Electronic Design

Charger Chip Supports Datacenter Memory System Backup Apps

How does a chip company maximize return on its non-recurring engineering investment? It could duplicate its intellectual property (IP) in a range of ICs aimed at similar applications, but with different feature sets tailored to those apps. Or, it can put the same IP into a narrower range of more versatile ICs.

Linear Technology saw that a certain class of its customers had intrinsically similar needs, but a wide variety of what were essentially I/O specifications. So the company took the second route with its LTC4110 battery-backup controller, a one-size-fits-all approach to implementing emergency backup in the datacenter and elsewhere (see the figure).

Most engineers associate batteries in the datacenter with uninterruptible power supplies, but disk-drive systems and tape arrays can have local backup batteries as well. Medical and “high-reliability” systems also use battery backup. Within that range of apps, “batteries” may use various chemistries. They could be simple or “smart.” Or they may be not chemical batteries at all, but supercapacitors.

WHAT IT DOES
In a typical application, Linear’s LTC4110 is placed between a main power supply that powers all or part of the system and the device or subsystem that requires battery backup. It switches automatically from the main power supply to the battery when battery backup mode is required. Additionally, it maintains the state of charge (SOC) of the battery at all times.

This controller can handle any kind of battery a subsystem designer wants to throw at it: lithium-ion (Li-ion), Li-ion polymer, leadacid, nickel-metal-hydride/nickel-cadmium (NiMH/NiCd), or supercapacitors—with rated voltages from 2.7 to 19 V. It also doesn’t care about input voltage, handling charge and discharge functions with input voltages from 4.5 to 19 V.

There are four normal modes of operation: battery backup, battery charge, battery calibration, and shutdown. Battery backup and battery charge are automatic standalone modes. The calibration mode involves communication with a host via an SMBus port. To minimize pin-count, the SMBus port can be configured to use any of three general-purpose I/O pins that are used as status indicators when the LTC4110 operates autonomously.

HOW IT WORKS
Internally, the LTC4110 operates as a high-efficiency, synchronous, pulse-width modulated, flyback battery charger with constant current and constant float-voltage regions of operation. For Li-ion batteries, the float voltage can be resistor- programmed to 4.2, 8.4, 12.6, or 16.8 V, depending on the number of series cells. Furthermore, it can be adjusted ±0.3 V/cell.

If the battery voltage exceeds 107.5% of the programmed float voltage during any stage of charge, the charger will pause until the voltage drops below a certain value, though the charge timer won’t be stopped. For nickel batteries, the constant-voltage function isn’t used.

For “smart batteries,” i.e., batteries with an integrated “gas-gauge” monitor and an SMBus for control, the internal auto-recharge function is inhibited. The external controller handles timing and selects the parameters for wake-up charge, preconditioning charge, and bulk charge.

Supercapacitors are slightly different. The IC then is dealing with devices that represent a short circuit at zero charge, and that would exhibit an exponential discharge into a resistive load, absent some kind of current throttling. The LTC4110 handles supercapacitor charging by modifying the standard Li-ion or sealed lead-acid (SLA) modes with dynamic charge-voltage adjustments. Current limiting flattens the discharge voltage curve.

EES GONE WILD
There’s more than that, though. Once Linear’s engineers were turned loose on the project, they came up with a number of features intended to make the IC more appealing than a patchwork solution made up of simpler parts. For instance, when it’s necessary to discharge the battery during calibration, the flyback charger is used in reverse, discharging the battery with a programmable constant current. But rather than directing this current into a resistor and generating excess heat, it directs the discharge current into the system load.

Also, a “shutdown” pin isolates the battery so the batterypowered subsystem can be shipped with a charged battery installed. Furthermore, it’s easy to combine multiple chips to form a redundant battery backup system or to increase the number of battery packs to achieve longer backup runtimes. The designers used low-loss ideal diode FETs to switch to connect the main supply or the battery to the backup load. This lets multiple LTC4110s work together in a scalable fashion to permit longer backup times, redundancy, and/or higher load currents. The flyback converter topology makes it possible to charge batteries with a termination voltage higher than the main supply voltage while providing high dc isolation to minimize parasitic drain on the battery.

The LTC4110 comes in a low-profile (0.75 mm), 38-pin, 5- by 7-mm quad flat no-lead (QFN) package. Pricing starts at $9.25 each in 1000-unit quantities.

LINEAR TECHNOLOGY • www.linear.com

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