ASSP optimises handheld power
01 July 2007
Last year, consumers bought over one billion mobile phones, 220 million notebook computers, 140 million MP3 players, 90 million digital still cameras and 10 million personal navigation devices
All share some level of commonality in terms of their internal systems configuration. They are all battery powered, typically with some type of Lithium-Ion (Li-Ion) battery as the primary power source, but with other input power sources available for battery charging purposes.
Secondly, they all have built-in storage capacity, usually some kind of ROM, DRAM or NAND flash and, in many cases, a hard disk drive or SDIO (secure digital I/O) card. A recent study by IDC determined that the world generated 161billion gigaByte (161exaBytes) of digital information in 2006.
Other products are hybrid versions, such as portable media players (PMP) or digital media broadcast (DMB) products. These also include a Li-Ion battery as their main power source and have a large memory storage capacity. A PMP or DMB product can play both MP3 and MP4 formats. Therefore, a single device can be used to listen to music or watch a movie from a DVD-CD, or from a website downloaded file. Typically, their storage medium allows the device to store over 150 hours of video or 1,200 hours of music.
Manufacturers of these PMPs are under increasing pressure to pack features into an already constrained form factor, while simultaneously gaining longer runtimes.
Power requirements
The internal electronics requires multiple low voltage output rails at varying power levels. The majority of the digital LSI (large scale integrated) ICs have an operating voltage of 1.5V or less. Memory and I/O voltage requirements can vary between 2.5V and 3.3V. Thus, it is becoming impractical to use multiple point-of-load (PoL) DC/DC converters directly from the Li-Ion battery. System designers are adopting a more integrated approach.
Most battery-powered handheld products have used an ASIC (application specific integrated circuit) to deal with the requirements of battery charging, powerpath control, providing multiple supply voltages, as well as protection features such as true output disconnect and accurate USB current limiting. A single device can meet all power management needs. However, ASICs are manufactured on a specific wafer fabrication process, making it difficult to maximise their performance for each of these functions. Additionally, it is common for a power management ASIC to take over 18months to produce from conception to delivery. During this time, the design needs could have changed three or more times.
ASSP for power management
Most battery-powered handheld products can be powered from an AC adapter, a USB (universal serial bus) cable, or a Li- Ion/polymer battery. Managing the power path control between these power sources presents a significant technical challenge. Until recently, designers have tried to perform this function discretely by using MOSFETs or op-amps, but face problems with hot plugging and large inrush currents. An ASSP (application specific standard product) could be used without the performance compromises normally associated with manufacturing an IC on a single wafer fabrication process.
Linear Technology’s LTC3555 seamlessly manages power flow between an AC adapter, USB cable and Li-Ion battery, while complying with USB power standards, from a 4mm x 5mm QFN package. It has a full featured Li- Ion/Polymer battery charger that can provide up to 1.2A of charge current plus three high efficiency synchronous buck converters to generate low voltage rails which most USB peripherals require. The device also provides an always-on 25mA low dropout linear regulator to power the real-time clock and low power logic circuitry. The entire product can be controlled via a simple I2C interface or simple I/O.
The DC/DC conversion is a relatively straightforward step-down (buck) converter function. Each of the LTC3555’s three on-chip buck converters operate under current-mode control and achieve efficiencies as high as 95 per cent with either I2C or pin-selectable Burst Mode or automatic Burst Mode operation. These DC/DC converters operate at a fixed 2.25MHz switching frequency that allows the use of very small external capacitors and inductors. The continuous output current levels of these buck converters are 1A, 400mA and 400mA, respectively, with output voltages programmable between 0.8V and 3.6V.
Delivery method
Instead of a charger-fed system, where the external power source does not power the loads directly, the AC adapter or USB port would be used to charge the battery, which then powers the loads. In the event that the battery has been deeply discharged or even missing, there will be a delay in getting power to the loads. This is because power cannot be taken from the battery until it has obtained the required minimum amount of charge. The LTC3555 eliminates this delay so that the handheld device can be powered up as soon as the wall adapter or USB power source is connected. In addition, the chip will take any available power not being used by the loads and use it to charge the battery.
Elimination of charging delays and simultaneous battery charging and powering of loads extend the effective runtime of the application and accelerate charging when attached to the USB cable. Another advantage of this power management technique is increased efficiency whenever the AC or USB power source is available. In these instances, an unnecessary stage of power conversion (i.e. battery charging) is eliminated.
The LTC3555 has a high efficiency switch mode PowerPath controller. Designed specifically for USB applications, the LTC3555’s PowerPath controller incorporates a precision average input current step-down switching regulator to make maximum use of the allowable USB power. As power is conserved, the device allows the load current on VOUT to exceed the current drawn by the USB port without exceeding the USB load specifications. The PowerPath switching regulator and the battery charger communicate to ensure that the input current never violates the USB specifications. Furthermore, the ideal diode guarantees that sample power is always available to VOUT even if there is insufficient or absent power at VBUS.
PowerPath switching
Whenever VBUS is available and the PowerPath switching regulator in enabled, power is delivered from VBUS to VOUT via SW. VOUT drives the combination of the external load and the battery charger. If the combined load does not exceed the PowerPath switching regulator’s programmed input current limit, VOUT will track 0.3V above the battery. By keeping the voltage across the battery charger low, efficiency is optimised because power lost to the linear battery charger is minimised; as a result, power available to the load is therefore optimised.
If the combined load at VOUT is large enough to cause the switching power supply to reach the programmed input current limit, the battery charger will reduce its charge current by an amount necessary to enable the external load to be satisfied. Even if the battery current is set to exceed the allowable USB current, the USB specification will not be violated because the switching regulator will always limit the average input current to ensure that this is the case. Furthermore, the load current at VOUT will always be prioritised and only excess available power will be used to charge the battery. If the voltage at BAT is below 3.3V, or the battery is not present, and the load requirement does not cause the switching regulator to exceed the USB specification, VOUT will regulate at 3.6V. If the load exceeds the available power, VOUT will drop to a voltage between 3.6V and the battery voltage. If there is no battery present when the load exceeds the available USB power, VOUT will collapse to ground.
The LTC3555 has an internal ideal diode and a controller for an optional external ideal diode. The ideal diode controller is 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. Furthermore, if power to VBUS (USB or wall adapter power) is removed, then all the application power will be provided by the battery via the ideal diode. The transition from input power to battery power at VOUT will be fast enough to allow only a 3uF capacitor to keep VOUT from drooping. This is made possible because the ideal diode consists of a precision amplifier that enables a large on-chip P channel MOSFET transistor whenever the voltage at VOUT is approximately 15mV (VFWD) below the voltage at BAT. The resistance of the internal ideal diode is approximately 180m2 and can be reduced to less than 50m2 with an optional resistor.
TONY ARMSTRONG is product marketing manager, Power Products Group, Linear Technology
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