LE AVAILAB Application Note 82 Using the Dallas Trickle Charge Timekeeper DESCRIPTION The Dallas Semiconductor/Maxim real-time clock (RTC) family contains a number of parts within an integrated trickle-charging circuit. This application note describes the operation of the DS1302 trickle charger. Most of the data in this note can be applied to other Dallas RTC trickle chargers, with a few circuit-specific changes. Figure 1. DS1302 PROGRAMMABLE TRICKLE CHARGER 1 of 9 052402 AN82 TRICKLE CHARGER The trickle charge circuit is shown in Figure 1 along with the trickle charge register. To enable the trickle charger, the desired path through the circuit must be selected and the appropriate pattern written to the trickle charge register. The trickle charge select (TCS) bits (bits 4 to 7) control the selection of the trickle charger. In order to prevent accidental enabling, only a pattern of 1010 enables the trickle charger. All other patterns disable the trickle charger. The DS1302 powers up with the trickle charger disabled. The diode select (DS) bits (bits 2 to 3) select whether one diode or two diodes are connected between VCC2 and VCC1 . If DS is 01, one diode is selected or if DS is 10, two diodes are selected. If DS is 00 or 11 the trickle charger is disabled independently of TCS. The RS bits (bits 0 to 1) select the resistor that is connected between VCC2 and VCC1 . The resistor selected by the resistor select (RS) bits is as follows: RS BITS 00 01 10 11 RESISTOR None R1 R2 R3 TYPICAL VALUE None 2kΩ 4kΩ 8kΩ If RS is 00 the trickle charger is disabled independently of TCS. The user determines diode and resistor selection according to the maximum current desired for battery or super cap charging. The maximum charging current can be calculated as illustrated in the following example. Assume that a system power supply of 5V is applied to VCC2 a and a super cap is connected to VCC1 . Also, assume that the trickle charger has been enabled with one diode and resistor R1 between VCC2 and VCC1 . The maximum current IMAX would, therefore, be calculated as follows: IMAX = (5.0V - diode drop) / R1 ~(5.0V–0.7V) / 2kΩ ~2.2mA Obviously, as the super cap charges, the voltage drop between VCC2 and VCC1 decreased and, therefore, the charge current decreases. See curves in Trickle Charge Characteristics. POWER CONTROL The DS1302 can be powered in several different ways. The first method, shown in Figure 2, illustrates the DS1302 being supplied by only one power supply. In Figure 2a, the power supply is connected to VCC2 (pin 1) and in Figure 2b the power supply is connected to VCC1 (pin 8). In each case, the unused power pin, VCC1 or VCC2 , is grounded. The second method, Figure 3, illustrates the DS1302 being backed up using a nonrechargeable battery connected to VCC1 . In these two cases the trickle charge circuit has been disabled. In the final case, Figure 4, the DS1302 is being backed up by connecting a super cap, Figure 4a, or a rechargeable battery, Figure 4b, to VCC1 . In this case, the trickle charge circuit has been enabled. 2 of 9 AN82 Figures 2a and 2b. SINGLE POWER SUPPLY OPTION Figure 3. NONRECHARGABLE BATTERY BACKUP Figures 4a and 4b. SUPER CAP OR RECHARGABLE BATTERY BACKUP 3 of 9 AN82 TRICKLE CHARGE CHARACTERISTICS Charging the Super Cap The maximum current, IMAX, required by the trickle charge circuit can be calculated by inserting the correct values selected in the trickle charge register into the following equation IMAX = (VCC2 - diode drop) / R Table 1 contains the values of IMAX for VCC2 values of 4.5V, 5.0V, and 5.5V; 1 diode drop and 2 diode drops; resistor values of 2000Ω, 4000Ω and 8000Ω. Also, the charging current can be modeled as a function of charge time. Both the super cap voltage and charging current as a function of time are represented in Figure 5. The equation to model the super cap voltage as a function of time is V(t) = VMAX [1 - e (-t / RC) ] where V(t) VMAX R C = = = = Super Cap Voltage (VCC2 - n Diode Drops), n = 1, 2 Internal Trickle-Charge Resistor Super Cap Capacitance The time needed to charge the super cap to 95% of VMAX is given in Table 2. Note that the time required to charge the super cap to 95% of the value of VMAX is independent of the value of VMAX. The equation, which models the charging current as a function of time, is given as I(t) = VMAX / R x e (-t / RC) where I(t) VMAX R C = = = = Charging Current (VCC2 - n Diode Drops), n = 1, 2 Internal Trickle-Charge Resistor Super Cap Capacitance 4 of 9 AN82 Discharging the Super Cap When modeling the DS1302 for the time to discharge the super cap, the DS1302 characterization data was used to observe that the ICC1T , timekeeping current through VCC1 , was linear. This implies that it is proper to represent the DS1302 as a resistive load, RL, through which the super cap is discharged. Using the data sheet spec of ICC1T max of 0.3µA at 2.0 VCC1 gives a value for RL of 6.7MΩ. Then the equation modeling the discharging of the super cap is given by V(t) = VMAX x e(-t / RLC) where V(t) VMAX RL C = = = = Super Cap Voltage (VCC2 - n Diode Drops), n = 1, 2 DS1302 Load Resistance Super Cap Capacitance The calculated values for the time required to discharge the super cap to 2V are given in Table 3 and a sample of the super cap voltage as a function of discharge time is given in Figure 6. Figure 7 shows the typical ICC1T current versus voltage at +25°C. 5 of 9 AN82 Table 1. CALCULATED VALUES OF IMAX VCC2 (V) 4.5 5.0 5.5 2000Ω 1 diode 2 diodes 1.90 1.55 2.15 1.80 2.40 2.05 4000Ω 1 diode 2 diodes 0.95 0.78 1.08 0.90 1.20 1.03 8000Ω 1 diode 2 diodes 0.48 0.39 0.54 0.45 0.60 0.51 UNITS mA mA mA Table 2. CHARGING TIME FOR SUPER CAP TO 95% OF V MAX CHARGE TIME Super Cap = 0.047 Super Cap = 0.47F Super Cap = 1.5F 2000Ω 4.7 46.9 149.8 4000Ω 9.4 93.9 299.6 8000Ω 18.8 187.7 599.2 UNITS minutes minutes minutes Table 3. SUPER CAP DISCHARGE TIME TO 2V VCC2 (V) 4.5 5.0 5.5 0.047F 1 diode 2 diodes 69.8 47.7 83.3 63.9 95.2 78.1 1 diode 698.3 832.8 952.5 0.47F 2 diodes 476.8 639.5 780.9 6 of 9 1 diode 2228.7 2657.9 3039.8 1.5F 2 diodes 1521.7 2040.9 2492.5 UNITS hours hours hours AN82 Charge Voltage (V) Figure 5. SUPER CAP CHARGING CHARACTERISTICS 7 of 9 AN82 Figure 6. SUPER CAP DISCHARGING CHARACTERISTICS 8 of 9 AN82 Figure 7. DS1302 AVERAGE ICC1T at +25°C 400 350 300 250 200 average 150 100 50 0 2 2.5 3 3.5 VCC1 (V) 9 of 9 4 4.5 5