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Maxim > Design Support > Technical Documents > Application Notes > Real-Time Clocks > APP 3816
Keywords: battery, cell, super cap, supercap, rechargeable
APPLICATION NOTE 3816
Selecting a Backup Source for Real-Time Clocks
May 26, 2006
Abstract:
Most Maxim
real-time clocks (RTCs) include a supply input for a backup power source. This alternate
supply source allows the RTC to maintain the current time and date while the main power source is absent. This
application note discusses the various types of alternate supplies that can be used, as well as some of the criteria
that a designer should consider when selecting a backup source.
Introduction
Maxim's first real-time clocks (RTCs) were designed so that a backup source, such as a primary (nonrechargeable)
lithium coin cell, could be used as the backup supply. Since then, Maxim has introduced additional RTCs with built-
in trickle chargers. There are changes that affect system requirements since the first RTCs were introduced,
including the shift to IR reflow in manufacturing and restrictions on transportation and disposal of lithium cells. The
following paragraphs discuss battery backup techniques and the advantages and limitations of commonly used
backup supply sources.
Backup Supply Operation
Early Maxim RTCs had a relatively simple voltage-comparator circuit to monitor V
CC
and switch between the V
CC
and V
BAT
supplies. The DS1307, for example, uses a comparator and a voltage divider to switch to V
BAT
when V
CC
drops below approximately 1.25 times the voltage on V
BAT
. Other RTCs, such as the DS1305/DS1306, switch when
V
CC
drops below the V
BAT
input voltage. When using these devices, care must be taken to ensure that the voltage
on V
BAT
never rises high enough to cause the device to inadvertently switch over to V
BAT
while V
CC
is at the
normal operating voltage. An external charging circuit must limit the maximum charging voltage to prevent such an
occurrence. Newer Maxim RTCs, which are designed to allow operation whether V
CC
is above or below the voltage
on V
BAT
, use an internal bandgap voltage reference to determine when V
CC
is too low for normal operation.
The following table lists the common supply technologies used for backup power. The table lists key parameters
that affect selection. The paragraphs following the table discuss each technology and their advantages and
drawbacks.
Table 1. Common Backup Supply Sources and Key Selection Criteria
Technology
Operating
Temperature
(°C)
PC Board
Attachment
Self-
Discharge
Rate
Disposal/Transportation
Restrictions
Charging
Circuit/Cycles
Backup
Time
Primary
Lithium
-30 to +80 Wave solder¹ Low High N/A Long
Capacitor -40 to +85 SMT High Low Simple/unlimited Short
Rechargeable
(NiCd/NiMh)
0 to +40² Hand solder³ Medium Medium Simple/≈500 Short
Reflowable
ML
-20 to +60 SMT Low High
Voltage 12 - >
1000
Medium
4
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