Summary
Nearly all electronic products now include an RTC function, so longer RTC battery life is desirable.
Problem description
This case concerns an industrial product with an RTC function. The RTC power supply circuit is shown below. After six months on the market, the product indicated the RTC battery needed replacement, far shorter than the designed 5-year life.
Cause analysis
After the product returned to the company, we installed a new RTC battery and measured current with a high-precision multimeter. The RTC operating current reached about 100 uA, much higher than the 5 uA we designed for. Suspected causes included:
- Leakage through diode D3, allowing current to flow back into the system power when the device is powered off.
- RTC chip behavior. The original RTC was NXP-PCF8563P, which specifies about 0.25 uA in backup mode; the RTC was later replaced with a domestic RTC chip.
- Leakage elsewhere on the RTC power rail, such as capacitor leakage.
- Effect of resistor R71.
Using elimination, we first removed D3. The current only dropped by about 1 uA, so D3 leakage was not the main issue. Removing capacitors did not reduce the current, which remained about 100 uA.
When we replaced the RTC with NXP-PCF8563P, current returned to normal at about 4 uA. Reinstalling the domestic RTC but changing the series resistor to 100 Ohm also restored normal current around 4 uA.
This led to two questions:
- What series resistance is appropriate on the RTC battery supply?
- Why does inserting a 10 kOhm series resistor increase, rather than decrease, RTC current?
1. Appropriate series resistor value for the RTC supply
There is debate online about the resistor value on the RTC supply: some use 0 Ohm, others 1 kOhm, 10 kOhm, etc. The resistor's purpose is current limiting. Consider coin cells: manufacturers require that the battery must not be shorted under any condition to avoid rupture or explosion. Therefore, if the downstream circuit is shorted, the series resistor must limit current to below the battery's maximum continuous discharge current.
For a typical CR2025 coin cell, using a worst-case maximum continuous discharge current of 3 mA, the minimum resistor is:
R = V / I = 3 V / 3 mA = 1 kOhm.
The resistor can be larger than this value, but it must not be smaller.
2. Why does a 10 kOhm series resistor increase current?
First, understand some RTC chip characteristics:
- RTC chips have two operating modes: normal operating mode and backup (battery) mode. The supply currents can differ by a large factor, up to several hundred times.
- In each mode, the RTC chip can be approximated as a current source: for battery mode it may draw around 1 uA, while in normal mode it may draw around 200 uA.
- RTC chips have a wide operating voltage range, typically able to operate from about 1.5 V to 5.5 V.
Treat the RTC as a current source. With a 10 kOhm series resistor, if the current through the resistor is 100 uA, the voltage drop across the resistor is 1.0 V. If the coin cell is at 2.6 V, the RTC supply node would be about 1.6 V. If the battery voltage is lower, the RTC node voltage is even lower. Because the RTC needs roughly the same current to operate, it may respond by lowering its internal impedance to try to obtain sufficient current, which can result in a larger current draw. Some domestic RTC chips may not clearly switch between normal mode and backup mode when battery power is used, causing them to remain in a higher-current mode while on battery. This explanation is a reasoned hypothesis; it was not confirmed in vendor documentation.
Solution
To extend battery life, the main objective is to reduce current in the RTC circuit. Major losses in the loop are from the series resistor, diode leakage, the RTC chip itself, and capacitor leakage.
1. RTC battery-mode current
Most RTC manufacturers provide battery-mode currents in the range of a few hundred nA to about 1 uA. For estimation, assume 1 uA for the RTC chip in battery mode.
2. Diode leakage
Diode loss is mainly leakage current. Select a diode with as low leakage as possible. For example, the BAS70 series shows leakage versus temperature and voltage; conservatively, estimate about 1 uA leakage.
3. Capacitor leakage
Capacitor loss is primarily leakage current. For RTC battery use, a 100 nF ceramic capacitor is sufficient for filtering; estimate leakage around 0.5 uA.
4. Resistor loss
Estimated total current = diode leakage + RTC current + capacitor leakage = 1 uA + 1 uA + 0.5 uA = 2.5 uA. A practical choice is a 1 kOhm series resistor. With 2.5 uA through 1 kOhm, the voltage drop is:
V = I * R = 1 kOhm * 2.5 uA = 0.0025 V.
Power dissipated in the resistor:
P1 = V^2 / R = 0.0025 V * 0.0025 V / 1000 = 0.00625 uW.
RTC chip power consumption (approximate):
P2 = U * I = (3 V - 0.2 V - 0.0025 V) * 1 uA = 2.7975 uW.
Resistor loss fraction = P1 / P2 = 0.089%, so resistor loss is negligible.
5. Battery life estimate
Using a typical CR2025 coin cell with nominal capacity 150 mAh and annual self-discharge about 1%, estimate battery life as:
T = 150 mAh * 95% / (diode 1 uA + RTC 1 uA + capacitor 0.5 uA) = 57,000 hours ≈ 6.5 years.
6. Final implementation
For a typical CR2025 coin cell example, use a lower-leakage diode such as a BAS70 series device with leakage around 1 uA, and place a series resistor only on the battery to prevent battery short-circuit, sized to limit short-circuit current to about 3 mA.
Conclusion
This article addressed two RTC questions:
- Whether to place a series resistor on the RTC battery and what value to choose.
- Factors to consider when evaluating RTC battery life.
