AN404 F-RAM RTC Backup Supply (VBAK pin) and UL Compliance.pdf

F-RAM RTC Backup Supply (VBAK pin) and UL Compliance
Author: Harsha Medu
Associated Part Family: FM31256, FM3164, FM31278, FM31276,
FM31L278, FM31L276, FM33256B
Related Application Notes: click here
AN404 discusses the F-RAM RTC processor companion’s backup power sources, the internal charging circuit, and the
associated issues.
The FM31xx and FM33xx families are the Integrated Processor Companion devices that feature a real-time clock
(RTC). The RTC provides the date/time information for the system and operates on either VDD or VBAK power. A
battery or a capacitor is attached to the VBAK backup power-supply pin. This application note discusses the backup
power sources, the internal charging circuit, and the associated issues.
RTC Backup
The FM31xxx and FM33xxx RTCs are designed to operate over long periods of time without the main VDD power
supply. A backup power source can be provided by a non-rechargeable battery (lithium coin cell) or a large value
supercapacitor as shown in Figure 1. Depending on the size and capacity, a lithium coin cell can provide continuous
RTC operation for many years. When VDD is powered up, the RTC and other associated circuits operate from the VDD
power source. There is essentially no current drawn from the backup source. If VDD is powered down, the RTC is
automatically switched over to the VBAK power source.
The F-RAM RTC operates at very low current (<1 uA). A 3-V coin cell is typically used as the VBAK power source. The
expected life of a lithium cell in hours is calculated using the following equation:
Expected Life (in hours) = 1000 x Rated Battery
Capacity (mAh)/Load Current (uA)
For example, a BR1225 rated at 48 mAh with a continuous load current of 1 uA (system powered down) will last
48,000 hours or nearly five and a half years. If VDD is powered up for some time, then the battery will last longer
because there is no backup current when VDD is applied. During this time, the battery’s self-discharge characteristic
dominates the battery life.
Figure 1. Two RTC Backup Options
with RTC
with RTC
In applications for which a battery cannot be used (due to space constraints, environmental restrictions, or added
costs), a capacitor can be used as a backup power source. A large value capacitor, such as a supercapacitor, is
generally used. The F-RAM companion devices have an integrated charging circuit that is used with the
supercapacitor. This provides the user with cost savings, reduced board space, and convenience. The internal circuit
charges the supercapacitor when VDD is applied. No external components, such as protection diodes or current
limiting resistors, are needed. The circuit provides a constant current to deliver charge to the capacitor. When V DD is
powered down, the RTC and other backed-up circuits are powered from the supercapacitor. See Figure 2.
Document No. 001-87420 Rev. *B
F-RAM RTC Backup Supply (VBAK pin) and UL Compliance
Figure 2. Simplified Diagram of Power Switch and VBAK Trickle Charger
2.5 V
+3.3 V
or +5 V
0.1 F
backed up
The length of time (in hours) that the RTC will run from the supercapacitor is calculated using the following equation:
Capacity of supercap (F) x (VINITIAL – 2.0 V) / (0.0036 x
Load Current (uA))
In this equation:
VINITIAL is the capacitor voltage that the VBAK pin is charged to when VDD is powered down.
2.0 V is the minimum operating RTC voltage as specified in the datasheet.
The factor 0.0036 converts seconds to hours and amps to microamps.
A 0.1-F capacitor provides approximately two days of backup and a 1-F supercapacitor provides 20 days (this
assumes VINITIAL is charged to the maximum voltage allowed for VBAK). The VBC bit in the Companion Control
Register (Register 0B, bit 2 for FM31256, FM3164, FM31278, FM31276, FM31L278, FM31L276 and Register 18,
bit 3 for FM33256B) must be set high to activate the charging circuit. The supercapacitor charging voltage on VBAK is
provided by an internal regulator. Typically, systems run from either 3.3 V or 5 V. In the case of a 5-V system, the
highest VBAK voltage is 3.75 V, the internally regulated voltage. In the case of a 3.3-V system, the highest VBAK is
reduced to 3.3 V by VDD.
The supercapacitor must be fully charged to get the maximum backup time during power down. The time (in hours) to
fully charge a discharged capacitor is calculated as:
Capacity of the supercapacitor (F) x (VCHARGED) / (0.0036
x Charge Current (uA))
In this equation:
VCHARGED is the maximum VBAK voltage. VCHARGED is 3.75 V for VDD = 5 V and 3.3 V for VDD = 3.3 V.
The factor 0.0036 converts seconds to hours and amps to microamps.
For a capacitor = 0.1 F, VDD = 5 V, and a charge current of 15 uA (charge current is 80 uA for FM31278, FM31276,
FM31L278, FM31L276, FM33256B and 15 uA for FM31256, FM3164), it takes almost 7 hours to fully charge the
supercapacitor. In the case of VDD = 3.3 V, it takes approximately 6 hours to fully charge the supercapacitor.
Note The FM31278 / FM31276 / FM31L278 / FM31L276 / FM33256B devices define a Fast Charge mode (FC bit –
register 0Bh, bit 5), which is capable of sourcing approximately 1 mA.
UL Compliance
Users of primary batteries (non rechargeable) need assurance that the charging circuit has low reverse current when
off and that it also has a protective resistor in series with the battery. This limits the charge current in the event of
malfunction in the charging circuit that can cause excessive battery current. The circuit diagram in Figure 3 shows
two 1.3-kΩ resistors in series with the P-ch current source. There are no other elements in the path to the VBAK pin.
The trickle charger current is provided by the P-ch shown as IBAKTC current and is activated only when the VBC bit is
set. The RTC includes an Oscillator and Timekeeping Block, which holds the time/day/date values. If VDD drops below
2.5 V, the RTC power is switched to the VBAK source. Under this condition, the Oscillator and Timekeeping Block pulls
no more than 1.4 uA (IBAK maximum at VBAK = 3.0 V), which is 100% tested at worst-case conditions.
When VDD is within its normal operating range, there is no current into or out of the VBAK pin, other than leakage
current. Cypress F-RAM expects IBAK to be < 1 nA under this condition.
Document No. 001-87420 Rev. *B
F-RAM RTC Backup Supply (VBAK pin) and UL Compliance
Note If a primary battery is used, the VBC bit should not be set (charger off). If the VBC bit is set (charger on), the
battery life may be reduced. The charge current provided by the FM31xxx/FM33xxx is low enough that it poses no
safety risk, such as excessive heat. However this condition should be avoided.
Figure 3. FM31xxx/FM33xxx Internal Circuits Relating to VDD and VBAK
AN404 describes the different backup source options for the F-RAM RTC processor companion. It explains the
internal charging circuit and associated issues to be taken care of while designing systems with the F-RAM RTC
processor companion.
Related Application Notes
You can refer to the following application notes for better understanding of the F-RAM Processor Companion devices.
AN407 - A Design Guide to I2C F-RAM Processor Companions – FM31278, FM31276, FM31L278, and
AN408 - A Design Guide to SPI F-RAM Processor Companion - FM33256B
AN400 - Generating a Power-Fail Interrupt using the F-RAM Processor Companion
AN401 - Charging Methods for the F-RAM RTC Backup Capacitor
AN402 - F-RAM RTC Oscillator Design Guide
Document No. 001-87420 Rev. *B
F-RAM RTC Backup Supply (VBAK pin) and UL Compliance
Document History
Document Title: AN404 – F-RAM RTC Backup Supply (VBAK pin) and UL Compliance
Document Number: 001-87420
Orig. of
Description of Change
New Spec.
Changed the title from “F-RAM RTC Backup and UL Compliance” to “F-RAM RTC
Backup Supply (VBAK pin) and UL Compliance”.
Added Fast Charge Mode.
Added Related Application Notes section.
Updated template
Document No. 001-87420 Rev. *B
F-RAM RTC Backup Supply (VBAK pin) and UL Compliance
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Document No. 001-87420 Rev. *B