Real Time Clocks (RTC) Power Backup Solutions

Over 1 billion real time clocks (RTC) are being sold each year with the ability to keep time when power fails. Clock chip manufacturers have been steadily lowering operating currents, which are now approaching 200 nA. This allows the use of Cymbet EnerChip thin film, rechargeable batteries as a replacement for traditional, bulkier power sources, such as coin cell batteries and super capacitors. The EnerChip can provide power from a few hours to a few weeks and can be recharged thousands of times. They offer the engineer new cost-lowering possibilities, such as reduced size, Pb-free reflow tolerance and automatic pick & place assembly methods.

EnerChip is designed for a range of currently available RTCs including:

• TI BQ
• Maxim DS
• ST Micro M41S; M41T
• Epson RTC; RA; RX
• OKI Semi MIL
• Intersil ISL 12
• EM Microelectronics V30

Most real-time clock (RTC) chips include an input for a backup power source and a built-in power-sense circuit which detects power failures and automatically switches to the backup power source. This allows the Real Time Clock to maintain the current time and date in the absence of main system power. However, even though the clock continues to operate in the background, no clock data may be read or written in backup power mode. The typical specified backup current of modern clock chips is of the order of 500 nA with the timekeeping oscillator on.

Real Time Clock (RTC) Chip Backup Power Guidelines
Electrically isolate the clock chip in the event of system power loss, so that the backup energy source does not have to power the rest of the circuit. This will limit the required backup current.

Select a low power clock chip. Many clock chips are now on the market designed specifically with battery backup power in mind.

Determine the true backup current. The backup current specified by clock chip manufacturers is often the worst-case maximum. The typical current may lower by a factor of 2, doubling the backup time.

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.