Bq40370

1. Overview: What is the bq40370?

The bq40370 is a Li-Ion / Li-Polymer battery pack manager with a CEDV (Compensated End of Discharge Voltage) gas gauge. It integrates:

• Analog front-end (AFE) for cell protection
• A 16-bit microcontroller with gas gauge algorithms
• I²C communication interface
• Internal temperature sensor (plus support for external thermistors)

It is designed for 2-series (2S) to 4-series (4S) battery packs, commonly used in:

• Power tools
• E-bikes / e-scooters
• UPS / backup systems
• Medical devices
• Portable electronics

Note: The bq40370 is a ROM-based device (firmware is fixed in ROM, not flash-programmable). Configuration is done via data flash parameters over I²C. bq40370

5. Protection Functions and Behavior

• Overcurrent (OC)
  • Continuous OC detection uses current-sense resistor voltage across sense amplifier.
  • If current exceeds OC threshold for longer than an OC blanking time (to ignore inrush), device transitions to limit or opens FETs.
  • Two-level response: a soft limit (current regulation) for moderate overcurrent and hard disconnect for sustained OC.
• Short-Circuit (SC)
  • Fast comparator monitors instantaneous sense voltage; on detection of extreme differential it triggers immediate FET turn-off within microseconds.
  • Latching behavior often used; device may require manual reset or automatic retry after a timed cooldown.
• Overvoltage (OV)
  • If pack voltage rises above OV threshold (e.g., during charging), charge path FET is turned off to prevent further charging.
  • Hysteresis ensures small oscillations around threshold don't cause repeated toggling.
• Undervoltage (UV)
  • When pack voltage falls below UV threshold, discharge FET is turned off to protect cell from overdischarge.
  • Recovery often requires pack to be externally charged above the release threshold or device-specific auto-retry.
• Thermal Protection
  • External NTC connected to TS pin(s) used to monitor pack or ambient temperature.
  • Multiple ranges: charge inhibit when too cold or too hot; discharge inhibit for extreme temperatures.
  • Temperature faults may be temporary (retry after conditions normalize) or latching depending on severity.
• Fault Reporting
  • FAULT / STATUS pins assert to signal the MCU with encoded or multi-line signalling.
  • Some versions use PWM or blink patterns; others use open-drain outputs that go low on fault.
  • Clear and descriptive flags for OV, UV, OC, SC, THERM, and other conditions.

3. Pinout (24-pin TSSOP – simplified)

| Pin(s) | Name | Description | |--------|------------|------------------------------------| | 1–4 | VC1–VC4 | Cell voltage inputs (VC1 = lowest) | | 5 | VSS | Ground (negative of lowest cell) | | 6 | REG25 | 2.5V regulator output (cap to GND) | | 7 | TS1 | Thermistor input 1 (battery temp) | | 8 | TS2 | Thermistor input 2 (optional) | | 9 | SDA | I²C data | | 10 | SCL | I²C clock | | 11 | BAT | Pack positive voltage (for AFE) | | 12 | PACK | Pack+ terminal (through protection FETs) | | 13 | DSG | Discharge FET gate drive | | 14 | CHG | Charge FET gate drive | | 15 | SRP | Sense resistor positive | | 16 | SRN | Sense resistor negative | | 17–18 | NC / ALERT | Alert output (optional) | | 19–24 | Other | Cell balancing control, test pins |

Refer to TI datasheet (SLUSC73) for full pinout. Analog front-end (AFE) for cell protection A 16-bit

4.2. CEDV Gas Gauge

• Computes Remaining Capacity (RM), Full Charge Capacity (FCC), State of Charge (SOC).
• Uses voltage, current, temperature, and internal resistance models.
• Requires configuration of EDV0, EDV1, EDV2 thresholds and CEDV coefficients (based on battery chemistry).

3. Sleep vs. Shutdown

If the battery has been over-discharged below the UV threshold, the bq40370 enters Shutdown mode. Connecting a charger to the PACK+ pin with >9V typically "wakes" it up by activating the BATFET. If this fails, check the REG25 pin. If REG25 is 0V with >6V on BAT, the chip is physically dead.

7. Typical Application Circuits

1. Single-cell Li-ion protection with external MOSFETs

• Battery connected to BATT
• Current-sense resistor (R_sense) between BAT- and GND or in series on negative side (depending on device)
• Gate drivers drive external charge and discharge MOSFETs (back-to-back recommended to reduce body diode conduction)
• TS pin connected to pack NTC
• FAULT pin to MCU interrupt for status
• Decoupling capacitors on VCC and BAT

2. Integrated-FET simplified pack protector

• Minimal external components
• External MOSFETs not required if device includes low RDS(on) FETs
• Sense resistor optional if internal current sensing is used; otherwise external required for higher accuracy
• Recommended input filter to avoid false triggers during transient loads

Schematic notes:

• Use back-to-back MOSFETs for true bidirectional blocking and lower body diode conduction.
• Place R_sense close to device pins; use Kelvin routing if available for accurate sensing.
• Add TVS or input surge protection on BATT for high-energy transients.

1. Introduction

bq40370 is a high-side battery protector intended for use with single-cell and multi-cell lithium-ion and lithium-polymer battery packs. It integrates overcurrent, overvoltage, undervoltage, short-circuit, and thermal protection functions along with MOSFET gate drivers and status reporting. This document provides a detailed technical reference, design guidance, typical applications, electrical characteristics, timing diagrams, protection behavior, PCB layout recommendations, thermal considerations, and example circuits using bq40370.

Introduction: What is the BQ40370?

In the world of battery management systems (BMS), most enthusiasts and engineers are familiar with standard fuel gauges like the BQ20Z65, BQ30Z55, or the modern BQ40Z50 series. However, a specific part number often pops up in niche repair forums, datasheet deep-dives, and high-reliability application notes: bq40370. It is designed for 2-series (2S) to 4-series

The bq40370 is a specialized gas gauge and protection Integrated Circuit (IC) manufactured by Texas Instruments (TI). Unlike standard consumer-grade fuel gauges, the bq40370 is designed for impedance tracking and is frequently found in high-end battery packs, medical devices, and industrial tools where accuracy under load is non-negotiable.

This article provides a comprehensive breakdown of the bq40370, covering its architecture, key features, programming requirements, common failure modes, and how it compares to other TI gauges.