bq24735: A Comprehensive Overview

The bq24735 is a high-efficiency, synchronous battery charger developed by Texas Instruments. Designed with precision and versatility in mind, this device is tailored for space-constrained, multichemistry battery charging applications. Here’s a closer look at its features, applications, and functionalities.

Features:

1. Turbo Boost Support: The bq24735 is designed to support Intel® CPU Turbo Boost Mode. This means that both the adapter and battery can provide power to the system simultaneously, ensuring that the system’s power demand is met even when it temporarily exceeds the adapter’s maximum power level.
2. SMBus Controlled: The device operates with an SMBus host-controlled NMOS-NMOS synchronous buck converter. Users can choose from programmable switching frequencies of 615-, 750-, and 885-kHz.
3. Safety First: The bq24735 comes with enhanced safety features. This includes protection against overvoltage, overcurrent, and short circuits in the battery, inductor, and MOSFET.
4. High Accuracy: The device boasts impressive accuracy levels. It offers ±0.5% charge voltage accuracy up to 19.2 V, ±3% charge current accuracy up to 8.128 A, and ±3% input current accuracy up to 8.064 A.
5. Programmable Features: Users can program various parameters, including input current, charge voltage, and charge current limits. Additionally, it has a programmable battery depletion threshold and a battery LEARN function.
6. Integrated Functionalities: The bq24735 comes with integrated loop compensation and soft start features. It also offers real-time system control on the ILIM Pin to limit charge current.

Applications: The bq24735 is versatile and can be used in a range of applications. These include:

• Portable notebook computers, UMPCs, ultra-thin notebooks, and netbooks.
• Handheld terminals.
• Industrial and medical equipment.
• Other portable equipment.

Description: The bq24735 stands out for its ability to support turbo boost. This feature allows the battery discharge energy to be directed to the system when the system power demand temporarily surpasses the adapter’s maximum power level. This ensures that the adapter doesn’t crash under high demand. The device uses two charge pumps to drive N-channel MOSFETs separately, enabling automatic system power source selection. The SMBus controlled input current, charge current, and charge voltage digital-to-analog converters (DACs) ensure high-regulation accuracies.

Pin Configuration and Functions:

1. ACN (1): Input current-sense resistor negative input.
2. ACP (2): Input current-sense resistor positive input.
3. CMSRC (3): ACDRV charge pump source input.
4. ACDRV (4): Charge pump output to drive both adapter input N-channel MOSFET (ACFET) and reverse blocking N-channel MOSFET (RBFET).
5. ACOK (5): AC adapter detection open-drain output.
6. ACDET (6): Adapter detection input.
7. IOUT (7): Buffered adapter or charge current output.
8. ILIM (10): Charge current limit input.
9. BATDRV (11): Charge pump output to drive battery-to-system N-channel MOSFET (BATFET).
10. GND (14): IC ground.
11. LODRV (15): Low-side power MOSFET driver output.
12. HIDRV (18): High-side power MOSFET driver output.
13. PHASE (19): High-side power MOSFET driver source.
14. BTST (17): This is the high-side power MOSFET driver power supply. Users should connect a 0.047-µF capacitor from BTST to PHASE, and a bootstrap Schottky diode from REGN to BTST.
15. PowerPAD™: This is an exposed pad beneath the IC. It serves as the analog ground and power ground star-connected only at the PowerPad plane. It’s crucial to solder the PowerPad to the board. Vias on the PowerPad plane should connect to both the analog ground and power ground planes. Additionally, the PowerPad serves as a thermal pad, helping to dissipate heat.
16. SDA (8): This is the data line for the SMBus interface. It’s used for bidirectional data transfer between the bq24735 and the system’s microcontroller.
17. SCL (9): This is the clock line for the SMBus interface. The system’s microcontroller uses it to synchronize data transfer with the bq24735.
18. REGN (16): This is the linear regulator output. It provides the necessary voltage for the internal circuits of the bq24735.
19. VCC (20): This is the main power supply input for the bq24735. It powers the internal circuits and logic of the device.
20. SRP and SRN (12 and 13): These pins are associated with the battery current sensing. SRP is the positive input, and SRN is the negative input. They help in monitoring and regulating the battery charge current.

With these pins, the bq24735 offers a comprehensive set of functionalities to ensure efficient battery charging and system power management. Each pin plays a vital role in the overall operation of the device, making it a versatile choice for various applications.

What does AC adapter detection open-drain output mean?

The term “AC adapter detection open-drain output” refers to a specific type of output configuration used in electronic circuits, particularly in the context of the bq24735 device’s ACOK pin. Let’s break down the term to understand it better:

1. AC Adapter Detection: This refers to the ability of the circuit or device to detect the presence or absence of an AC adapter. In the context of battery chargers or devices that can be powered by both batteries and AC adapters, this detection is crucial. It helps the device decide whether to draw power from the battery or the AC adapter.
2. Open-Drain: An open-drain (or open-collector in the case of bipolar transistors) configuration refers to an output that can either be in a low state (grounded) or left floating (open). It doesn’t actively drive the output high. Instead, to achieve a high state, an external pull-up resistor is used. When the open-drain output wants to signal a “high,” it stops conducting, and the external resistor pulls the voltage up to the desired level. When it wants to signal a “low,” it conducts, effectively grounding the output.
3. Output: This simply means that the described configuration is an output from the device, sending signals or information to other parts of the circuit or to other devices.

In the context of the bq24735’s ACOK pin, the “AC adapter detection open-drain output” means that the ACOK pin will either ground itself (indicating one state, such as “no AC adapter detected”) or leave itself open (relying on an external resistor to pull it to a high voltage, indicating the opposite state, such as “AC adapter detected”). The exact behavior (which state is indicated by grounding and which by floating) will be detailed in the device’s datasheet.

SRP (Pin 13) and SRN (Pin 12):

• Functionality: These pins are associated with battery current sensing. They play a crucial role in monitoring and regulating the battery charge current.
• SRP (Sense Resistor Positive): This is the positive input for battery current sensing. It is connected to one end of a sense resistor, which is typically placed in series with the battery’s positive terminal. The voltage at this pin represents the positive side of the voltage drop across the sense resistor.
• SRN (Sense Resistor Negative): This is the negative input for battery current sensing. It is connected to the other end of the sense resistor, which is closer to the battery’s negative terminal. The voltage at this pin represents the negative side of the voltage drop across the sense resistor.
• Operation: The differential voltage between SRP and SRN is proportional to the battery charge current. This differential voltage is used by the bq24735 device to monitor the battery charge current in real-time. By measuring the voltage drop across the sense resistor (between SRP and SRN), the device can determine the current flowing into the battery and make necessary adjustments to ensure optimal charging.
• Filtering: It’s recommended to place a 0.1-µF ceramic capacitor between SRP and SRN for differential-mode filtering. This helps in reducing noise and ensuring accurate current sensing.
• Importance: Accurate battery current sensing is vital for ensuring safe and efficient battery charging. By monitoring the charge current, the bq24735 can prevent overcharging, optimize the charging rate, and prolong battery life.

In summary, the SRP and SRN pins are essential components of the bq24735’s battery current sensing mechanism. They enable the device to monitor and regulate the battery charge current, ensuring safe and efficient charging of the battery.

CMSRC (Pin 3):

• Functionality: The CMSRC pin is associated with the ACDRV charge pump, which is responsible for driving the adapter input N-channel MOSFET (ACFET) and the reverse blocking N-channel MOSFET (RBFET).
• Description:
  • ACDRV Charge Pump Source Input: The CMSRC pin serves as the source input for the ACDRV charge pump. The charge pump is a type of DC to DC converter that uses capacitors as energy storage elements to create either a higher or lower voltage power source. In the context of the bq24735, the charge pump is used to generate a higher voltage to effectively drive the gates of the N-channel MOSFETs (ACFET and RBFET).
  • Driving MOSFETs: The ACDRV charge pump, with CMSRC as its source input, produces an output voltage at the ACDRV pin. This output voltage is used to drive both the adapter input N-channel MOSFET (ACFET) and the reverse blocking N-channel MOSFET (RBFET). By driving these MOSFETs, the bq24735 can control the flow of power from the AC adapter and the battery to the system.
• Protection: To limit the inrush current on the CMSRC pin, it’s recommended to place a 4-kΩ resistor from CMSRC to the common source of ACFET (Q1) and RBFET (Q2). This ensures that the current flowing into the CMSRC pin is controlled, preventing potential damage to the device or the MOSFETs.
• Importance: The CMSRC pin, in conjunction with the ACDRV charge pump, plays a crucial role in the automatic system power source selection mechanism of the bq24735. By controlling the MOSFETs, the device can seamlessly switch between the AC adapter and the battery as the primary power source, ensuring efficient power management and optimal device operation.

In summary, the CMSRC pin is integral to the operation of the ACDRV charge pump in the bq24735 device. It facilitates the driving of key MOSFETs, enabling the device to efficiently manage power sources and ensure safe and optimal operation when powered by an AC adapter or a battery.

ACDRV (Pin 4):

• Functionality: The ACDRV pin is responsible for driving both the adapter input N-channel MOSFET (often referred to as ACFET) and the reverse blocking N-channel MOSFET (often referred to as RBFET). These MOSFETs play a crucial role in determining the source of system power, whether it’s from the AC adapter or the battery.
• Operation:
  • The bq24735 device uses two charge pumps to separately drive the N-channel MOSFETs (ACFET, RBFET, and BATFET) for automatic system power source selection.
  • When certain conditions are met, such as when the voltage on the ACDET pin is between 2.4 V and 3.15 V, the voltage on the VCC pin is above the UVLO (Under Voltage Lock Out) threshold, and the voltage on the VCC pin is 275 mV above the voltage on the SRN pin, the ACDRV pin generates a voltage that is 6 V above the CMSRC pin. This ensures that both the ACFET and RBFET can be turned on, allowing the system to be powered by the AC adapter.
  • A resistor (typically 4-kΩ) is often placed between the ACDRV pin and the gate of the ACFET and RBFET. This resistor limits the inrush current on the ACDRV pin, ensuring stable operation.
• Importance: The ACDRV pin plays a pivotal role in determining the source of power for the system. By controlling the ACFET and RBFET, the bq24735 can seamlessly switch between the AC adapter and the battery as the primary power source. This ensures that the system remains powered even if one source becomes unavailable or is below the required power level.
• Protection: The ACDRV pin, in conjunction with other pins and functionalities of the bq24735, offers protection mechanisms. For instance, if the AC adapter is not present or if there’s a fault, the ACDRV pin ensures that the system can still draw power from the battery by controlling the associated MOSFETs.

In summary, the ACDRV pin in the bq24735 device is integral to the automatic system power source selection mechanism. It ensures that the system can be powered efficiently and safely, either from the AC adapter or the battery, based on the prevailing conditions and requirements.

ACOK (Pin 5):

• Functionality: The ACOK pin stands for “AC Adapter OK.” It serves as an indicator for the presence or absence of a valid AC adapter.
• Type: The ACOK pin is an open-drain output. This means that internally, it can either pull the output to ground (a low state) or leave it floating (open). It doesn’t actively drive the output to a high state. To achieve a high state, an external pull-up resistor is typically used.
• Operation:
  • High State (Adapter Present): When a valid AC adapter is detected (based on conditions like the voltage on the ACDET pin, voltage on the VCC pin, etc.), the ACOK pin stops conducting, leaving itself open. The external pull-up resistor then pulls the voltage up to the desired level, indicating that an AC adapter is present and is considered “OK” or valid.
  • Low State (Adapter Not Present or Invalid): If the conditions for a valid adapter are not met, the ACOK pin conducts, effectively grounding the output. This indicates the absence of an AC adapter or that the connected adapter is not valid.
• Usage: The ACOK pin can be connected to other system components, such as a microcontroller or LED indicator, to provide real-time feedback about the AC adapter’s status. For instance, it can be used to light up an LED when an AC adapter is connected and working correctly or to alert the system to switch to battery power if the AC adapter is removed or not functioning properly.
• External Components: A typical setup involves connecting a pull-up resistor from the ACOK pin to a supply rail. This ensures that the ACOK pin can achieve a high state when the internal open-drain output stops conducting. A value of 10-kΩ is commonly used for this pull-up resistor.
• Importance: The ACOK pin provides a simple yet effective way for the system to monitor the status of the AC adapter. It ensures that the system can make informed decisions based on whether external power is available, optimizing power management and user experience.

In summary, the ACOK pin of the bq24735 device serves as a real-time indicator of the AC adapter’s status. Its open-drain configuration, combined with external components like a pull-up resistor, allows it to provide clear feedback on whether the system is receiving valid external power or not.

ACDET (Pin 6):

• Functionality: The ACDET pin is responsible for adapter detection. It helps the bq24735 device determine the presence or absence of an AC adapter.
• Description:
  • Adapter Detection Input: The ACDET pin serves as an input that is used to detect the presence of an AC adapter. By monitoring the voltage level on this pin, the bq24735 can determine whether a valid adapter is connected.
  • Threshold Programming: The valid input threshold for adapter detection can be programmed by connecting a resistor divider from the adapter input to the ACDET pin and then to the GND pin. This setup allows users to define the voltage level at which the device recognizes the presence of an AC adapter.
  • Operational Conditions: When the voltage on the ACDET pin is above 0.6 V and the VCC is above the undervoltage lockout (UVLO) threshold, several functionalities become active. These include the presence of the REGN LDO (Low Dropout Regulator), activation of the ACOK comparator, and the IOUT being active. These conditions indicate that a valid adapter is present and ready to start the charging process.
  • Adapter Validity Indication: If the voltage on the ACDET pin is between 2.4 V and 3.15 V, the voltage on the VCC pin is above the UVLO threshold, and the voltage on the VCC pin is 275 mV above the voltage on the SRN pin, it indicates that a valid adapter is present. This condition signals the device to start the charging process.
  • ACOK Interaction: The ACDET pin works in conjunction with the ACOK pin (AC adapter detection open-drain output). When the conditions for a valid adapter are met, the ACOK pin is pulled HIGH, indicating the presence of a valid adapter. If any of the conditions are not met, the ACOK pin is pulled LOW, signaling the absence of a valid adapter or an error condition.
• Importance: The ACDET pin is crucial for ensuring that the bq24735 device can accurately detect the presence of an AC adapter. This detection is vital for the device to decide when to initiate the charging process and to ensure safe and efficient operation.

In summary, the ACDET pin plays a pivotal role in adapter detection for the bq24735 device. It ensures that the device can accurately determine the presence of an AC adapter and make informed decisions regarding the charging process.

IOUT (Pin 7):

• Functionality: The IOUT pin provides a buffered output that represents either the adapter or the charge current, depending on the configuration set by the system’s SMBus command ChargeOption().
• Operation: The voltage at the IOUT pin is 20 times the differential voltage across the sense resistor. This means that the IOUT pin provides an amplified representation of the current, either from the AC adapter or the charge current going to the battery. By amplifying the sensed current by a factor of 20, the device allows for easier and more accurate monitoring of the current levels.
• Usage: The IOUT pin can be connected to an analog-to-digital converter (ADC) or other monitoring circuits in the system. This allows the system’s microcontroller or other control circuits to monitor the current levels in real-time and make necessary adjustments or decisions based on the current flow.
• Decoupling: To ensure accurate readings and reduce noise, it’s recommended to place a ceramic decoupling capacitor (typically 100 pF or less) from the IOUT pin to GND. This capacitor helps filter out high-frequency noise and provides a stable voltage reading at the IOUT pin.
• Importance: The IOUT pin is crucial for real-time monitoring of the system’s current levels. By providing an amplified representation of the current, the bq24735 allows the system to keep track of power consumption, charging rates, and overall system health. This information can be used to optimize power usage, ensure safe charging, and prolong both the device’s and battery’s lifespan.

In summary, the IOUT pin of the bq24735 device offers a convenient and accurate means to monitor the system’s current levels. Whether it’s tracking the input current from the AC adapter or the charge current to the battery, the IOUT pin provides valuable insights into the system’s power dynamics.