Abstract

The Analog real-time clock (RTC) family includes several devices with integrated trickle-charging circuitry. The trickle charger charges a secondary battery or supercapacitor (supercap). The supercap maintains the operation of the clock when the primary battery runs out. The energy stored in the capacitor maintains the clock operation for a period determined by several factors. This design solution discusses how to calculate the backup time based on the supercap size.

Introduction

As more battery powered devices take over our daily lives, manufacturers are always looking for ways to extend the battery life to minimize the "low-battery" anxiety most people feel. This uneasiness has become so pervasive that there is now a phobia associated with it. Nomophobia (short for 'no mobile phone phobia') is the anxiety caused by a mobile phone running out of battery. While the term was first used to describe the anxiety associated with not having a working cell phone, it is now easily applied to all battery devices. How about your fitness watch running low on battery during your long run? How will you know the distance, pace, and heart-rate data if you run out of battery before you reach your destination? Even worse, what if your run is on trails, you have no cell coverage, and your satellite messenger battery is running low?

To extend battery life, manufacturers use the latest low power devices, which draw minimal current from the battery. Analog Devices' nanoPower Technology^TM was developed with the singular focus of minimizing quiescent current. Practically, any part where the current consumption in the standby or active mode is less than one microamp is considered a nanoPower device. Examples of nanoPower products include voltage regulators, operational amplifiers, supervisors, real-time clocks, and microcontrollers. Building the system with nanoPower devices ensures it has the longest battery life and minimizes the dreaded "low-battery" anxiety.

Let us look at the example of real-time clocks. Real-time clock (RTC) ICs keep track of time in electronic circuits. Maintaining accurate time is critical, especially under periods of severe system stress or when the power of the main device is off. During these times, RTCs must provide robust performance and often draw power from an auxiliary battery or supercapacitor. As expected, power consumption is a key factor in most RTC designs. Keeping the RTC powered after the primary battery runs out allows the end-product to maintain time and critical data.

Trickle-Charger Operation

Many RTCs include a trickle charger to charge a secondary battery or supercapacitor. This secondary battery, or supercap, powers the RTC when the primary battery power source runs out of juice. The energy stored in the supercap serves as the backup source to maintain clock operation for a period, which is determined by the amount of charge stored in the supercap.

Figure 2 shows the trickle charging circuit of the MAX31341B/C, low-current RTC with power management. The D_MODE[1:0] bits in the Pwr_mgmt_reg (0x56) register and D_TRICKLE[3:0] bits in the Trickle_reg (0x57) enable the trickle charger and determine the charging path, including the resistor value in that path. Keep in mind that other RTCs may have a different charging path and register configuration. So, consult the appropriate data sheet.

To calculate the amount of time the RTC runs, we need additional info from the data sheet. Figure 3 includes the relevant data needed. The minimum timekeeping voltage is given as 1.0V. This is the voltage where the oscillator still functions and allows the RTC to continue keeping time. The maximum time keeping current is 390nA if the operating voltage is 3.6V. It is somewhat lower at lower operating voltages.

Charging the Supercapacitor

What does this all mean?

Let us assume that a system power supply of 3.0V is applied to V_CC, the trickle charger is enabled, and the path with the 3K resistor with the Schottky diode is turned on. The maximum charging current is calculated as follows:

I_MAX = (V – V_D – V_SD - V_BAT)/R

where,

V_D = Diode voltage drop
V_SD = Schottky diode voltage drop
V_BAT = Voltage of the supercap being charged
R = Resistance selected in the charging path

As the battery/supercap charges, the battery voltage increases, and the voltage across the charging path decreases. Therefore, the charging current also decreases.

Using the above equation, it is seen that the maximum charging current can be calculated as follows. In this example, we assume the switch in parallel with the diode is closed (V_D = 0V), the supercap is fully discharged (V_BAT=0), and the Schottky voltage drop is 0.2V:

I_MAX = (V_CC – V_D – V_SD-V_BAT)/R
= (3.0V – 0.2V – 0V)/R
˜ (3.0V – 0.2V)/3kO
˜ 0.93mA

The maximum charging current depends on the supercap selected. Select the appropriate path diode and resistor based on the maximum current required for the capacitor charging.

Calculating Backup Time

So, how long does this RTC stay operational given, for example, a 0.2F supercap? The discharge time is given by:

T = -ln(V_BACKUPMIN/V_BACKUPMAX)[(V_BACKUPMAX/I_BACKUPMAX) × C]

From the MAX31341B/C data sheet (Table 1), we can get the minimum oscillator operating voltage as well as the maximum V_BACKUP current (I_BACKUPMAX). In this example, V_BACKUPMAX is 3.0V and I_BACKUPMAX is 370nA. Assuming the capacitor value is 0.2F and is charged to 3.0V, the I_BACKUPMAX is 370nA, and the minimum oscillator operating voltage is 1.0V, the backup time is calculated as:

T = -ln(V_BACKUPMIN/V_BACKUPMAX)[(V_BACKUPMAX/I_BACKUPMAX) × C]
T = -ln(1.0/3.0)[(3.0/370x10-9) × 0.2]
T = 1,781,533s or 494.9hrs

It should be clear by now that using an RTC with the lowest timekeeping current allows for the longest operating time.

A standard RTC drawing 600nA, while technically still considered a nanoPower device, reduces the time from 494hrs to 305hrs. Figure 3 shows a typical implementation of the real-time clock with the supercap or rechargeable battery connected to the trickle charger.

Conclusion

Our continued reliance on battery-powered devices has come at a cost – an increased level of anxiety as to whether our device battery will run out mid-use. To help alleviate this anxiety, or Nomophobia, select nanoPower devices such as the MAX31341B/C RTC, which has one of the lowest timekeeping currents of any RTC on the market.

Learn more:

1. MAX31341B Low-Current, Real-Time Clock with I²C Interface and Power Management
2. MAX31341C Low-Current, Real-Time Clock with I²C Interface and Power Management
3. Design Considerations for Maxim Real-Time Clocks
4. Battery Life Calculator for RTCs