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# BQ25792: Output Power of BQ25792EVM

Part Number: BQ25792
Other Parts Discussed in Thread: TPS25750EVM, TPS25750,

I am following TPS25750EVM design and I will connect a 4S 4.5A lipo battery. I have some questions regarding output power of this EVM.

1. What will be the system voltage and current when input source is present and not present. The 100W PD adapter is connected as a input source (see attached).

I am curious about the output power of this EVM. I will use it as a sink to charge the battery and get the output voltage at SYS pin. Correct me if I am wrong about the term source/sink.

Let's say Tps25750 negotiate a power with adapter at 20V 5A.

• What will be the input to BQ25792 in that case? I believe the input power can be only 20V 3.3A to BQ.
• If that is true then what will be the SYS pin voltages and current considering 4S lipo battery connected with BQ.

Thanks

• Hi Ali,

When input source is not present, V(SYS)=V(BAT)-I(SYS)*RdsonBATFET where the battery is sourcing current to SYS.

When input source is present, pulling its 3.3A max input current:

if V(BAT) < MINSYS (3.5V default) then V(SYS)max=3.5V+~0.2V.  If output power [I(SYS)*3.5V] > I(VBUS)*VBUS*efficiency, then the battery supplements the converter so V(SYS)=V(BAT)-I(SYS)*RdsonBATFET.

If V(BAT)>MINSYS, then V(SYS)=V(BAT)+I(SYS)*RdsonBATFET until V(BAT) reach VBATREG.

When hotplugging/swapping a full charged 4S battery to the EVM, be very careful of transient spikes above the 20V max voltage of the BATP and BAT pins.  In your final design, if hotplugging/swapping will occur, I recommend adding a ~18-21V TVS diode to suppress such spikes.

Regards,

Jeff

• Hi @jeff

It means

• When input source is not present then V(SYS) = VBAT ideally ignore the drops. or simply connects the battery to the system.
• When input source is present, Did you consider 3.5V by assuming single cell battery?

Also, let's talk about ideal case and ignore losses. The BATTERY is 4S 12V to 16.8V

Case1: When Input source is present and it is capable to provide power to the load then battery will disconnect from the system. In that Case, What will be the output voltages? Is it regulated voltages or will change according to the load. I am not talking about overload here. Let's say 20V 3.3A(66W) and load is 60W.

Case3: When input source is present and load is over then Battery supplement mode turns on. In that case, what will be the system voltages?

About hotplugging/swapping, In my product, Battery will always connected with the system but i might add up some ESD protection because i have to connect SYSTEM output with the SYSPWR of TPS65988EVM.

Thanks

• HI Ali,

When input source not present, battery is connected to system.

When input source is present and can provide the load at SYS, V(SYS) ideal = max (MINSYS=12V default or battery voltage), in other words SYS is only regulated with discharged battery and then follows battery up to regulation = 16.8V.

If input source can't provide the load then battery supplements the load so ideally V(SYS)=V(BAT) with both the converter and the battery providing I(SYS).

Regards,

Jeff

• With that said, I can't expect VSYS more than 16.8V in my case but Can current go till 5A if input power and Battery is capable to provide? Or it is also limited due to some limitations?
This is my last question about it.
Thanks

• Hi Ali,

Correct on both counts.

Regards,

Jeff

• Thanks a lot. It really help me.