bq51020EVM + bq500212AEVM: why current limited?

a couple of other details...

Driving a battery-equivalent load directly from receiver is indeed not 'proper'.  Our study is to determine how various system configurations affect efficiency etc.  As soon as we characterize receiver+load performance we will add bq24250EVM and repeat.

here is behavior of TX+RX versus load; i guess question is whether we should expect 1 A of current.

Hello cm,

The bq51x2x devices are supposed to work with every TX-EVM no matter what the input voltage is.

Are your jumpers set as in the User`s Guide test procedure described?

Which resistance do you measure from JP1 pin FIX to GND if you remove the jumper? The current limit is set by the resistors between ILIM and GND (R4+R2+R6). Since R6 is a potentiometer the adjusted resistance value could be too high.

Does your power supply of the TX have a current limit?

The bq51x2x GUI can read out the I2C register and send commands (make IC adjustments) over the I2C interface.

Best Regards,
Markus Fleck

Thanks, Markus, for your comments.

Jumpers are set as per data sheet.

We didn't measure the resistance as suggested but we did vary the potentiometer (both directions) and this would decrease the current but not increase it.

We tried both a wall AC-DC transformer (3 A capacity) and a variable DC supply (5 A) and neither show limitation.

Perhaps I'm looking at this wrong.  The Qi spec is that it is capable of 5 V and 1 A.... but maybe this is not simultaneous.  5V/1A output implies a load of 5 ohms.  We've tried a variety of loads and do see an increase in current as load is decreased.  But smaller resistance loads seem to pull down the output voltage.  For example, a 5 ohm load has power input to transmitter of 5V/0.6A but output from receiver is 2.85V/0.6A.  The equal currents seems more than coincidence and would seem to imply that transmitter is not drawing enough current... or the receiver is communicating a self-protecting power limitation.

So is the spec 5 V and 1 A, but not both simultaneously?  I can see how the load could pull down the output voltage as I believe the receiver was not designed to drive small impedances- to charge a discharged Li+ battery (where impednace will be a few ohms) requires one to use something like the BQ24250EVM (which we will add next after we characterize Qi function).  Regardless of battery charger, how does one get 1A or 5W out of receiver?

Hello cm,

The device is able to supply 5W which means it can definitely provide 1A @ 5V.

Could you please measure the resistance from FOD to GND (p8. in Users Guide, RFOD, R6 Set up). It would give us more clarity if we can exclude this as reason for your issue.

Also helpful is to look at the behaviour of the RECT voltage and Output voltage/current with a scope.

Best Regards,

Markus Fleck

Hi Markus- Sorry I got pulled into another priority. Now back to this issue.

Set up system again and obtained V/I in of 5.02V/630mA and out was 3.18V/606mA into 5 ohm load.

Checked R from FOD to GND and it was 1317 ohms, so lowered it to 261 ohms. System behaved similarly but now input current was 718 mA and the system would shut down after 5 to 10 seconds.

Increased R of FOD-GND to 495 ohms, and input current dropped to 700 mA or so and system was stable.

Observed VRECT through most of these measurements. Generally it was a DC level about 0.5 V or so above VOUT. Sporadically on top of VRECT were 0.5 V bursts of 2 kHz around 28 ms long- Bursts occurred somewhat randomly but always separated by at least 40 ms. I assume this is the communications between XMTR and RCVR. Also observed VOUT with scope, it was a DC level with no features.

I also connected GUI to receiver using USB interface module I received with BQ24250EVM board but it did not seem to communicate as readings of voltages showed zeroes and trying to assert current limits did nothing.

What should I try next?