USB technology has increased the convenience of electronic devices. Now it is possible to charge a device from the same USB port that performs the data transfer, eliminating the need for a separate wall adapter. However, there are power limitations (2.5W maximum) when the USB is used for charging the device's battery. A USB-based battery charger must extract as much power from the USB as efficiently as possible, to meet the stringent space and thermal constraints of today’s power-intensive applications. Managing power flow within the product is another issue. Larger batteries require either higher charging current or additional time to charge to their full capacity. This need for high-efficiency charging, combined with the high level of feature integration required of the battery charger IC, need to save board space and increase product reliability.

The new generation of power management ICs, such as the LTC3576 from Linear Technology, are equipped with features such as PowerPath control, as well as other advanced integrated functional blocks such as a USB OTG boost converter and efficient programmable switching regulators to simply and easily solve the design challenges mentioned above. In fact, in many systems one PMIC is sufficient to power the entire system, which is achieved through utilization of selective integration level to offer a compact solution without any performance compromises.

A key feature of the LTC3567 is PowerPath control. This automatic load prioritization offers the ability to autonomously and seamlessly manage power flow between multiple input sources such as USB ports, wall adapters and the battery, all while preferentially providing power to the system load. In a traditional battery-fed charging system, the user must wait until there is sufficient battery charge and voltage level available to obtain system power. Conversely, PowerPath control allows the end product to operate immediately when plugged in, regardless of the battery's state of charge, commonly referred to as instant-on operation. The power delivered from VBUS to VOUT is controlled by a 2.25MHz constant frequency bidirectional switching regulator operating in step-down mode. To meet the maximum USB load specification, the switching regulator contains a measurement and control system that ensures that the average input current remains below the level programmed at CLPROG.

USB On-The-Go OTG

For USB OTG applications, the "double duty" bi-directional PowerPath switching regulator acts as a step-up converter to deliver power from VOUT to the VBUS to both run the application and charge the battery. The power from VOUT can come from the battery or the output of the external high voltage switching regulator. As a step-up converter, the bidirectional switching regulator produces 5V on the VBUS and is capable of delivering a minimum of 500mA. USB OTG can be enabled by either the external control pin, ENOTG, or via an I2C interface.

The input current limit is programmed by the ILIM0 and ILIM1 pins or by the I2C serial port. The input current limit has four possible settings ranging from the USB suspend limit of 500μA up to 1A for wall adapter applications. The table shows the current limit settings using ILIM0 an ILIM1. When the switching regulator is activated, the average input current will be limited by the CLPROG programming resistor according to the expression as shown. The IVBUSQ is the quiescent current of the LTC3576, VCLPROG is the CLPROG servo voltage in current limit, RCLPROG is the value of the programming resistor and hCLPROG is the ratio of the measured current at VBUS to the sample current delivered to CLPROG. You can refer to the Electrical Characteristics table for values of hCLPROG, VCLPROG and IVBUSQ.

The WALL, ACPR and VC pins can be used in conjunction with an external high voltage step-down switching regulator such as the LT3480 or the LT3653 to minimize heat production when operating from higher voltage sources. Bat-Track control circuitry regulates the external switching regulator’s output voltage to the larger of (BAT + 300mV) or 3.6V. This maximizes battery charger efficiency while still allowing instant-on operation when the battery is deeply discharged. When high voltage is applied to the external regulator, WALL will rise toward this programmed output voltage. When WALL exceeds approximately 4.3V, ACPR is brought low and the Bat-Track control of the LTC3576 overdrives the local VC control of the external high voltage step-down switching regulator. Therefore, once the Bat-Track control is enabled, the output voltage is set independent of the switching regulator feedback network.

The LTC3576 each have an internal ideal diode as well as a controller for an optional external ideal diode. Both the internal and the external ideal diodes are always on and will respond quickly whenever VOUT drops below BAT. If the load current increases beyond the power allowed from the switching regulator, additional power will be pulled from the battery via the ideal diode. If power to VBUS (USB or wall adapter) is removed, then all of the application power will be provided by the battery via the ideal diodes.

If the LTC3576 are configured for USB suspend mode, the bidirectional switching regulator is disabled and the suspend LDO provides power to the VOUT pin. This LDO will prevent the battery from running down when the portable product has access to a suspended USB port. Regulating at 4.6V, this LDO only becomes active when the switching converter is disabled. To remain compliant with the USB specification, the input to the LDO is current limited so that it will not exceed the low power or high power suspend specification. If the load on VOUT exceeds the suspend current limit, the additional current will come from the battery via the ideal diode. The low quiescent current always-on LDO regulator is used to provide power to a system pushbutton controller, standby microcontroller or real time clock. The LDO is powered from Vout, and therefore will enter dropout at loads less than 20mA as Vout falls near 3.3V.

When a battery charge cycle begins, the battery charger first determines if the battery is deeply discharged. If the battery voltage is below VTRKL, typically 2.85V, an automatic trickle charge feature sets the battery charge current to 10% of the programmed value. If the low voltage persists for more than 1/2 hour, the battery charger automatically terminates. Once the battery voltage is above 2.85V, the charger begins charging in full power constant-current mode. The battery charger has a built-in safety timer. Once the battery charger detects that the battery has reached the fl oat voltage, the four hour safety timer is started. After the safety timer expires, charging of the battery will discontinue and no more current will be delivered.

The LTC3576 may receive commands from a master device using 2-wire I2C interface. A bus master signals the beginning of a communication to a slave device by transmitting a START condition. A START condition is generated by transitioning SDA from high to LOW while SCL is HIGH. When the master has finished communicating with the slave, it issues a STOP condition by transitioning SDA from LOW to HIGH while SCL is high. The bus is then free for communication with another I2C device. The I2C serial port can be disabled by grounding the DVCC pin. In this mode, the LTC3576/LTC3576-1 are controlled through the individual logic input pins EN1, EN2, EN3, ENOTG, ILIM0, ILIM1, SDA and SCL.

The LTC3576/LTC3576-1 contain three general purpose 2.25MHz step-down constant-frequency current mode switching regulators. Two regulators provide up to 400mA and a third switching regulator can provide up to 1A. All three switching regulators can be programmed for a minimum start-up output voltage of 0.8V and can be used to power a microcontroller core, microcontroller I/O, memory, disk drive or other logic circuitry. All three switching regulators have I2C programmable set points for on-the-fl y power savings.

When two on-the-go devices are connected, one will be the A device and the other will be the B device depending on whether the device is connected to a micro A or micro B plug. The A device provides power to the B device and starts as the host. Figure 1 shows an on-the-go device using the LTC3576 acting as the A device. Additional capacitance can be placed on the VBUS pin of the LTC3576 when using the overvoltage protection circuit. When an on-the-go device using the LTC3576 becomes the B device, as in Figure 1, it must send out a data line pulse followed by a VBUS pulse to request a session from the A device. The on-the-go device designer can choose how much capacitance will be placed on the VBUS pin of the LTC3576 and then generate a VBUS pulse that can distinguish between a powered down on-the-go A device and a powered down standard host.

Thank you for taking the time to view this presentation on New Generation Switching Power Manager with USB On-the-Go. If you would like to learn more or go on to purchase some of these devices, you may either click on the part list link, or simply call our sales hotline. For more technical information you may either visit the Linear Technology site – link shown – or if you would prefer to speak to someone live, please call our hotline number, or even use our ‘live chat’ online facility.