BQ24250: Poor efficiency when supplying system without battery

Filipe Martins

Part Number: BQ24250

Hello,

I'm currently designing a battery powered system with the BQ24250 device.

Everything seems to work properly when a battery is plugged, but when the battery is absent the efficiency dramatically drops and the BQ24250 builds up temperature.

Basically when the battery is operating the current draw is ca. 690mA. When the battery is absent and the circuit is supplied from the (USB) charger the current is ca. 802mA and the IC gets very hot. Notice I'm using 4 layers and the BQ24250 thermal plane (QFN24 package) is connected to ground in the second and forth layer with vias. The inductor used for the BQ24250 buck converter stage is far from saturation (ratings: 1uH / 12mR / 4.5A). In these conditions I'm also noticing the Vsys collapses significantly with the load: it drops as low as 2.7V.

The IC is configured as follows: ISET: 270R, ILIM: Short to ground, EN1: High, EN0: Low, VDPM: center tap with R1:270kR, R2:100kR

From my understanding I'm not having the announced high efficiency, am I missing something?

Any support would be very appreciated.

Thank you.

over 5 years ago

0 Jeff F over 5 years ago

TI__Guru**** 166396 points

Hi Filipe,

Are you able to take a scope shot of VBUS, I(Inductor), VSYS and ISYS? If not, how about VBUS, VSW and VSYS?

Regards,

Jeff

0 Filipe Martins over 5 years ago in reply to Jeff F

Prodigy 90 points

Hello Jeff,

Please find some of the requested waveforms. I'm afraid this week I can't provide much more scope pictures, but let me know if you need more.

Battery only:

Charger only:

0 Jeff F over 5 years ago in reply to Filipe Martins

TI__Guru**** 166396 points

Hi Filipe,

The sinusoidal oscillation is worrisome. That implies that the converter is not stable. Can you send your schematic?

Regards,

Jeff

0 Filipe Martins over 5 years ago in reply to Jeff F

Prodigy 90 points

Hello Jeff,

Attached a screenshot of the schematics.

Thank you

0 Jeff F over 5 years ago in reply to Filipe Martins

TI__Guru**** 166396 points

Hi Filipe,

We require a 2.2uF capacitor close between the IN and GND pins. If you add that capacitor, does the oscillation go away?

Regards,

Jeff

0 Filipe Martins over 5 years ago in reply to Jeff F

Prodigy 90 points

Hello Jeff,

Not shown in the picture, but I've a pi filter in the charger input with 4.7uF, 220Ω@100MHz ferrite and 2.2uF.

Best regards

Filipe

0 Jeff F over 5 years ago in reply to Filipe Martins

TI__Guru**** 166396 points

Filipe,

I wouldn't expect the pi filter to cause the issue but I can test tomorrow in the lab, assuming I can find a similar ferrite bead. I can't tell from our scope plot but is the VBUS oscillation drooping to your VINDPM point? Does changing the VINDPM point set per the external resistors using I2C register change the oscillation?

Regards,

Jeff

0 Jeff F over 5 years ago in reply to Jeff F

TI__Guru**** 166396 points

Hi Filipe,

I modifed an BQ24250EVM to include the pi filter but I only had access to a 270ohm not 220ohm ferrite bead. I do not see the oscillation that you see, even if the charger is in VINPDM. Can you give me more detail about the system load, for example is it an inductive motor? Can you order an EVM and test using the EVM?

Regards,

Jeff

0 Filipe Martins over 5 years ago in reply to Jeff F

Prodigy 90 points

Hello Jeff,

Thank you for providing these scope images, these look indeed pretty different from what I'm getting.

The load in my circuit is a boost converter that steps up the Vsys into 5V. This 5V rail is supplying an LCD application with backlight.

The EVM module is on its way to my location, then I'll have a look at both designs and spot differences. Will keep this post up to date with my findings.

Thank you.

0 Filipe Martins over 5 years ago in reply to Filipe Martins

Prodigy 90 points

Hello,

I found an error in my schematics: The boot capacitor should be tied to the SW pins and not to ground. When reworking the PCB this gets the circuit working properly.

Thank you very much for the support.

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BQ24250: Poor efficiency when supplying system without battery