bq24171: Icharge

Hi Ikon,

Can you have the customer measure the voltage at the Iset pin please?

There may be a few things going on that are causing the voltage to drop:

• The BJT for A25_DET may be pulling Viset down just a little if there is any current flowing. Switching to an NMOS may help.
• Can you get a scope trace of the Iset pin voltage and the Vref pin voltage? There may be some switching noise at Vref, which carries over into Iset.

Hi David,

Above is resolved.

On another note, customer is testing another feature on the BQ24171 EVB (pre-charge (10%) current). Refer to datasheet on threshold of fast charge transition.

The EVB has a simple potential divider to set this threshold. If customer want the battery go to fast charge after Vbat reaches 6.5V, therefore R34 = (100K/(1.45/6.5)) – 100K = 355K. Please confirm if this calculation is correct.

Thanks.

Ikon,

From the first schematic, it seems that the customer intends the charging voltage to be ~8.38 V. Changing R34 to 355k would increase their charging voltage to ~9.55 V. Using just a simple voltage divider to change the fast charge transition will not work for that reason - the charging voltage will track the transition voltage.

If I am understanding the situation correctly, the customer wants to a charge voltage of ~8.38V, but wants to extend the precharge phase to 6.5 V. Is that correct?

Hi David,

Bingo.

Customer Vbat spec is 6V to 8.4V.

Cheers.

Ikon,

I see. So if they are using 2 Series Li-ion cells, the BQ24650 would not begin fast charging current until ~2.9 V/cell. This is by design - if you check for any number of cells the 1.45 V always gives ~2.9 V/cell. Fast Charge current is usually not applied until 2.9-3 V. If it is applied much later, it is almost like slow charging the cell, which can be detrimental to Li-ion.

I will dig  a little deeper, however, to see if there is a way to shift that number up to reach the 6V number you are asking for.

Hi David,

From spec if Icharge set to 2.7A, I pre-charge will be 0.27A.
But  I can’t get pre-charge current during Vbat 6 to 8.4V.(always charge 2.7A)
From spec highlight FB must be < 1.35  for pre-charge setting or

If from calculation:

6v=FB*(1+300/100)
FB=1.5
Does this mean that the design will never go to pre-charge state?

Thanks.

Ikon,

That is correct. Once the battery voltage reaches 5.9 V, the Vfb will be ~1.475 V, which may cause a transition to the pre-charge state. If the battery only ever drops to 6 V, it will use the fast charge current because Vfb > Vlowv.