Wireless Charging: A New Wireless Battery Charger Transmitter from STMicroelectronics

A new wireless battery charging transmitter from STMicroelectronics tries to raise the bar with faster charging times with higher efficiencies.

Wireless charging is a growing trend. Although we may see many more wireless charging applications in the future, smartphones, tablets, and wearables seem to be the most popular and common today. All wireless power transfer systems, also known as wireless charging systems, require a transmitter and a receiver. Wireless-charging protocols make it possible for wireless power transfer to be governed by communication between the transmitter and receiver.
STMicroelectronics, or known simply as ST, has released the STWBC-EP, a high-efficiency wireless power transmitting IC. The STWBC-EP transmitter is able to control the amount of energy transferred to the receiver by modulating the duty cycle, amplitude, or frequency of the transmission. This IC is a single coil transmitter optimized for applications requiring up to 15W of power. And by generating the correct amount of power, the highest levels of end-to-end efficiencies are achieved, even during light load conditions. Also, this IC is able to charge devices up to three times faster due to its "stepwise increase to higher power levels," according to the STWBC-EP flyer.

Figure 1. A typical wireless charging system; the STWBC-EP is the transmitting controller. Image taken from the datasheet.

A Reference Design Is Available

If you're new to the wireless charging arena, or if you would like a little extra help with your designs, ST offers the STEVAL-ISB044V1, which is their wireless charger transmitter evaluation kit/reference design that uses the STWBC-EP transmitter controller.

Figure 2. ST's wireless charging reference design (STEVAL-ISB044V1) uses the STWBC-EP transmitting controller. Image courtesy of the reference design's user guide.

Supports USB Vin and Provides 15W, with a Caveat

As advertised, this IC is capable of providing up to 15W of power to a receiver load and also supports USB voltage inputs. However, as stated in section 5.5 (Input power supply management: VMAIN, QC_IO), 15W is only available when the USB wall adaptor is capable of providing 12V when requested to do so by the STWBC-EP. Unfortunately, the maximum power available when using USB 5V or 3.3V is not listed.

Figure 3. Package information, from the datasheet.

Requires Filtering

According to section 5.1 (Power supplies: VDD, VDDA, VSS, VSSA, VOUT) of the datasheet, "VDD and VDDA should be correctly filtered to allow the correct operation of the device." Unfortunately, the datasheet offers no explanation for how to correctly filter these pins. Additionally, section 5.2 (DC/DC converter: ...) calls for a "second order passive filter" for properly generating DCDC_DAC_REF, and a "filtered current sensor" must be connected, as noted in section 5.4 (Wireless power functions: ...), to the ISENSE pin, but again no additional information is provided. It's all very strange that no guidance on the filtering requirements is available.
Since this is, after all, a new device, maybe ST is still working out the filtering details and will update their datasheet when all the wrinkles have been ironed out. Perhaps the proper filtering schemes can be gleaned from the reference design, but this approach would be inefficient and time-consuming. We'll make a note in this article if we become aware of any such updates.

Not Tested in Production

If you're planning to use this part in a new design, it might be worth your while to call ST to inquire about the items listed as "Data based on characterization result, not tested in production." See image below. This note is not uncommon; I have observed similar notes on other recently released semiconductor products. What exactly does this note mean, especially considering that it is attached to numerous specifications given in the  STWBC-EP datasheet? Can we trust these specs, or not?
In any event, I imagine that ST will be happy to keep you updated on their production testing results if you contact them and request whatever new information they have.

Figure 4. An example of "not tested in production" specs, from the datasheet.

Have you had a chance to use or test this new wireless battery charger transmitter or its reference design/evaluation kit? If so, leave a comment and tell us about your experiences.