Dual Input Li+ Charger and Smart Power Selector Eliminates the Need for External MOSFETs
SUNNYVALE, CAMaxim Integrated Products introduces the MAX8677A, dual input USB/AC-adapter linear charger with Smart Power Selector for portable devices powered by rechargeable single-cell Li+ batteries. This charger integrates all of the power switches required for charging and switching the load between the battery and external power, thus eliminating the need for external MOSFETs. MAX8677A is ideal for portable applications, such as smartphones, PDAs, portable media players, GPS navigation devices, digital still cameras, and digital video cameras.
The MAX8677A can operate either from separate inputs for USB and AC-adapter power or from a single input that accepts both. When external power is connected, the Smart Power Selector allows the system to operate with no battery or a deeply discharged battery. It automatically switches the system load from the battery and makes the best use of limited USB or adapter power while charging the battery from input power not used by the system. All necessary current-sense circuitry is included on-chip with the integrated power switches.
The DC input's current limit is adjustable up to 2A, and both DC and USB inputs support 100mA, 500mA, and USB-suspend modes. Charge current is adjustable up to 1.5A to support a wide range of battery capacities. Other MAX8677A features include thermal regulation, overvoltage protection, charge status and fault outputs, power-OK monitors, a battery thermistor monitor, and a charge timer.
The MAX8677A is available in a space-saving, thermally enhanced, 4mm x 4mm, 24-pin TQFN package that is specified for the extended temperature range (-40 degrees Celsius to +85 degrees Celsius). Prices start at $1.95 (1000-up, FOB USA). An evaluation kit is available to speed designs.
When it comes to power management for nomadic designs, there are myriad ICs from quite a few chip vendors to choose from. This one, from Maxim Integrated Products , eliminates the lossy external pass element common to many regulator designs, and brings a number of slick and safe functions to the power party.
What's more, it apparently ups the ante as far as its competitors go, offering higher input voltage capabilities, built-in current limiting, and lower drop-out voltages.
As Maxim's press statement (on the left) notes, the digitally controlled MAX8677A , in its thermally enhanced TQFN (thin quad flat no-lead) package, includes on-chip MOSFET s. The pass MOSFET exhibits only 40-milliohms of ON -resistance. In operation, the MOSFET can throttle loads up to 3-A, and charge a battery at up to a 1.5-A rate.
That contributes to low power operation. Of course, the package's paddle must be soldered to your circuit board with multiple vias packed under the IC's exposed “paddle.” That's needed to ensure thermal contact in order to minimize heat rise.
Your heatsink ensures that maximum charging current is maintained over the widest range of external conditions. Note that Maxim's package can continuously dissipate about 1.6-W on a single-layer board, and over 2-W on a multi-layer substrate. Beyond that, a thermal limiter reduces a battery's charge rate, and any external power-source current, to prevent overheating.
A Variety Of Power Sources
With its smart battery charger capabilities, as well as power flow control, the MAX8677A looks like just the ticket for designs that need to operate transparently from a variety of power sources.
In operation, the IC's DC and USB (Universal Serial Bus) inputs can be separately connected to an AC adapter output and a USB port, or the DC input can be a single input that connects to either an adapter or USB. The applications diagram shown here shows a typical circuit using separate DC and USB connectors.
The IC's so-called Smart Power Selector controls the power distribution from an AC adapter or a USB source, routing it to a system's battery and load. The battery is charged with any available power not used by the system load. If your system's peak demand exceeds the input current limit, supplemental current can be automatically taken from the battery.
What's nifty about this chip is that you don't have to shut down any of a system's features to limit current when a USB source is connected. When a DC or USB source is at its limit, the system's battery can supply extra current to maintain the desired load, and your system plays without alteration or loss of functions. In effect, the Smart Power Selector distributes power between external inputs, the battery, and the load.
The Smart Power Selector can distribute power with both an external power supply (either from the USB port, or from an AC adapter) and the system's battery. When load requirements are less than the input current limit, the battery is charged with residual power from the input. When the system load exceeds the input current limit, the battery supplies current to the load.
However, if the battery is connected and there isn't any external power input, the system is powered from the battery, and when an external input is connected and there isn't any battery, the system is powered from the external power input.
The on-chip MOSFET is what connects the load to the battery when no voltage source is available either from the DC source or the USB port. However, when an external source is detected, the MOSFET stops conducting, and your system is then powered from the valid input source through an input limiter.
An LDO Regulator
The limiter acts as a LDO (low drop-out) regulator. In addition to its function of passing power to the system and charger loads, it performs several additional functions, all calculated to minimize system power.
For one thing, it can act as a voltage limiter. If an input voltage is above an over-voltage threshold, the chip enters an over-voltage lockout mode. In OVLO mode, the chip and downstream circuitry is protected from any over-voltage conditions.
This protection is good up to 14-V from your DC source, and up to 8-V from the USB power source. If both sources are in over-voltage, input switches on the chip open and the charger turns off.
The limiter also provides input over-current protection. When doing this, the limiting can be selected to match the source. So, the source can be 100-mA USB source, or even a 0.5-A USB source, or the product's AC adapter. If the load exceeds the input current limit, the output drops, at which point the battery supplies the extra load current needed to run the system. This scheme is actually protected by a Maxim patent.
The MAX8677A also includes a circuit that can protect against transients. Its soft-start feature prevents input transients stemming from a USB or AC adapter. When a DC or USB input is connected, input current limiting is ramped from zero to a set limited value. If DC is connected with no USB power present, input current ramps in 1.5-ms. If DC is connected with USB already present, input current ramps in 50-µs.
If USB power is connected with no DC present, input current ramps also ramps in 50-µs, and if the USB power is applied with DC already present, the USB input is ignored. However, if an adapter is plugged into DC while USB is already powered, the input current limit re-ramps from zero back up to the DC current limit so that the AC adapter doesn't see a load step.
During this transition, if the input current limit falls below the system load current, the battery again supplies additional current needed to support the load. Beyond that, you can add a capacitor to the scheme to support the load during input power transitions.
Maxim's smart dual-input MAX8677A looks like it could cut component count in a space constrained portable system, and generate less heat than other approaches, thanks to its MOSFET's Rds ON-resistance. That should result in longer battery life.
Moreover, designing-in an '8677A promises to ensure safe operation by ensuring an AC adapter isn't overloaded. Additionally, you can dispense with a power-hungry sense resistor (and eliminate another discrete component).
Pretty cool for less than $2 a pop, eh?
Click here for a datasheet (in Adobe Acrobat .PDF format).
For further info, contact Maxim Integrated Products, Inc., 120 San Gabriel Dr., Sunnyvale, Calif. 94086. Phone: 408-737-7600. Fax: 408-737-7194.
Maxim Integrated Products , 408-737-7600, www.maxim-ic.com