MP2760 not stable with dead/no battery

Battery Management
1

Hello,

I am designing a handled device powered by a 1S LiPo battery. We use the MP2760GVT-010A to manage the battery charging from a 4.8V input.

Our design is in production at the moment but we discovered a severe issue with charging a dead battery. Previously, we made these tests:

  • Apply 4.8V at charger input with no battery ==> VSYS is powered and stable
  • Apply 4.8V at charger input with dead battery ==> VSYS is powered and stable

However, we recently discovered it is not always working. We tested it on several devices, some from production, some from engineering samples, some assembled in house, and the verdict is the same. It might work or not. Sometimes it does not work, sometimes it does not work until we remove the 4.8V input, sometimes it works if we plug the battery after the 4.8V input. All cases observed on each device.

This happends:

  • with and without load on VSYS
  • with and without host I2C communication.

Unfortunately I cannot upload pictures (Schematic, oscillo view, …) but I’ll try to explain.

Checking with an oscillo, we the charger is cycling the power-up sequence, with around 400ms period.

VSYS has kind of a saw teeth profile, reaching the target VSYS then directly falling down.

Zooming on VSYS where it reaches its target value, we see that BGATE is activated to start the charging process. However, it never reaches its rated 6V, quickly drops to less than 1V then turns back off. This means the BAT MOSFET is never activated (we could observe no movement on VBAT).

Furthermore, INT is never pulled low and ACOK is aloways low. However, I am not sure about the error reporting on the INT pin as it is disabled by default and the MCU maybe has not the time to enable it.

So my question is, what could cause such behavior?

Otherwise, the system runs perfectly fine in all other situation. We have tens of devices in test on field for more that 2 months now and there is no issue in nominal conditions. The problem only appears when battery is fully depleted.

Thanks in advance four your insights.

2

Hello, welcome to the MPS forums!

I was looking at this issue and wondered about it for quite some time.

While it would certainly be easier to look at your schematic to help resolve this issue, I would like to start the debugging process by giving a few preliminary checks as is stipulated by the MP2760 datasheet for your application:

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  • Be sure a 4.7uF capacitor is connected from VCC to AGND, ensure that VCC pin is 3.6V
  • Place the battery as close to the BATT pin as possible to mitigate any IR losses.
  • Ensure that two 22uF ceramic capacitors are bypassed to GND in place of C3 and C10 as denoted in Figure 21.
  • The ACOK and INT pins must both be pulled up to VCC with an external 10kOhm resistor.
  • For pins BST1 and BST2 ensure that there are 100nF bootstrap capacitors are placed in the way that C5 and C6 are denoted in Figure 21.
  • At the SYS pin, ensure that four 22uF ceramic capacitors and one 1uF ceramic capacitor are connected from SYS to PGND as close to the IC as possible.

Please verify that all of these initial items are consistent with your schematic.

From the way that you described your issue though, it seems that there may be an issue regarding initial inrush current when charging a very deeply discharged battery in your case. The VSYS signal as seen on your oscilloscope is rising and falling seems to suggest that your charger is trying to charge and is forced to reset due to some issue (verifying the initial list should narrow that down.

I know I threw a lot at you, but hopefully this helps with your debugging process.

Best,
Krishan