Using a LTC3555 buck converter to manage portable media device power flows - Embedded.com

Using a LTC3555 buck converter to manage portable media device power flows

This “Product How-To” article focuses how to use a certain product in an embedded system and is written by a company representative.

In 2006, consumers bought over 1 billion cellphones, 220 million notebook computers, 140 million MP3 players, 90 million digital still cameras and 10 million personal navigation devices. All these products have something in common in terms of their internal systems configuration.

First, they are all battery-powered, typically with a type of Li-ion battery as the primary power source but with other input power sources available as alternatives and also for batterycharging purposes. And second, they all have built-in storage capacity, usually having a kind of ROM, DRAM or NAND flash and in many cases, HDD or secure digital I/O card.

A recent study by technology research firm IDC determined that the world generated 161 billion Gbytes (161exabytes) of digital information in 2006, an equivalent of information in 2 billion iPods.

However, another category of products not mentioned are hybrid versions of two or even three of these product functions, such as portable media players (PMPs) or digital media broadcast (DMB) products. These products also include a Li-ion battery as their main power source and have a large memory storage capacity. They are fast becoming a significant player in the CE area.

A key advantage of either a PMP or DMB product is that they can play both MP3 and MP4 formats. Thus, a single device can be used to listen to music or watch a movie from a DVD-CD or from a downloaded file. Their storage medium typically allows the device to store over 150hrs of video or 1,200hrs of music.

Like the makers of any battery-powered handheld products, however, PMP manufacturers are under ever increasing pressure to pack all of their features into an already constrained form factor while also gaining longer runtimes.

Since most PMPs have the functionality of both a video player and an MP3 player, the internal electronics requires multiple low voltage output rails at varying power levels. The reason for this is clear – majority of the digital LSI ICs have operating voltage of 1.5V or less. At the same time, 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 DC/DC converters directly from the Liion battery, and system designers are adopting a more integrated approach.

ASIC solution
Most battery-powered handheld products have used an ASIC to deal with the requirements of battery charging, power-path control, providing multiple supply voltages, and protection features such as true output disconnect and accurate USB current limiting. The reasons for adopting this approach are clear—they can obtain a single device that meets all of their power management needs. However, this approach has drawbacks, too.

First, ASICs are manufactured on a specific wafer fabrication process, making it difficult to maximize their performance for each of these functions. Second and becoming more important in these times of short, dynamic design cycles is the long lead-time associated with ASIC definition and development. It is common for a power-management ASIC to take over one and a half years to produce from conception to delivery. During this time, the design needs for a particular product could have changed three or more times.

MOSFETs et al.
Most battery-powered handheld products can usually be powered from an AC adapter, a USB cable or a Li-ion/polymer battery. However, 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 a bunch of MOSFETs, op amps and such, but have faced tremendous problems with hot plugging and having large inrush currents, which cause big system problems.

A commonality of features and functions inside many battery- powered handheld products exists. And an ASSP could be used without any of the usual performance compromises normally associated with manufacturing an IC on a single wafer fabrication process.

Figure 1: Each of the LTC3555's three on-chip buck converters operate under current-mode control and achieve efficiencies as high as 95 percent with either I2C or pin-selectable or automatic burst mode operation.

Linear Technology has introduced the LTC3555, which has the performance and functionality for these applications. The LTC3555 manages power flow between an AC adapter, USB cable and Li-ion battery, while complying with USB power standards.

Housed in a 4mm x 5mm QFN package, it has a fullfeatured 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.

Furthermore, the LTC3555 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 application schematic of the LTC3555 (Figure 1 above ) illustrates how it accomplishes multiple functions. 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 percent with either I2C or pin-selectable 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.

The LTC3555's method of power delivery differs from existing battery and power management ICs, which are charger-fed systems. In such systems, the external power source does not power the loads directly. Instead, the AC adapter or USB port would be used to charge the battery, which then powers the loads.

When 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.

With the LTC3555, this delay is eliminated 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.

These two benefits – elimination of charging delays, and simultaneous battery charging and powering of loads—extend the effective application runtime and accelerate charging when attached to the USB cable. Another advantage of this power management technique is increased ef- ficiency whenever the AC or USB power source is available. In these instances, an unnecessary stage of power conversion (i.e. battery charging) is eliminated.

High efficiency
Unlike its predecessor, the LTC3455, which had a linear PowerPath controller, 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 maximize the allowable USB power.

Because power is conserved, the LTC3555 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 from BAT to Vout guarantees that ample power is always available to Vout even if there is insufficient or absent power at Vbus. Whenever Vbus is available and the PowerPath switching regulator is enabled, power is delivered from Vbus to Vout via SW (Figure 2 below).

Vout drives the combination of the external load (switching regulators 1, 2 and 3 from Figure 1) 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 optimized because power lost to the linear battery charger is minimized. As a result, power available to the load is optimized.

Figure 2: When Vbus is available and the PowerPath switching regulator is enabled, power is delivered from Vbus to Vout via SW.

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 prioritized 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 (right-hand side of Figure 2 above) 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, all the application power will be provided by the battery via the ideal diode.

The transition from input to battery power at Vout will be fast enough to allow only a 3 microFarad 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 180 milli-ohms and can be reduced to less than 50 milli-ohms with an optional resistor.

Designers of battery-powered handheld products have a number of options available to ensure that battery life is optimized for their particular configuration. A performance-optimized, multifunction ASSP can provide the necessary voltages and power levels to provide optimum system performance while ensuring that the power drain on the battery is minimized during normal operation.

Tony Armstrong is Product Marketing Manager, Power Products Grou, Linear Technology Corp.

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