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September 2011 Doc ID 5226 Rev 2 1/6
AN922
Application note
Using a Super Cap to back up the M41T56, M41T00
M41T11, M41T81, M41T94, and M41ST84W (16-pin)
Introduction
The M41T56, M41T00, M41T11, M41T81, M41T94, and M41ST84W (16-pin) real-time
clocks (RTCs) from STMicroelectronics are used by applications designers who need a
single chip device that offers fast SRAM storage and an integrated real-time clock (the
M41T00 and M41T81 provide the real-time clock only). Many of their designs switch in a
battery to maintain the data and keep the clock running when the external power supply falls
below specification (or is completely absent). When the battery is depleted, though, the
designer or user can be faced with the issues of replacement and disposal (see the
application note AN1011, “Battery technology used in NVRAM products from ST”).
This document describes a more maintenance-free way to sustain the data and clock in
systems that only experience short breaks in the power supply (on the order of days). A
Super Cap can be used as a type of secondary cell (a rechargeable battery) and can
therefore provide an alternative solution to using a primary cell. Figure 1 on page 2 (for the
M41T56) and Figure 2 on page 4 (for the M41T00, M41T11, M41T81, and M41T94 as well
as for the 16-pin M41ST84W) show two typical circuit arrangements. Since the Super Cap is
limited to a certain maximum charging current, a series-limiting resistor may also be
required (please consult the datasheet for the Super Cap).
In this document, the reliability, leakage current, and charging cycle limitations of the Super
Cap have not been taken into account. Please consult the datasheet of the Super Cap for
details.
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