\$ – Andy aka Apr 27 '13 at 22:07
  • \$\begingroup\$ I have edited the question with the circuit \$\endgroup\$ – James Apr 27 '13 at 22:56
  • \$\begingroup\$ I've edited my answer to suggest how you'd connect the n-channel FET \$\endgroup\$ – Andy aka Apr 27 '13 at 23:14
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I solved this by switching the boost regulator to the TPS61202, a very similar part with UVLO programmable by a resistor divider. Similar efficiency ratings and lower price are added bonuses.

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