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[FAQ] BQ25171-Q1: Configuration Pins and Design Questions

Part Number: BQ25171-Q1

The device can be configured to charge Li-Ion, NiMH and LiFePO using external pins.  Can the configuration be changed during operation? Any other things I should note?

    • What are the device input protections? --- see Data Sheet (SLUSDK0A) section 7.3.4
      • Max input voltage without damaging device is 40 V.
      • Device will enter standby mode when above the overvoltage threshold of 18 V until the input voltage recovers below the threshold.
      • Device does not have IN-DPM (voltage fold back protection).
    • Can I dynamically change ISET during charging?
      • The ISET pin is monitored while charging and changes to R-ISET during operation will immediately change the charge current.
      • The VSET and CHM_TMR pins are not continuously monitored during charge. Changes to these pins following startup will not be acknowledged by the device until the input supply or /CE pin is toggled.
    • What is the relation between ISET and the charge current?
      • Voltage developed on the R-ISET resistor maps to output current at a 300 to 1 ratio.
      • ISET can be monitored from an external circuit to track charge current.
    • What is the charging profile for NiMH battery applications?
      • Device does not support fast charging NiMH.
      • The duration of the time based charge is set by CHM_TMR and the current is set by ISET.
      • Device charges NiMH battery in constant current mode only, option for additional intermittent charges is set by VSET pin.
      • Intermittent charging enables a shortened charge cycle 25% of the charge timer set by CHM_TMR and is initiated when battery voltage falls below VRECHG.
    • Does the product have internal thermal regulation and shutdown?
      • Device monitors internal junction temperature independent of TS pin battery temperature monitor.
      • Thermal regulation limits charge current when IC temperature exceeds 125°C.
      • Thermal shutdown turns off unit if IC temperature exceeds 150°C. Device resumes charge when temperature falls below TSHUT falling threshold of 135°C.
    • How can efficiency of charge be improved and power dissipation be reduced?
      • The closer Vin is to Vout the better the efficiency and lower the power dissipation. Be aware Vin will need to be larger than Vout + VDO (dropout voltage) to account for voltage loss internal to the IC.
      • Required power dissipation in IC is greatest during fast charge with a low battery voltage and a high input voltage.
    • How does the TS pin function in battery temperature applications?
      • Device in normal operation when TS pin voltage is between Vcold and Vhot, typically corresponds to 0°C to 45°C.
      • TS pin voltage outside the acceptable range results in a recoverable fault. Device enters standby state until battery temperature returns to normal condition.
      • High and low thresholds can be modified using series and parallel resistors Rs and Rp between the TS pin and the NTC thermistor. Refer to Data Sheet table 7-3 for recommended resistor values for various temperature ranges.
    • Does the device regulation charge current for a deeply discharged battery?
      • For Li-Ion battery voltage less than VBAT_SHORT (2.2 V), and LiFePO4 battery voltage less than 1.2 V the output current is limited to 16 mA. Device is in trickle charge phase.
      • For Li-Ion battery voltage between VBAT_SHORT and VBAT_LOWV (2.2 V and 2.8 V), and LiFePO4 battery voltage between 1.2 V and 2.0 V output current is limited to the precharge current set to 20% of ISET value.
    • How do I disable automatic recharge?
      • Under standard operation device automatically restarts the charge cycle once the battery falls below the recharge threshold (VRECHG).
      • Disable the charger by pulling the /CE pin high from a host device to prevent automatically starting a charge cycle.
      • Insure your system has a method to reenable charging when desired as device will not monitor battery voltage when disabled.
    • What is the device behavior in an open battery scenario? --- see Data Sheet Figure 8-3
      • When no battery is present the device will cycle in and out of charging with an output voltage equal to the VSET regulation voltage.
    • FAQ Provided by Garrett Kreger