BQ24780S: Design and optimization questions

Hello,

1. Can the ACDRV, CMSRC, and BTDRV resistors (R12, R13, R28) be replaced by 5.1kOhm resistors? If no, why exactly 4kOhm?

>>  5.1 kOm is okay to use.  Purpose of these resistors is to limit current to protect the pin if there is a voltage overshoot during hot plug or other transient event, so I wouldn't recommend reducing the value as this will reduce the protection.  Be careful raising the value too high as it will slow the turn-on of the associated FETs.

1. What is the purpose of C12 and R7? Can those components be omitted?

>>  C12, C13 and R7 form a soft start circuit for the Q1 and Q2 (AKA ACFET and RBFET)  If you remove the soft start circuit, the FETs will turn on very fast.  The system caps C2-C5 an C10 will charge from battery voltage to adapter voltage very quickly which will result in a large inrush current which may cause the adapter to overcurrent and disconnect.  You can make modifications to this circuit to adjust timing, but I would not recommend removing it altogether unless there is another soft start circuit in the system.

1. Is it safe to replace all FETs (ACFET, RBFET, BATFET, High-side FET, Low-side FET) with CSD17579Q3A? This would be a more cost-effective FET that the proposed CSD17308Q3.

>>  This should be fine.  The EVM uses the same FETs for the switching FETs (Q4/Q5) as it does for these FETs, and I agree that they are probably a little over-spec'ed for the ACFET, RBFET and BATFET.

1. As far as I understood BATPRES can be pulled permanently high. In case of battery connection, charging can still be started by setting REG0x14().
2. Is this correct or do I necessarily need the hardware schematic proposed in the reference design?

The /BATPRES is mainly used to exit Learn mode, although it also stops switching (charging or hybrid boost) on a low to high transition.  Halting charging helps prevent OVP on the BAT+ pin when the impedance suddenly goes HI-Z, although there is also a BATOVP function on the device.  So if you aren't using learn mode and you are fine with relying on the BATOVP (or don't believe the battery will ever be removed) then there is no problem with pulling /BATPRES permanently high.   

1. Does BATPRES need to be pulled high to REGN or 3.3V?

The /BATPRES has a maximum input high threshold of 2.1V, so pulling to 3.3V is fine.

1. I am planning to connect my 3.3V LDO to SYS and BAT bus. That means without battery, 3.3V is only available, after ACFET and RFET are on. Will this be an issue for the pins that need 3.3V (e.g. ILIM or SDA/SCL)?

It might be an issue on the SCL/SDA if you had no battery and ACFET was turned off due to a fault condition such as ACOV, although this is a fairly unusual circumstance.  On the BQ24800 EVM, there is a similar EVM and it is placed after the diode-OR that leads into VCC.  I would recommend placing here so that it can be powered directly off of adapter even if ACFET is disabled, and the diode-OR is already in place. 

1. Is the cell configuration (4S) detected automatically?

No, the maximum charge voltage needs to be written into register 0x15 at startup.  This voltage is then used in other calculations where needed, such as the battery depletion threshold. 

1. Do the input/output filter capacitors need to have 1210 footprint, or can they be 0805 with same voltage rating?

0805 should be okay.

1.  Is the capacitor C22 on ILIM relly necessary?

It is just a filter to filter out high frequency noise.  If you remove it, you may see less precision on your ILIM set point as a result.  But the device can function without it, just a matter of noise/precision.

1. Can you give an example when I should use the independend comparator IN/OUT? Currently I have them unconnected.

If you aren't using the independent comparator, you may tie the input to GND and float the output.  The independent comparator is basically just a free extra  comparator that you get with the chip.  You don't need to use it.  A common use would be for /BATPRES detection as on the EVM or to sense an overcurrent/overvoltage error condition on the board.   

Regards,

Steve

>>>> I understand that C13 is used for soft-start and R13, R14 will limit inrush current. But are you sure C12 and R7 are part of this soft-start? Aren't they there for something like MOSFET false-turn on protection? It would be great if you could give some further explanation.

These components help to control the ramp of the soft-start circuit when it turns on.  You are right that the C13 alone would slow the charging of the ACFET gate, which would slow the turn on, but there is still a very narrow threshold voltage range which would result in a "hockey stick" turn on.  The C12 and R7 help to give the voltage rise at CMSRC a more linear behavior by holding the ACFET gate right at the threshold as CMSRC rises. 

This circuit was adapted from http://www.ti.com/lit/an/slva156/slva156.pdf.  Figure 2 isn't exactly equivalent because it is a PFET, but if RT connects to ACDRV instead of GND, then you can replace the PFET with an NFET and  RT <----> R13, RGD <----> R7, CGD <----> C17 and CT <----> C13.   Then note the explanation in the application note "The RT and CT determine the ramp time, and CGD provides a smooth, linear ramp of the output voltage."

>>>> I still want to use learn mode though for battery calibration. Is this still possible with /BATPRES high? 

You can still use learn mode, but you need to tie /BATPRES low.  It is an inverted signal, so tying it high will indicate that a battery is not present.  I think from your description you want to tie /BATPRES to GND.

>> The /BATPRES has a maximum input high threshold of 2.1V, so pulling to 3.3V is fine.

>>>> But would REGN also be fine? It looks like this is also used in reference design. 

/BATPRES has a 6.5V max recommended operating voltage, so it could be tied to REGN, but you could exceed that voltage possibly with different system transients on the board.  To be safe, I would use a resistor divider off of the 6V REGN as is done on the reference design to drop the voltage to 3.0V. 

That said, I think you want to tie to GND.  I believe the TPS3898A has an open-drain output and this is why EVM pulls the output up with REGN, but with a battery attached, that signal will be driven low. 

>>>> This is clear now. However, in BQ24800 EVM the diode-OR is not connected to Battery (DNP), so how does 3.3V volt start if only battery is connected and adapter is not present?

The BQ24800 EVM has a diode-OR from Adapter and Sys as opposed to the BQ24780S which has a diode-OR from Adapter and BATT.  The reason for this is related to the new battery-only boost feature of that device.  But even with the diode-OR from Adapter and Sys, the VCC is still powered from battery configuration across the body diode of the BATFET.  The battery goes across the BATFET body diode then across the Schottky diode into VCC. 

In your case since using BQ24780S that does not have battery-only boost, you should keep the Schottky diodes at Adapter and Batt, but still put your pull-up LDO behind the diode-OR

EDIT: also for other charger families such as BQ257xx you provided some design guideline pdfs. Is this also planned for BQ24780S (and newer BQ24800)?

The primary reference for BQ24780S design is the EVM User Guide as well as the application section of the datasheet.  The EVM User Guide has a list of layout tips, and the schematic provided in the User Guide shows the recommended external components.