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Posted on 17 July 2019

Power Management IC Simplifies Design for Bluetooth Headsets

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The perfect solution for space limited applications

This article details recent developments in power supply circuits enabling simplified designs compared to discrete solutions. The PMU discussed also reduces the board space needed significantly for Bluetooth headsets.

By Thomas Schaeffner, Texas Instruments, Freising

 

Recently Bluetooth headsets have become popular not only as an interface for mobile phones while making a phone call but also in a Stereo version for listening to music. There are very tiny models already available on the market, which are pretty much the same size than standard (wired) stereo inear headphones. For such applications, the size of all electronics including the battery is most critical. A second care about is high efficiency to get a maximum of operating time with a given battery charge. A new family of power management units (PMUs) by Texas Instruments is available for these applications. The TPS65720 family is available in two package options: TPS65720YFF in wafer chip scale package and TPS65721RSN in QFN package. TPS65721 is a more generic device while TPS65720 was designed for smallest space. TPS65720 comes in a 2mm x 2mm chip scale package with a 0.4mm ball grid and enables designs requiring a minimum of board space. This is not only achieved by the small package but also by reducing the size and the amount of external components needed. TPS65721 in QFN package has a higher output current on its dcdc converter for example, noise cancelling headset designs or other applications powered by a single LiIon cell.

The power supply functions needed in Bluetooth headsets includes mayor blocks like:
• Li-Ion battery charger with power path
• Step-down converter for generating a regulated voltage with high efficiency
• An LDO for powering the application (bluetooth) processor
• Common functions like reset, start-up, GPIOs and digital interface

All these functions have been integrated into TPS6572x giving the user a single chip power solution with minimum size.

TPS65720 ON/OFF functions

Portable systems are usually turned on by pressing a button for a certain time. For a PMU just providing enable pins, this functionality can not easily be implemented as there is debounce needed on such a button. Once the application is turned on, one may want to turn off again by pressing the same button again. As the main processor is not alive, this functionality has to be realized in hardware or by adding a small low power microcontroller for this function. This however will add cost and requires additional board space which is not an option for space limited applications like bluetooth headsets. TPS6572x provides this function internally, so no additional microprocessor is needed. An enable circuitry with two inputs and a simple state machine inside TPS6572x serves this function and only requires a typical current of 12uA.

The enable button (ON-key) is tied to an input called /PB_IN. Once /PB_IN is pulled low by the ON-key, the state machine will activate all blocks necessary and the dcdc converter and LDO will be started. The PMU would turn off again once /PB_IN is released, so two more input are used to keep DCDC converter and the LDO alive, when /PB_IN is released. HOLD _DCDC1 keeps the dcdc converter enabled when pulled HIGH while input HOLD-LDO1 serves the same function for the LDO.

Once enabled, the I2C interface is active an interrupts can be unmasked to signal state changes of e.g. “ON-key pressed”, voltage applied to the charger, “charge done,” etc.

Figure 2 shows the block diagram. The first block to discuss is the charger with its power path. A power path is an array of switches separating the battery from the load such that the battery is connected to the load if there is no input voltage to the system from an external power supply. If there is an input voltage at pin AC, the voltage at AC is connected to the load and the battery is recharged via a separate path. This allows charging the battery independently of load current as long as the input source can provide the current needed for both load and battery. In case the current capability of the supply (typically a mains adapter or USB cable) is not high enough for powering the load and providing the full charge current, the current used to charge the battery is reduced to supply the current needed for the load and maintain the voltage at the output of the power path on pin SYS. This functionality is called dynamic power path management (DPPM). TPS6572x implement such a charger with power path that allows charging a LiIon cell with up to 300mA. There is an input current limit with a maximum of 100mA and 500mA as well as a suspend mode which reduces the input current to 50uA typically.

Block diagram of TPS6572x

The output of the power path / charger (SYS pin) is used to power a step-down converter that is optimized for high efficiency at low output current. It automatically switches between PFM and PWM mode during operation but can be forced into PWM mode by an I2C command for lowest output voltage ripple.

In a typical Bluetooth application, the output of the dcdc converter is used to power an LDO that provides the supply voltage to the system at 1.85V. To keep the total efficiency high, the LDO needs to be operated with a minimum voltage differential. The LDO in TPS6572x is a 200-mA LDO optimized for low dropout voltage and low quiescent current, so it can be supplied at 2.05V from the step-down converter and still be able to post-regulate the voltage to generate a low noise supply for the bluetooth chip. Typical Bluetooth systems operate in sleep mode for a certain time requiring less than 100uA of supply current, wake up and take some tens of mA while they are transmitting data and go to sleep again afterwards. The low quiescent current of TPS65720 allows to keep the efficiency of the system at a high level even at low output current. Figure 3 gives the efficiency of the total system with the LDO powered by the step-down converter.

Total efficiency for the power system

While the step-down converters output voltage is externally adjustable with a resistor divider, the output voltage of the LDO is fixed at 1.85V for the TPS65720. However, the I2C interface allows for changing the LDO voltage in TPS65720 in a wide range from 0.8V to 3.3V. In TPS65721, the voltage of the LDO is externally adjustable as well.

Four GPIOs can be used to extend the ports of the processor as general purpose inputs or outputs. Two of them can be configured as a current sink, such that a LED connected from a supply voltage to the GPIO is driven by a constant current of 5mA with no need for a current limiting resistor. This allows a precision current for a wide supply voltage rails and minimizes the external part count.

The I2C interface with its extensive register set allows a wide variety of charging options and also to define a set of status Bits that will generate an interrupt, so the processor can react on events like: “battery fully charged”, “voltage applied to the charger” or “user-keypressed”.

In general, TPS6572x is designed to operate with small external components like 0402 size capacitors. The small size of the 2mm x 2mm chip scale package in additional to few and small external components makes TPS65720 a perfect solution for space limited applications like bluetooth headsets.

 

 

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