Abstract: Most Dallas Semiconductor 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 a designer should consider when selecting a backup source.

Key Words : Battery, cell, super cap, supercap, rechargeable

Introduction

The first Dallas Semiconductor 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, Dallas has introduced additional RTCs with built-in trickle chargers. Changes that affect system requirements since the first RTCs were introduced include the shift to IR reflow in manufacturing and restrictions on transportation and disposal of lithium cells. The following paragraphs discuss backup techniques and the advantages and limitations of commonly used backup supply sources.

Backup Supply Operation

Early Dallas Semiconductor 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 Dallas 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

1. Primary lithium cells may be wave soldered as long as the cell temperature does not exceed +85°C. Cells may be placed in a holder or hand soldered after reflow (tabbed cells).

2. Ambient temperature during charging. The allowed ambient temperature during discharge may be higher.

3. Batteries may be placed in a holder or hand soldered after reflow (tabbed batteries).

4. Total backup time is dependent upon the depth of discharge between each charging cycle.