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# BQ24800: About Charge/Discharge current limit

Part Number: BQ24800

(1). Charge current limit is derived below equation as Datasheet Chapter 8.4.1.1.

Ichg = VILIM/(20*RSR).

If VILIM is fixed value and RSR is fixed value , How is Ichg toleranse +/- ??%?

For example , VILIM is 1.2V(fixed value) and RSR is 5mohm(fixed value), Ichg is 12A center. In that case, 12A is changed by something tolerance ?

We are thinking that above torelance is Charge Current Regulation Accuracy +/- 2% and, in addition, something else +/- ??% (Charger's inside Amplifier and Comparator etc.).

Please tell me the details of the variation factor except RSR resistor and VILIM voltage.

We must do that it is necessary to calculate the set value based on all the variation factors.

(2). Discharge current Reglation is derived below equation as Datasheet Chapter 8.4.4.

Idchg = VILIM/(5*RSR).

If VILIM is fixed value and RSR is fixed value , How is Idchg toleranse +/- ??%? as above (1) question.

Please tell me the details of the variation factor except RSR resistor and VILIM voltage.

(3).Chapter 8.2 Functional Block Diagram's descripton is Chg gain x16 and Dischg Gain x4.

However, (1)chg calcuration gain is 20 and (2) dischg calcuration gain is 5.

Please teach cause of above difference.

(4). When ILIM pin enable by setting the REG 0x38H[7] to 1,
Charge/Discharge current limit will be selected to the lower one between ILIM pin setting and REG 0x14h and REG 0x39h?

For example, ILIM pin setting: Charge limit 3A/Dischage Limit 12A, and REG 0x14: Charge 5A / Reg 0x 39h: Discharge 8A,
As the result, Charge/Discharge current limit is set to 3A/8A? (Chager select low one automatically?)
Or Charge/Discharge current limit will be depended to only ILIM pin? (exlusive control?)

(5). ILIM pin has Dvider for VILIM setting as below schematic.
If ILIM pin disable by setting the REG 0x38H[7] to 0,
Is it no problem? Or ILIM pin must be floating etc. by removing divider?

• Hello Norimichi,

Both Charge Current and Discharge Current tolerances are outlined in the Regulation Accuracy in the Electrical Characteristics section. Given parameters are set to typical values, SRN>2V, and RSR=10mOhm,ChargeCurrent set to 4069mA has a Regulation Accuracy of +/-2%. Similar characteristics are explained for Discharge Current(Ex: DischargeCurrent set to 8192mA has a Regulation Accuracy of +/-2%)

3. Referring to IDCHG Gain and IADP Gain, REG0x12[4] and REG0x12[3] set these two gain values respectively, IADP Gain can be set to 20x or 40x, and IDCHG can be set to 8x or 16x gain. This is outlined in Table 6. The calculations referenced do not correspond to accurately setting these two gain values.

4. The lower of ILIM voltage and 0x14(for charge) or 0x39(for discharge) reference sets actual regulation limit - if bit is set high, lower voltage will be selected, if bit is set low, ILIM pin programmed value will set limit.

5. There is no problem with the schematic if ILIM pin is disabled, the divider will be ignored.

Sincerely,

I have an another question.

What is discharge current limit use for?

DIscharge current limit is set by REG 0x39 or ILIM pin.

I had been thought that this discharge current limit is used to PROCHOT function of IDCHG.

However, PROCHOT  IDCHG Threshold is set by 0x3DH[15:10] from datasheet. I don't know purpose of setting REG 0x39H.

Could you teach the purpose of the dIscharge current limit REG 0x39, please?

>3. Referring to IDCHG Gain and IADP Gain, REG0x12[4] and REG0x12[3] set these two gain values respectively, IADP Gain can be set to 20x or 40x, and >IDCHG can be set to 8x or 16x gain. This is outlined in Table 6. The calculations referenced do not correspond to accurately setting these two gain values.

I'm sorry. Do you mean that Chapter 8.2 Functional Block Diagram's gain description is partial? (All gain has not been described) Is it correct?

Best regards,

• Hello Norimichi,

If the ILIM pin is disabled, the discharge current limit value is set by REG0x39, otherwise the ILIM pin will select the lower value. It is used to set the maximum discharge current. Referring to 8.4.4, once the battery discharge current is limited, the input current goes up to meet the system current requirement. The user can assert PROCHOT to detect input current increase and request CPU throttling to lower the system power.

As far as the gain descriptions, the VLIM equations you referred to in your initial question, (EX: VILIM = 20 × (VSRP – VSRN)) are not the values for IDCHG and IADP Gain, if you would like to set these two gain values, refer to my response above.

Sincerely,

• >It is used to set the maximum discharge current. Referring to 8.4.4, once the battery discharge current is limited, the input current goes up to meet the system current requirement.

How does the charger limits the battery discharge curent? Could you teach behavior of Charger, please?

We think that even If stop the BATFET, battery will supply the high current consumption because BATFET has body diode.

Best regards,

• Today is a US holiday, please check it back after Tuesday.

• Hello Norimichi,

Discharge Current limit is achieved by the following: To enable discharge current regulation, set REG0x37[15]=1, this enables battery discharge current regulation. To set the discharging current limit, set DischargeCurrent() (REG0x39) to desired discharge current limit value. The device provides a discharge current limit range of 512 mA to 32.256 A, with 512-mA resolution.

Sincerely,

1. REG0x37[15]=1 of batery discharge current regulation is descriped to chapter 8.4.4 (P.30). It is use for Hybrid boost mode and Only boost mode.Therefore, is discharge current regulation by REG 0x37[15] and REG 0x39 effcted only to Hybrid boost mode and Only boost mode? Is it correct? (It can decrease of current consumption during Hybrid boost mode and battery only boost mode only, is it correct?)

2. If the above REG 0x39 discharge current limit is trigged, boost convert's ON duty will be reduced, then as the result boosted voltage will be dropped?

3. If REG 0x37[15] is 0, Is the above REG 0x39 discharge current limit not trigged? Is it correct?

4. Is the above REG 0x39 discharge current limit not related PROCHOT directly? Is it correct?

5. If REG 0x39 setting is lower than 0x3D[15:10](PROCHOT IDCHG Threshold), what is occur?

Best reagrds,

• Hello Norimichi,

We are conducting further research to answer your questions. I will get back to you by the end of the week with updates.

Sincerely,