SRAM Backup

Static RAM (SRAM) has always been attractive to engineers because of its speed, low cost, reliability, freedom from refresh, and unlimited number of read/write cycles. The major drawback has always been its volatility. Remove power, and all data is lost.

Solid state, thin film, EnerChip™ rechargeable batteries by Cymbet have emerged as a new battery backup power source for SRAM data. EnerChp batteries provide a combination of the best characteristics of two traditional power backup technologies, namely lithium coin cell batteries and super capacitors. EnerChips provide a flat output voltage of 3.8V and hold their charge for years like a lithium coin cell. EnerChips are rechargeable and like a super capacitor, have a very high number of charge and discharge cycles. The Cymbet EnerChip is specifically designed to provide back-up power to widely available SRAMS such as:

• Texas Instruments BQ
• Cypress CY
• Dallas/Maxim Max
• Samsung K
• Micron MT
• Renesas R1

Recharge is simple from a regulated 4.10 V source, with no external current limiting components. Charge from zero to 80% of full capacity is 30 minutes. Built-in resistance makes the devices short-circuit proof. EnerChips™ are much smaller than either coin cell batteries or super capacitors. Rugged solid state construction allows them to be automatically board inserted and lead-free reflow soldered. Currently available SRAM backup times before recharge range from a few hours to several weeks, depending on the battery chip capacity and SRAM current drain.

SRAM Backup Guidelines

In view of the limitations of other technologies, SRAM is still the high speed memory of choice, and design engineers are still faced with the need to come up with creative backup solutions for applications where that memory needs to retain data in the absence of system power. The following are some design guidelines: Electrically isolate the SRAM in the event of system power loss. This will limit the retention current drain to the SRAM, so that the backup energy source does not have to power the rest of the circuit.

Minimize the size of the SRAM that has to be backed up, since data retention current is approximately proportional to the number of cells. Store static program data in Flash or EEPROM. Locate non-essential operating data in SRAM that is not backed up.

Select a low power CMOS SRAM which is rated for low power consumption and low data retention current with battery backup power in mind. Such SRAM is aimed at mobile applications. Data retention current may be under 1 µA for a 16 Mbit chip.

Determine the true SRAM data retention current. Many SRAM manufacturers de-rate the typical retention current by a factor of 10 and publish the worst case retention current.

Determine the true required backup period. This may be hours or days to satisfy most applications. In most of the civilized world, over 99% of power outages are less than 1 hour in duration. The backup period in hours will be the capacity of the backup energy source in µAh divided by the expected data retention current in µA.

Determine the cost of losing data so as not to over-design the solution. Losing financial data from an electronic cash register could be costly; however, losing radio station presents from a car radio would not be.

Selecting the best SRAM Backup Power Solution

The best backup power solution for SRAM will depend on the application, in particular the required duration of the backup. In view of their unique characteristics, EnerChip solid state, thin film, rechargeable batteries make ideal short-term SRAM backup.