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| BQ25601 IC pinout, charging circuit and repair test points explained for practical battery charging troubleshooting and board-level diagnosis. |
BQ25601 IC Boot Sequence, Pinout and Circuit Repair Guide
The BQ25601 is a highly integrated single-cell Li-ion and Li-polymer battery charger and power-path management IC from Texas Instruments. It combines a synchronous buck charger, battery power-path control, USB input management, OTG boost operation, battery temperature monitoring, protection functions, and I²C-based configuration in a compact 24-pin WQFN package.
For electronics repair work, this IC is important because a charging problem is not always caused by the battery or USB connector. A damaged input capacitor, shorted PMID rail, incorrect TS network, faulty switching components, poor PCB grounding, or a failed BQ25601 can produce very similar symptoms.
This guide explains the BQ25601 circuit operation, pin functions, startup sequence, common charging faults, practical repair checks, and important measurement points.
Note: Always compare the board schematic, component values, PCB layout, and IC revision with the official Texas Instruments datasheet before applying repair voltage or replacing components.
BQ25601 Main Features and Circuit Overview
The BQ25601 is designed for one-cell Li-ion and Li-polymer battery systems and supports a maximum charging current of up to 3 A. It uses a 1.5-MHz synchronous buck charging architecture and supports a 3.9 V to 13.5 V operating input range, with a higher absolute maximum input rating specified by Texas Instruments.
Its integrated architecture reduces the number of external components normally required around a battery charger IC. The device contains internal MOSFETs, current-sensing circuitry, loop compensation, BATFET control, USB input management, and power-path functions.
The most important functional blocks are:
VBUS input and reverse-blocking section
PMID intermediate power rail
Switching buck charger
SW switching node and bootstrap circuit
SYS system power output
BAT battery connection
REGN internal supply
TS battery temperature sensing
SCL and SDA I²C communication
PG and STAT status outputs
QON and CE control functions
BATFET and NVDC power-path management
Understanding these rails makes BQ25601 circuit repair much easier because each section can be tested separately instead of treating the entire charging circuit as one block.
BQ25601 IC Pinout and Pin Functions
The BQ25601 uses a 24-pin WQFN package measuring approximately 4 mm × 4 mm. The official datasheet identifies the pin functions and recommends specific external capacitor and thermistor connections around several pins.
Input and Power Pins
Pin 24 — VBUS: This is the main charger input. The USB or external adapter supply enters through this pin. Texas Instruments recommends placing a 1-µF ceramic capacitor close to VBUS and ground.
Pin 1 — VAC: This pin senses the input voltage and must be connected to VBUS.
Pin 23—PMID: PMID is an important intermediate power rail connected to the internal reverse-blocking MOSFET and high-side switching section. A 10-µF ceramic capacitor is recommended between PMID and ground.
Pins 15 and 16 — SYS: SYS is the system power output. The internal current-sensing network is located between SYS and BAT. A 20-µF capacitor is recommended close to SYS.
Pins 13 and 14 — BAT: These pins connect to the positive battery terminal. The internal BATFET and current-sensing path are associated with the SYS-to-BAT section, and a 10-µF capacitor is recommended close to BAT.
Switching and Driver Pins।
Pins 19 and 20 — SW: The SW node is the high-frequency switching point of the buck converter and connects to the external inductor.
Pin 21 — BTST: BTST provides the bootstrap supply for the high-side switching driver. A 0.047-µF bootstrap capacitor is connected between SW and BTST.
Pin 22 — REGN: REGN is an internal supply output used by the gate-driver circuitry. A 4.7-µF ceramic capacitor is recommended between REGN and ground.
Control, Monitoring and Communication Pins
Pin 5 — SCL: I²C clock input.
Pin 6 — SDA: I²C data input/output.
Pin 9 — CE: Charge-enable input. According to the datasheet, pulling CE low enables battery charging.
Pin 12 — QON: Controls BATFET enable and reset functions. It is also associated with exiting ship mode and performing a system power reset.
Pin 11 — TS: Battery temperature sensing input. It normally works with an NTC thermistor network and allows the charger to suspend charging when the battery temperature is outside the permitted range.
Pin 3 — PG: Open-drain power-good output that indicates whether a valid input source is detected.
Pin 4 — STAT: Open-drain charging-status output. It can indicate charging, charge completion, sleep mode, and certain fault conditions.
Pin 7 — INT: Open-drain interrupt output used to notify the host processor about charger status or fault events.
Pins 8 and 10 — NC: These are no-connect pins and should remain floating as specified by the datasheet.
Pins 17 and 18 — GND: Ground connections.
The exposed thermal pad is also connected to ground and is intended to help conduct heat into the PCB through an appropriate ground-plane and via structure.
BQ25601 Charging Circuit and Boot Sequence Explained
The phrase “boot sequence” can be confusing in a charger IC because the BQ25601 is not a processor that boots an operating system. In practical circuit-repair terms, the startup sequence is the order in which input power, internal rails, power-path control, battery detection, and charging regulation become active.
Step 1: USB or Adapter Voltage Reaches VBUS
When a compatible external source is connected, input voltage reaches the VBUS pin. The IC checks whether the source is within its operating conditions and manages the input through its internal reverse-blocking and power-path circuitry.
The device supports USB sources as well as higher-voltage adapters within the specified operating range.
Step 2: Input Qualification and VINDPM Control
The charger monitors the adapter voltage and input conditions. Its VINDPM function limits the charging load when excessive input loading would cause the source voltage to collapse.
This is particularly useful with weak USB sources, long cables, or adapters that cannot provide the required current. Instead of allowing the input rail to fall uncontrollably, the charger reduces the charging demand.
Step 3: Internal REGN Supply Becomes Active
The REGN rail powers internal driver-related circuitry. A technician checking a completely dead charging circuit should therefore inspect the surrounding REGN capacitor and associated PCB area for shorts or damage.
A shorted REGN capacitor can prevent normal charger operation and can sometimes make a good IC appear faulty.
Step 4: PMID and Power-Path Section Become Operational
PMID is an important internal power node. It sits between the input-side power path and the switching charger architecture.
If VBUS is present but PMID is missing, heavily loaded, or shorted, the investigation should move toward the input protection path, internal reverse-blocking section, PMID capacitor, and switching stage.
Step 5: SYS Power Becomes Available
The NVDC power-path architecture allows the system to remain powered from the adapter and battery in a controlled manner. TI specifies instant-on operation under conditions including a deeply discharged or absent battery.
This is one reason the BQ25601 is more than a simple battery charging IC.
Step 6: Battery Condition Is Evaluated
The charger monitors battery voltage and temperature. If the battery is deeply discharged, the device can begin with a pre-conditioning stage before moving into higher-current charging.
The TS pin is also important here because the battery NTC network determines whether charging is permitted under the configured temperature conditions.
Step 7: Charging Enters the Appropriate Regulation Phase
The BQ25601 uses pre-conditioning, constant-current, and constant-voltage charging phases. Charging terminates when the required battery and current conditions are satisfied, and a new cycle can begin after the battery voltage falls below the recharge threshold.
Problem: BQ25601 IC Not Charging the Battery
A no-charge condition should not immediately lead to IC replacement. Start from the input side and follow the power path logically.
Solution: Check VBUS Before Testing the IC।
Measure the voltage directly at the VBUS pin rather than only measuring the USB connector.
If the connector shows approximately the expected adapter voltage but VBUS at the IC is missing, inspect the following:
USB connector solder joints
Input fuse or protection component
Series resistors or ferrite components
ESD protection
Reverse-polarity or overvoltage protection components
VBUS ceramic capacitor
PCB tracks between the connector and charger IC
This approach prevents unnecessary BQ25601 replacement.
Solution: Check for a Short on the main power rails.
With power removed, measure resistance to ground on VBUS, PMID, SYS, BAT, and REGN where the circuit topology allows a meaningful comparison.
A very low resistance does not automatically prove that the BQ25601 is defective because external capacitors, MOSFETs, ICs, and other loads can also create low-resistance paths.
A useful repair habit is to isolate sections before making a final diagnosis.
Problem: VBUS Is Present, but Charging Does Not Start.
Solution: Inspect CE, TS, and Battery Conditions
A charger can have correct input voltage and still refuse to charge because charging is disabled or a battery safety condition is active.
Check the CE control condition and confirm that the battery temperature-sensing network is correctly connected.
The TS input uses an NTC-based temperature qualification circuit. An open thermistor, incorrect resistor network, broken PCB track, or contaminated board area can result in charging suspension.
Also inspect the actual battery voltage. A deeply discharged, internally damaged, or disconnected battery can produce symptoms similar to a charger fault.
Problem: Battery Voltage Is Present, but SYS Is Missing.
Solution: Investigate the Power-Path Section
SYS is one of the most important measurement points when repairing a BQ25601-based board.
If BAT is present but SYS does not behave normally, examine the following:
BATFET operation
SYS capacitors
Power-path loading
Short circuits on the system rail
QON control conditions
External system circuitry connected to SYS
Because the BQ25601 integrates the BATFET and power-path control, a SYS fault may originate inside the IC or from a downstream system load.
Problem: The IC Gets Hot Immediately
Solution: Check for External Shorts Before Replacing the IC
An overheating charger IC is often treated as a failed component, but the load connected to one of its rails may actually be causing the problem.
Disconnect the power source and inspect the board for:
Shorted ceramic capacitors
Damaged switching components
Shorted SYS loads
Abnormal PMID resistance
Contamination around the IC
Poor soldering or solder bridges
Damaged PCB traces
If the external rails are healthy and the IC still develops abnormal current draw without a reasonable operating explanation, IC failure becomes more likely.
Problem: USB Charging Works Intermittently
Solution: Inspect the Input Network and VINDPM Behaviour
Intermittent charging can be caused by voltage drop rather than a defective charger.
Check the USB connector, cable, input protection path, VBUS capacitor, and source quality.
The BQ25601 includes input current regulation and VINDPM functionality specifically to manage source limitations. Its programmable input-current control supports a broad range of current limits, while VINDPM helps prevent excessive input-voltage collapse.
A useful diagnostic test is to monitor the voltage at the charger IC while the circuit attempts to increase charging current. A large voltage drop between the connector and VBUS can reveal a high-resistance connection that is invisible during a no-load test.
Problem: Battery Charges Slowly
Solution: Check Input Capability, battery condition, and charging settings.
Slow charging does not necessarily mean the BQ25601 is defective.
The actual charging performance depends on the adapter, cable, programmed input current, battery condition, thermal conditions, and system load.
The IC supports programmable charging and input-current settings through its control system, including I²C configuration.
A worn battery can also show a large voltage increase while accepting limited current. For that reason, battery health should be evaluated separately from charger-IC performance.
Problem: OTG 5V Output Is Missing
Solution: Check the Boost Section and External Load
In OTG mode, the BQ25601 operates as a boost converter and can provide up to 1.2 A output under the specified conditions. TI specifies a nominal 5.15 V VBUS output for OTG operation.
Check whether:
OTG mode is correctly enabled
The battery has sufficient voltage
VBUS is not shorted
The switching section is intact
The boost output capacitor is healthy
The external USB load is within the supported current range
The device also includes output short-circuit protection and soft-start behaviour for capacitive loads.
BQ25601 I²C Control and Software Configuration
One advantage of the BQ25601 is that many charging parameters can be controlled through its I²C interface.
The host processor can configure and monitor functions such as charging parameters, input current limits, power-management behaviour, and OTG operation.
This means a board can have electrically correct power rails but still behave unexpectedly because of incorrect firmware configuration or register settings.
For advanced repair, an oscilloscope or I²C analyzer can be useful when checking communication between the application processor and charger IC.
However, hardware voltage measurements should come first. A missing supply rail cannot be fixed through software configuration.
Practical BQ25601 Circuit Repair Workflow
A disciplined measurement sequence can save a large amount of repair time.
Step 1: Inspect the PCB
Look for cracked components, corrosion, burnt areas, missing capacitors, damaged connectors, previous rework, and solder bridges around the QFN package.
Step 2: Test Resistance With Power Removed
Check the main power rails for suspicious low resistance and compare the readings with a known-good board when available.
Step 3: Apply a Current-Limited Power Source
Use an appropriate current-limited bench supply or safe laboratory setup rather than repeatedly connecting an unknown damaged board to a high-current adapter.
Step 4: Measure VBUS
Confirm that the input voltage actually reaches the BQ25601.
Step 5: Measure PMID and REGN
These measurements help determine whether the IC's internal power and switching section are starting normally.
Step 6: Measure SYS and BAT
Compare the system rail and battery rail while observing the battery condition and connected load.
Step 7: Check TS, CE, PG, and STAT
Control and status signals can explain why charging is disabled even when the primary power rails appear normal.
Step 8: Check the Switching Area
Inspect the SW node, inductor, bootstrap capacitor, and related passive components for physical or electrical damage.
A fast switching waveform should be measured with appropriate oscilloscope technique because an incorrectly connected probe can itself disturb a high-frequency switching node.
Step 9: Check I²C Communication.
For boards that use host-controlled configuration, verify SCL and SDA activity and confirm that the processor can communicate with the charger.
Step 10: Replace the IC Only After Section-Level Diagnosis
BQ25601 replacement should be the result of evidence rather than the first troubleshooting step. This is especially important on multilayer portable-device PCBs where a hidden short can damage a replacement IC immediately.
Important Repair Tips for BQ25601 Boards
Tip 1: Do not assume that the USB connector voltage is the same as the IC VBUS voltage. Measure at the IC.
Tip 2: A shorted capacitor near VBUS, PMID, REGN, SYS, or BAT can create a charging fault that looks like an IC failure.
Tip 3: Always inspect the battery NTC/TS network before concluding that the charger is defective.
Tip 4: Do not inject an arbitrary voltage into BAT, SYS, PMID, or REGN. These are controlled power rails with different electrical functions.
Tip 5: Use a current-limited supply during diagnosis to reduce the risk of turning a repairable board fault into additional component damage.
Tip 6: When replacing a QFN charger IC, PCB pad condition and thermal-pad soldering are just as important as the IC itself.
Tip 7: Use the official TI datasheet for exact pin identification because similar charger ICs can have different pinouts even when their functions appear similar.
BQ25601 Applications and Where It Is Commonly Found
The BQ25601 is designed for portable single-cell battery-powered equipment. Texas Instruments lists applications including smartphones, mobile-phone accessories, and medical equipment.
Its integrated power-path and OTG features also make the architecture useful for other compact battery-powered products where charging, system power management, and USB power functions need to coexist.
Important Correction About the “69A2 IC” Reference
The supplied script includes a “69A2 IC Pinout” entry in the table of contents, but the technical description and specifications are for the BQ25601.
Because pin names and package connections are device-specific, it would be unsafe to treat “69A2” as a BQ25601 pinout without an independent datasheet or board marking confirming that component.
For this reason, this guide uses the verified BQ25601 24-pin WQFN pin configuration from the Texas Instruments documentation rather than repeating an unverified IC identification.
Note: For accurate repair, identify the exact charger IC marking first, then match every measurement with its official datasheet and the actual PCB schematic.
Official BQ25601 Documentation
The official Texas Instruments product page contains the current product information, datasheet, package information, and related evaluation resources for the BQ25601.
Official Website: Texas Instruments — BQ25601 Product Page
https://www.ti.com/product/BQ25601
Official Datasheet:
https://www.ti.com/lit/ds/symlink/bq25601.pdf
Disclaimer: This article is for educational and electronics troubleshooting purposes; always follow the manufacturer’s datasheet and proper electrical safety procedures before repairing or powering a damaged circuit.
