BQ25185 Community Reference Notes

BQ25185-notes.md

BQ25185 Community Reference Guide

This guide is a summary of collective knowledge, best practices, and troubleshooting tips for the Texas Instruments BQ25185 linear battery charger, based on community forum discussions. It is intended to supplement, not replace, the official datasheet.

1. Overview & Key Specifications

The BQ25185 is a standalone, single-cell (1S) Li-Ion/Li-Po/LiFePO4 linear battery charger. Its key characteristic is the integrated power path, which allows it to power a system load while simultaneously and independently charging the battery.

• Charger Type: Linear Charger (standalone, resistor-configurable)
• Battery Chemistry: Single-Cell (1S) Li-Ion, Li-Po, LiFePO4
• Key Feature: Dynamic Power Path Management (DPPM)
• Input Voltage: 3V to 18V
• Max Charge Current: Up to 1A
• Package: WSON (not a BGA package)
• Important Notes:
  • The BQ25185 is not an automotive-qualified device and does not have a wettable flank package.
  • It is the recommended modern, standalone alternative to the discontinued BQ25181.

2. Core Features Explained

Power Path Management (DPPM)

The Dynamic Power Path Management (DPPM) feature intelligently manages power distribution from the input source.

• System Priority: The IC prioritizes powering the system load connected to the SYS pin.
• Current Allocation: If the total current drawn by the system load and the battery charging exceeds the input current limit (ILIM), the IC will automatically reduce the charging current to ensure the system remains powered.
• Supplement Mode: If the system load exceeds ILIM entirely, the charging current is reduced to zero. If the VSYS voltage then drops below the battery voltage (VBAT), the battery will discharge to help power the system. This prevents the input source from collapsing.

Input Voltage Management (VINDPM)

This feature is crucial when using high-impedance or weak power sources like solar panels or current-limited adapters.

• BATTRACK VINDPM: The device attempts to draw the maximum possible current from the source, but it will throttle the current back if the input voltage drops to a specific threshold (typically VBAT + 330mV). This prevents the source voltage from collapsing below the level needed for charging.
• Charging Requirement: For charging to occur, VIN must be greater than VBAT by at least this VINDPM threshold (approx. 300mV).

Protection Mechanisms & Safety

The BQ25185 integrates several protection features, but some are notably absent.

• Integrated Protections:
  • Battery Undervoltage Lockout (BUVLO): Disconnects the battery from SYS if VBAT falls below the threshold (typically ~3.0V) to prevent deep discharge.
  • Battery Overcurrent Protection (BATOCP): Protects against excessive discharge currents from the battery. This can be triggered by large inrush currents to SYS capacitance.
  • Battery Short Protection (BATSC): Protects against a shorted battery.
• Missing Protections:
  • CRITICAL: No Reverse Polarity Protection. Connecting a battery backward to the BAT pin will permanently damage the IC. Use a keyed connector or an external protection circuit.
• Safety Timer:
  • The IC has a fixed 6-hour maximum charge safety timer (tMAXCHG). This cannot be adjusted.
  • If the timer expires, charging stops, and a fault is indicated (STAT1 & STAT2 LOW).
  • The timer can be reset by toggling the /CE pin or by cycling the input power.

3. Pin Configuration & Best Practices

Correctly configuring the external pins is essential for proper operation.

ISET (Charge Current)

• Function: Sets the fast charge current.
• Configuration: A single resistor (RISET) to GND sets the charge current. ICHG = KISET / RISET (where KISET is typically 300 AΩ).
• Best Practice:
  • Use a ±1% tolerance resistor for accuracy.
  • For low charge currents (< 50mA), an RC circuit in parallel with RISET is recommended to ensure stability. A common starting point is a 2kΩ resistor in series with a 4.7nF capacitor.

ILIM/VSET (Input Limit & Voltage Set)

• Function: This pin has a dual role, setting both the input current limit and the battery regulation voltage (VBATREG).
• Configuration: A single resistor to GND sets both parameters according to a table in the datasheet.
• Examples:
  • 18kΩ: 500mA ILIM / 4.2V VBATREG
  • 13kΩ: 1100mA ILIM / 4.2V VBATREG
  • 5kΩ: Sets VBATREG to 3.65V (for LiFePO4)
• Warning: Using a resistor value that is not specified in the datasheet table can lead to incorrect or unpredictable operation.

TS/MR (Temperature Sense / Manual Reset)

• Function: Monitors battery temperature via an NTC thermistor and can be used for manual reset/mode entry.
• CRITICAL BEST PRACTICE: If not using the temperature sense function, this pin MUST be connected to GND through a 10kΩ resistor. Leaving the pin floating will suspend charging, as the device will interpret it as a temperature fault.
• NTC Details: The internal thresholds are designed for a standard 10kΩ NTC with a beta (β) value of 3435.
• Mode Control: Holding the pin low can trigger entry into Factory Mode (a low-power state), which is exited by connecting VIN.

STAT1 / STAT2 (Status Indicators)

• Function: Open-drain outputs that indicate charger status.
• Implementation: Require pull-up resistors or can directly drive the cathode of an LED (with the anode pulled up to a voltage rail). They can be left floating if unused.
• Status Codes:
  • Charging: STAT1 = HIGH, STAT2 = LOW
  • Charge Complete: STAT1 = LOW, STAT2 = HIGH
  • Fault (Timer, OCP, etc.): STAT1 = LOW, STAT2 = LOW
  • No Battery Connected: STAT1 = HIGH, STAT2 = Blinking. This is caused by the VBAT capacitor charging and discharging.

/CE (Charge Enable)

• Function: Enables or disables the charging function. Active LOW.
• Configuration:
  • To always enable charging: Connect the /CE pin directly to GND.
  • To disable charging: Pull the /CE pin HIGH.
  • Toggling /CE is a reliable way to clear faults and reset the safety timer.

SYS / BAT / VIN (Power Pins)

• Capacitance: Follow datasheet recommendations closely. Typically 1µF on VIN and BAT, and 10µF on SYS.
• Warning: Excessive capacitance on the SYS pin (e.g., > 100µF) can cause a large inrush current from the battery when VIN is absent, potentially triggering the BATOCP fault and disconnecting the battery from SYS.

4. Thermal Management

As a linear charger, the BQ25185 dissipates power as heat. This is its primary operational constraint.

• Heat Source: The power dissipated is a function of the voltage drop across the IC and the current passing through it. It becomes significant with high input voltages (e.g., 12V) and/or high charge currents.
• Power Dissipation Formula: PDISS = [(VIN - VSYS) * (ISYS + IBAT)] + [(VSYS - VBAT) * IBAT]
• Thermal Regulation (TREG): If the internal junction temperature becomes too high, the device will automatically reduce the charge current to prevent overheating. This is a normal protective behavior, not a fault.
• Design Best Practices:
  1. Use a Pre-Regulator: For high VIN (e.g., > 5.5V), use an efficient buck converter to step the voltage down to ~5V before the BQ25185's VIN pin. This drastically reduces power dissipation in the charger.
  2. PCB Layout: A good thermal layout is crucial. Use large, continuous copper pours connected to the IC's thermal pad on multiple layers.
  3. Thermal Vias: Use an array of thermal vias under the IC to transfer heat from the top layer to internal and bottom ground planes.

5. Common Issues & Troubleshooting

Symptom	Probable Cause & Solution
Charger gets very hot and charge current is lower than expected.	The device is likely in thermal regulation (TREG). This is expected with high VIN (e.g., 12V). Improve PCB thermal design or add a pre-regulator buck converter.
No charging occurs; STAT pins may indicate a fault.	Check the TS/MR pin. If not used for an NTC, it must be tied to GND with a 10kΩ resistor. A floating TS/MR pin is a common cause of failure.
With no battery, STAT2 is solid ON instead of blinking.	This is unexpected behavior and often indicates a damaged IC. Perform an "A-B-A swap" (test a known-good IC on the board) to confirm if the IC or the board is the issue.
Charging does not stop when /CE is pulled HIGH.	The capacitance on the SYS or BAT pins may be too low, causing oscillations. Ensure datasheet-recommended capacitor values are used (e.g., try increasing CSYS to 22µF).
SYS output is very low (~1.2V) even with a healthy battery.	This is a known issue. It can be caused by the device entering Factory Mode (if TS/MR was held low) or a BATOCP fault from inrush current. Cycle VIN power or disconnect/reconnect the battery to clear the state.
Safety timer expires before the battery is fully charged.	This can happen in designs with a series resistor on the battery line (for intrinsic safety). The IR drop across the resistor prevents the battery from reaching the termination voltage quickly, causing the timer to expire.
Device enters a non-responsive state with SYS off.	Check for BATOCP. High inrush current to the SYS capacitors can trigger this fault, disabling the BATFET. Reduce SYS capacitance or manage the load's turn-on slew rate. Exit BATOCP by toggling VIN.

6. Advanced Use Cases

Paralleling Chargers

It is possible to parallel multiple BQ25185 chargers to increase the total charge current for a single battery.

• Constant Current (CC) Mode: The charge currents will add together.
• Constant Voltage (CV) Mode: To prevent instability where both chargers fight to regulate the voltage, their VBATREG settings must be staggered. For example, set one to 4.1V and the other to 4.05V. The higher-set charger will handle the final CV phase and termination.
• System Load: Connect the main system load to the SYS pin of only one of the chargers.

Solar Charging

The BQ25185 can be used with solar panels, primarily due to its VINDPM feature.

• VINDPM helps maximize power extraction by preventing the panel's voltage from collapsing under load.
• Efficiency: As a linear charger, it will be inefficient if the solar panel voltage is much higher than the battery voltage.
• Recommendation: For better performance and efficiency in solar applications, a switching charger with true MPPT (like the BQ25895 or BQ25798) is a more suitable choice.

7. Alternatives & Related ICs

• BQ25180: The I2C-controlled sibling of the BQ25185. It comes in a BGA package and offers more granular control (e.g., adjustable timers, lower charge currents, ship mode). It does not have the "BATTRACK VINDPM" feature.
• Switching Chargers (e.g., BQ256xx, BQ2589x): If high efficiency, low heat, or higher charge currents are required, a switching (buck) charger is the recommended alternative.
• For JEITA Profile Support: The BQ25185 does not support the JEITA charging profile. For this, consider the I2C-controlled BQ25180 or a standalone switching charger like the BQ25616J.