TPS25751: TPS25751 with two chrager ICs BQ25792

Hi, 

Can you provide a block diagram of your full system? The integrated I2C control of TPS25751 is limited to only be able to configure one BQ257xx part, not multiple BQs. For multiple BQ257xx, you may need to consider using an MCU to control the two BQ parts separately (power management). 

Thanks and Regards,

Raymond Lin

Hi Raymond,

thanks for your reply - see the block diagram attached. There's a microcontroller included in our system, but the charging system should also work without a host (device is turned off, power cable is plugged to charge the batteries). In this state, the two (or at least the charge controller without connection to the TPS25751) would charge with the default values from BQ25792 datasheet Table 9-9, correct?

Kind regards,

Eike

Hi Eike, 

What are the requirements from the USB Type-C/PD port? Is this design intended to be DRP (Dual Role Port) or is this going to be sink-only? What are the ports power (PDO) capabilities? 

Also in your block diagram you show three power lines coming from TPS25751 feeding into both BQ25792 and the power path controller, is this all connected to TPS25751 PPHV/PP_EXT? 

How would the MCU control the two BQ25792 based on the negotiated contract? For example, if TPS25751 negotiates a PD contract of 5V/3A as a sink (charging battery), would the current be divided between the two BQ? 

hektopascal said:

In this state, the two (or at least the charge controller without connection to the TPS25751) would charge with the default values from BQ25792 datasheet Table 9-9, correct?

 Looping in the BQ team for their input. 

Thanks and Regards,

Raymond Lin

Hi Raymond,

thanks for your feedback. We are currently still at the beginning of the concept phase of the system. The port is intended to be sink-only.

In my first block diagram, the simplest idea was to split the PPHV in parallel between battery charger 1, battery charger 2 and system (managed by the power path controller). As you mentioned, this of course makes it difficult to divide the available power from the PD controller. I would therefore like to have a PD controller that can communicate with two battery chargers. An external microcontroller could be used to aks for the available power from the PD controller and then distribute it to the battery chargers and system load using a calculation/algorithm. However, this seems very complex and time-consuming to me.

In the meantime, we have come to the point with our system requirements that we can say that the backup battery does not have to have the same capacity as the main battery. We can therefore design the backup battery to be much smaller and charge it with less power. I am therefore inclined to turn the original idea of a parallel circuit into a series circuit as shown in the attached block diagram.

The battery charger of the main battery should communicate directly with the PC controller and be configured. The battery charger of the backup battery is set to a fixed charging current of less than 1 A using ILIM pin. The backup battery is only charged when a power supply unit is plugged in (CH_EN pin). The system should function without a host, the microcontroller is only used for diagnostics.

Best regards,

Eike

HI Eike,

Default charge current is set in the register as 1A.  ILIM_HIZ resistor divider clamps converter input current limit (power) so with VBUS=5V and ILIM_HIZ clamp = 1A, the max input power is 5W so max output power, split between SYS load and battery charger is 5W*efficiency.

Regards,

Jeff