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I have a circuit where a typical 3.7V LiPo battery supplies power through a bq24074 Charger IC, which should in theory short that voltage straight to the output of the charge-protect circuit, which then can be attached to a load. For a load, I'm having an electronic load drain constant current of 200mA. The flow is as I described:

[LiPo] ---(Vbatt)---> [Charge IC] ---(Vload)---> [Electronic load]

Voltage is collapsing when load is attached

When the load is turned on (draws 200mA), the battery voltage remains relatively unphased, but the voltage between the Charger IC and the Electronic load drops to 1.7V from 4.0V. I followed the application schematic from the datasheet almost exactly, only changing resistors that control charge current rates.

The application I have for this only draws around 15-25mA, and at those draws the voltage collapse is much smaller so there's no urgency in solving this. That said, in the future I want to implement applications that have infrequent large spikes in current consumption, so it begs the questions:

  • In the situation I described, why might the load be causing the voltage to collapse?
  • (and bonus question for curiosity's sake, since google wasn't helpful:) In general, why might a load cause voltages to collapse?
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  • \$\begingroup\$ I've included a link to the datasheet, in the first sentence click on "bq24074 Charge Protect IC." I used the standalone charger circuit shown on page 36. \$\endgroup\$ – Austin Fennacy Aug 17 '18 at 5:19
  • \$\begingroup\$ Page 36 contains two circuits, Fig 41 and Fig 42. Which one did you use? And where do you connect your load? \$\endgroup\$ – Ale..chenski Aug 17 '18 at 6:54
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    \$\begingroup\$ Show please your full schematics. How the pin SYSOFF is connected? \$\endgroup\$ – Ale..chenski Aug 17 '18 at 7:06
  • \$\begingroup\$ @AliChen I used "Figure 41. Using bq24074 in a Standalone Charger Application," and I connected my load to OUT (pins 10 & 11) \$\endgroup\$ – Austin Fennacy Aug 17 '18 at 18:12
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The standalone circuit on page 36, Fig.41, has one missing connection, pin SYSOFF. This seems to be the datasheet mistake.

enter image description here

The spec says about SYSOFF pin:

System Enable Input. Connect SYSOFF high to turn off the FET connecting the battery to the system output. When an adapter is connected, charging is also disabled. Connect SYSOFF low for normal operation. SYSOFF is internally pulled up to VBAT through a large resistor (approximately 5 MΩ). Do not leave SYSOFF unconnected to ensure proper operation.

SO it looks like the transitor Q2 (see functional diagram section 9.2) is floating, partially open, which causes the voltage to drop.

Connect the SYSOFF to ground for proper operations.

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  • \$\begingroup\$ So SYSOFF is bq24075 and bq24079 only, and I'm using bq24074 which does not have that pin... The idea of leaving something floating felt like the right track though, so I re-read the Pin Functions in the datasheet, and I still can't find the issue. Is it possible that joints with poor connection / not enough solder might cause issues? Everything looks good to me, but I did do it by hand without a stencil, not sure if that's relevant \$\endgroup\$ – Austin Fennacy Aug 17 '18 at 18:09
  • \$\begingroup\$ I've also checked the Thermal Pad connection, it's connected to the bottom of the board with "vias" (not sure if that's the right term when they're exposed..?) and I've applied more solder there just in case the Thermal Pad was floating. Best I can tell, it is not floating. \$\endgroup\$ – Austin Fennacy Aug 17 '18 at 18:39
  • \$\begingroup\$ @AustinFennacy, did you try a different battery cell? Did you charge it through the full charging cycle, so all internal watchdogs and timers end up in right state? Do you have the right value for thermal protection circuitry, TS pin? \$\endgroup\$ – Ale..chenski Aug 17 '18 at 18:58
  • \$\begingroup\$ Good ideas, I'll try going through a full charge cycle first and see if that helps. Just swapped in another battery which was also subject to voltage collapse, if a full charge cycle doesn't help then I'll try a 3rd battery that's fresh out of the box. Will post later when charge cycle etc is complete. \$\endgroup\$ – Austin Fennacy Aug 17 '18 at 19:18
  • \$\begingroup\$ @AustinFennacy, and what about TS pin? \$\endgroup\$ – Ale..chenski Aug 17 '18 at 21:13

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