FM31256-GTR Datasheet, PDF (7/27 Pages) Cypress Semiconductor – Integrated Processor Companion with Memory

Backup Power
The real-time clock/calendar is intended to be
permanently powered. When the primary system
power fails, the voltage on the V_DD pin will drop.
When V_DD is less 2.5V, the RTC (and event counters)
will switch to the backup power supply on V_BAK. The
clock operates at extremely low current in order to
maximize battery or capacitor life. However, an
advantage of combining a clock function with FRAM
memory is that data is not lost regardless of the
backup power source.
The I_BAK current varies with temperature and voltage
(see DC parametric table). The following graph
shows I_BAK as a function of V_BAK. These curves are
useful for calculating backup time when a capacitor
is used as the V_BAK source.
Figure 7. I_BAK vs. V_BAK Voltage
The minimum V_BAK voltage varies linearly with
temperature. The user can expect the minimum V_BAK
voltage to be 1.23V at +85°C and 1.90V at -40°C.
The tested limit is 1.55V at +25°C. The minimum
V_BAK voltage has been characterized at -40°C and
+85°C but is not 100% tested.
FM31256/3164 Processor Companion
Trickle Charger
To facilitate capacitor backup the V_BAK pin can
optionally provide a trickle charge current. When the
VBC bit, register 0Bh bit 2, is set to 1 the V_BAK pin
will source approximately 15 µA until V_BAK reaches
V_DD or 3.75V whichever is less. In 3V systems, this
charges the capacitor to V_DD without an external
diode and resistor charger. In 5V systems, it provides
the same convenience and also prevents the user from
exceeding the V_BAK maximum voltage specification.
In the case where no battery is used, the V_BAK pin
should be tied according to the following conditions:
For 3.3V systems, V_BAK should be tied to V_DD.
This assumes V_DD does not exceed 3.75V.
For 5V systems, attach a 1 µF capacitor to V_BAK
and turn the trickle charger on. The V_BAK pin
will charge to the internal backup voltage which
regulates itself to about 3.6V. V_BAK should not
be tied to 5V since the V_BAK (max) specification
will be exceeded. A 1 µF capacitor will keep
the companion functions working for about 1.5
second.
Although V_BAK may be connected to V_SS, this is not
recommended if the companion is used. None of the
companion functions will operate below about 2.5V
 Note: systems using lithium batteries should clear
the VBC bit to 0 to prevent battery charging. The
V_BAK circuitry includes an internal 1 K series
resistor as a safety element.
Figure 8. V_BAK (min.) vs. Temperature
Document Number: 001-86391 Rev. *B
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