I have a product with a PF1550. I want to charge the battery using constant current of 25 mA max for high temperatures.
In order to do that, I set a CC 100 mA charge limitation for the JEITA1 NORMAL temperature stage and a CC-75% for COOL/WARM temperature stages.
My first tests (using a potentiometer instead of real battery CTN) show no 25 mA charging stage but a direct 100 mA to 0 mA at HOT temperature (around 2770 Ohms in my case, close to my HOT=+60°C setting).
I wonder to know if PF1550 is capable of handling the JEITA1 COOL/WARM -75% CC setting when NORMAL stage is already at the lowest CC=100mA charge setting.
I tested to setup higher NORMAL charge currents (200mA, 400mA and 800 mA), keeping the CC-75% for WARM temperature range. I can see a lower charge current during the WARM temperature range with those settings, something like 100mA, 100mA and 200mA respectively.
As per the datasheet of PF1550, with CC mode charger can charge from 100mA to 1Amp only.
Please refer to section 8.8.4 (JEITA thermal control), Figure 23 (CC charge current and CV charge voltage adjustment), and Table 159 -Register CHG_CURR_CNFG - ADDR 0x0E. You can also refer to section-13 Application design.
In my understanding of the datasheet, it says that JEITA1 is able to lower CC charge by 75% over the JEITA-uncontrolled NORMAL temperature stage (which can be chosen by CHG_CC among 19 possible limitations between 100 mA and 1 A). It does not say that JEITA lowered CC is also constraint by those 19 NORMAL CC limitations.
I mean, reading datasheet I see no sentence clearly saying that JEITA is unable to lower CC of a NORMAL CC=100mA setup.
In fact, table 48 notes even say that an internal CC=50 mA charging limitation exists (auto-selected when charger input current is limited to 100 mA).
Are you planning to limit the current SW based then the min CC current is 100mA? Kindly refer to "Table 6. Input currents" of the datasheet to charge the battery with CC current <100mA. Kindly limit the VBUSIN input current to limit the charge current for <100mA.
I am planning to limit battery charge current above 45 °C, in order to comply with my 25 mA battery specifications above this temperature.
Unfortunately, in my opinion limiting VBUSIN current will not only limit battery charge current but also limit the current delivered to VSYS, hence to the entire system. This does not suit my need since I want my processor and its peripherals to be well powered, and some can drain power.
You can only choose between JEITA 1 (charging current and battery regulation voltage will be reduced at battery temperature > TWARM and < TCOOL) and JEITA 2 (charging current will be reduced at battery temperature > TWARM and < TCOOL. Charger voltage is not changed).
It is not possible to stop the charge at 45C (TWARM), only at THOT which can be adjusted to stop the charge at 55C by setting to ‘1’ the THM_HOT (bit 5) on register 0x92.
I understand. My willing is not to stop charging over 45 °C  but simply to lower charge current down to 25 mA (-75% of 100mA NORMAL charge current I am using).
Unfortunately it looks like JEITA current downgrades (-75% in my case) are mostly subject to the ~19 available charge current regulations (going from 100 mA to 1 A).
 In the past I used to stop charging at +45°C by acting on thermal interrupts and stopping charger outside of NORMAL thermal range. I do not want to do this anymore, but I need to comply with my battery specification (25 mA charge max over 45°C), and I am failing to accomplish this.
According to the theory, when thermistor reach the WARM threshold (45C) and setting the THM_CNFG[1:0] is set to 0x02, the charging current and CV voltage are adjusted based on CC_ADJ and CV_ADJ.
I’m guessing you have the system configured to charge at 100mA by setting Register 0x8E, bits 4:0 (0x8E = 00000).
Try using CC_ADJ bit value to 25% (00). And check the THM_SNS bit when Thermistor temperature is above Warm threshold (45C). Table 168 of the datasheet (https://www.nxp.com/docs/en/data-sheet/PF1550.pdf)
You can check this bye reading register 0x87 (Register CHG_SNS), bit 6 (THM_SNS).
Also change the CV_ADJ value to 00 (60mV of voltage adjustment).
You are guessing well.
Actually, CC_ADJ=0b00 (and CV_ADJ=0b11 by the way) is already my setting and until a few days I was using CHG_SNS bit/interrupt to manually stop charger outside of NORMAL bounds and it worked fine (using this same 100mA NORMAL temperature limitation). Ignoring CHG_SNS bit and letting PF1550 autonomous is the way I discovered my WARM=100mA issue.
My CV_ADJ is currently set to 0b11 (-200 mV). Do you think this can impact charging current? I can try 0b00 (-60mV) in next hours / today.
I tested with CV=-60mV with the same result. Charging current never goes under 100 mA during warm temperature stage.
But meanwhile, opened discussion with battery manufacturer make us think that a finer-grained temperature derating can be computed and may be available to us soon. We hope this derating to cover our needs. I will let you know as soon as I have it, likely this week.
For everybody to know the end of the story,
I failed to lower CC charge by 75% outside NORMAL (during COOL and WARM stages) with my NORMAL CC=100mA setup (smallest possible value). Charge continues at CC=100mA until COLD and HOT.
I finally fixed my battery specification issue by discussing the the battery manufacturer which edited a finer-grained datasheet of its battery, including temperature deratings (describing different possible max charge current for each low and high temperatures), then adjusting PF1550 HOT and COLD to not exceed temperatures under which CC=100mA is possible with my battery.