AN3060

Switchover voltage

Doc ID 16253 Rev 2

9/23

4

Switchover voltage

4.1

Does the switchover voltage depend on the battery level? Or

is it always a fixed value? What is the maximum battery

voltage for an RTC?

Figure 5.

Typical switchover circuits

communications to the host; it is write protected.

Switchover connects the backup supply - usually a battery, but sometimes a capacitor or

Fixed reference

Most ST serial real-time clocks use a fixed, precision reference for the switchover threshold.

Battery reference

Some older RTCs use the battery as a reference. One diode drop is applied, internal to the

RTC, to the battery pin, and that level determines the switchover threshold. Typically, the

diode drop is 0.5 V, but it can vary from 0.3 to 0.8 V. For brand new lithium coin cell

batteries, the voltage output may be as high as 3.5 V. (Once under load for a while, this will

drop to approximately 3.0 V and remain there for the useful life of the battery.) At low

temperatures, the diode drop will tend toward the minimum value, 0.3 V. This means that,

with new batteries and low temperatures, the switchover threshold may be as high as

3.5 - 0.3 = 3.2 V. The implication is that parts employing battery references may be

problematic in 3.3 V applications, and will be unsuitable for 3.0 V applications. In those

cases, users should choose parts with fixed references. For example, the M41T81S with

VCC

VSO

VBAT

VCC-INT

VCC

VSO

VBAT

VCC-INT

With fixed reference

With battery reference

AM04723v1