ST AN922 Application note
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.
September 2011 Doc ID 5226 Rev 2 1/6
www.st.com
Calculating the values of the circuit components for the M41T56
The minimum battery voltage for this device is 2.5 V, while the maximum battery supply
voltage is 3.5 V. This gives the maximum delta voltage swing across the capacitor (1.0 V).
AN922
Note: Charging the capacitor above 3.5 V will result in a higher power-fail deselect voltage (V
trip point, and may cause inadvertent deselection of the device at nominal V
V
PFD
1.25 V
×=
BAT typ()
values.
CC
The voltage divider provides a bias on the transistor; the resistor divider is calculated
according to the ratio of V
CC
to V
. Limit the maximum voltage charge on the capacitor
BASE
using the formula:
V
BAT
V
–=
BASEVBE
Derive the maximum voltage as follows:
V
BASE
Maximum supply voltage is 3.5 V; VBE is typically 0.6 V, so the typical value of V
MaximumSupplyVoltage VBE+=
is
BASE
4.1 V.
Recommended starting values for R1 and R2 are R1= 22 kΩ and R2 = 100 kΩ (with
V
= 5 V). Since the battery current, “I
CC
,” is limited to a maximum value of 550 nA, the
BAT
capacitance and the duration of “power-out time” can be calculated using the formula:
ΔV
IC
------- -
×=
Δt
where I = 550 nA, ΔV = 1.0 V, C = capacitance is in “Farads,” and Δt = “power out time” is in
“seconds.”
PFD
)
Using a 100,000 µF capacitor, for example, the equation would be:
550nA 0.1F
Solving for Δt, the maximum power down time is about 181,818 seconds. This is just over
two days.
Figure 1. External connections to the M41T56
R1
V
BASE
R2
C
2/6 Doc ID 5226 Rev 2
×=
1.0V
------------
Δt
M41T56
V
V
BAT
V
SS
+5V
CC
AI02481
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