Frequent Questions

  • [FAQ] BQ25173: Why does the BQ25173 charge a supercapacitor slowly or at reduced current, and how can the maximum charge current be achieved?

    Part Number: BQ25173 Slow charging or reduced current from the BQ25173 is almost always caused by thermal regulation (TREG) . When the supercapacitor starts from 0 V and the input voltage is, say, 5 V, the full input-to-output voltage drop appears across…

  • [FAQ] BQ25173: Can two BQ25173 devices be operated in parallel for higher charge current, and what is the recommended method?

    Part Number: BQ25173 Yes, two BQ25173 devices can be paralleled to increase total output current (up to 1.6 A combined). The recommended method uses a slight offset in the output voltage setpoints between the two devices: one unit is set slightly higher…

  • [FAQ] BQ25173: How should multiple supercapacitors be balanced when charged in series with the BQ25173?

    Part Number: BQ25173 The BQ25173 does not have built-in cell balancing . For series-connected supercapacitors, external balancing resistors placed in parallel across each capacitor are the standard recommended approach. A single-cell supercap (rated…

  • [FAQ] BQ25172: How should the TS pin be configured if no NTC thermistor is used, and can the TS pin be used as a charge enable/disable?

    Part Number: BQ25172 If no NTC temperature monitoring is needed, connect a 10 kOhm resistor from TS to GND . This biases the TS pin within the normal operating range (~350–380 mV with the internal 38 µA current source), preventing any false hot/cold…

  • [FAQ] BQ25172: How is charging current set and dynamically changed, and what does the ISET pin voltage indicate?

    Part Number: BQ25172 Charge current is set by a resistor to GND on the ISET pin: IOUT = KISET / RISET, where KISET = 300 ±10% (Ohms·mA) for the BQ2517x family. For example, RISET = 600ohms → IOUT ≈ 500mA. The ISET pin also serves as a real-time current…

  • [FAQ] BQ25172: Can VSET or TMR resistor values be chosen freely to get any charge voltage or time, or are only specific values valid?

    Part Number: BQ25172 The VSET and TMR pins on all BQ2517x devices operate as digital bin systems , not analog. At startup, a calibrated current pulse is applied to the pin and the resulting voltage is measured and compared to a lookup table of predefined…

  • [FAQ] BQ25172: Why does the charge timer run longer than expected, and what causes the safety timer to extend? BQ25171 BQ25172 BQ25176J BQ25171-Q1

    Part Number: BQ25172 Other Parts Discussed in Thread: BQ25176J The most common cause of the charge timer running longer than programmed is thermal regulation (TREG) . When the die temperature reaches the TREG threshold, the device: (1) reduces output…

  • [FAQ] BQ25172: When does the BQ25172 (NiMH) start or refuse to start a charging cycle, and how does the recharge voltage threshold (VRECHG) work?

    Part Number: BQ25172 At power-up and after each charge cycle, the BQ25172 checks the battery voltage against VRECHG = 1.33V/cell : Battery voltage < VRECHG: Charging starts immediately (full charge cycle). Battery voltage > VRECHG: Battery is…

  • [FAQ] BQ25172: How does the BQ2517x terminate a NiMH charge cycle, and why doesn't it use −DeltaV detection? BQ25172, BQ25171-Q1

    Part Number: BQ25172 Other Parts Discussed in Thread: BQ25171-Q1 , The BQ2517x NiMH chargers (BQ25172, BQ25171-Q1) use a timer-based termination method. Charging ends when: The safety/charge timer expires (set by TMR resistor; range 4–22 hours…

  • [FAQ] BQ25172: Why does charge current appear lower than configured, and how can this be verified?

    Part Number: BQ25172 Reduced charge current is most commonly caused by thermal regulation (TREG) limiting output current to protect the die. Other causes include: A current meter (ammeter) placed in series with the output — even a low-resistance…

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