White Paper Rolf Schütz Product Engineer at Micro Crystal Grenchen, Schweiz [email protected] Markus Hintermann Technical Marketing / Sales Manager at Micro Crystal [email protected] Date: December 2015 Headquarters: Micro Crystal AG Mühlestrasse 14 CH-2540 Grenchen Switzerland Tel. Fax Internet Email Revision N°: 1.0E 1/21 +41 32 655 82 82 +41 32 655 82 83 www.microcrystal.com [email protected] Micro Crystal Temperature compensated Real-Time Clock with back-up power supply White Paper Inhalt 1. Summary .......................................................................................................................................................... 3 2. Temperature compensated RTC Module RV-8803-C7 ................................................................................. 4 3. Power back-up with Supercap. ...................................................................................................................... 5 3.1. Super Capacitor ........................................................................................................................................ 5 3.1.1. Supercaps tested ................................................................................................................................ 6 3.1.2. Life time calculation ............................................................................................................................. 6 3.1.3. Self discharge ...................................................................................................................................... 7 3.2. Application diagram ................................................................................................................................. 8 3.3. VDD-Operation ............................................................................................................................................ 9 3.3.1. Maximal inrush current ........................................................................................................................ 9 3.3.2. Charging current of the Supercap ....................................................................................................... 9 3.3.3. Internal resistance of the Supercap .................................................................................................. 10 3.3.4. Dimensioning of R1 ........................................................................................................................... 11 3.3.5. Schottky-Diode .................................................................................................................................. 11 3.4. VBACKUP-Operation ................................................................................................................................... 12 3.4.1. Verification of the time tracking accuracy ......................................................................................... 13 3.4.2. VBACKUP discharge characteristic ....................................................................................................... 14 3.4.3. Leakage current of Supercaps .......................................................................................................... 15 3.4.4. Current consumption RV-8803-C7 ................................................................................................... 16 3.4.5. Leakage current of the Schottky-Diode ............................................................................................. 16 3.4.6. Back-up time calculation ................................................................................................................... 17 3.4.7. Buffered back-up time ....................................................................................................................... 18 4. Conclusion ..................................................................................................................................................... 20 5. Document version ......................................................................................................................................... 21 2/21 Micro Crystal Temperature compensated Real-Time Clock with back-up power supply White Paper 1. Summary The White Paper describes the combination of the Real-Time Clock Module RV-8803-C7 with an eco-friendly Supercap*. The low power consumption of the RTC Module RV-8803-C7 allows for the first time to keep the actual time over extended periods by using a simple Supercap back-up power supply. This eco-friendly solution permits the user to continue tracking time with high precision even during power down condition. The back-up circuit requires, beside the Supercap and the RTC Module RV-8803-C7, just one Schottky-diode plus a resistor to limit the inrush current during charging. *) Capacitors with ultra-high capacitance typically in the range of 20’000 µF to several F are commonly referred to as Supercap, Ultracap, Gold Cap, double-layer cap, multilayer cap and there more. In this paper they are generally referred to as Supercaps. 3/21 Micro Crystal Temperature compensated Real-Time Clock with back-up power supply White Paper 2. Temperature compensated RTC Module RV-8803-C7 The temperature compensated RTC Module RV-8803-C7 from Micro Crystal currently offers the highest accuracy of ±3 ppm across the entire industrial temperature range of -40 to +85°C. This corresponds to a maximum deviation of ±0.26 seconds per day with a current consumption of merely 240 nA at 3 V. This exceptionally low power consumption, in parallel with the full functionality of temperature sensing and frequency compensation, even down to a supply voltage of 1.5 V, prolongs the system power autonomy considerably. Asides from the lowest power consumption in combination with the highest accuracy, it also features the smallest SMD ceramic package with remarkable dimensions of only 3.2 x 1.5 x 0.8 mm. It allows the device to be used in a wide field of applications where accurate timing is required, also in power-down mode. Examples: Portable medical systems Automotive applications POS-terminals Utility metering Embedded modules Data loggers White Goods Key parameters of the RTC Module RV-8803-C7: Ultra-miniature ceramic SMD package: 3.2 x 1.5 x 0.8 mm Highest accuracy (±3 ppm) over the whole industrial temperature range of -40 to +85°C Wide supply voltage range: 1.5 to 5.5 V Lowest power consumption of just 240 nA / 3 V I²C-bus interface AEC-Q200 qualified Time accuracy as function of ambient temperature: 20 T0 = 25°C (± 5°C) Δ t/t [ppm] 0 -20 4096 Hz to 64 Hz -40 1 Hz and Clock / Calendar -60 -80 Xtal 32.768 kHz 2 -0.035 * (T-T0) ppm (±10%) -100 -120 -140 -160 -180 -50 -40 -30 -20 -10 0 10 20 30 40 Temperature [°C] 50 60 70 80 90 4/21 Micro Crystal Temperature compensated Real-Time Clock with back-up power supply White Paper 3. Power back-up with Supercap. The Supercap is the predestined solution for the secondary supply of RTC applications, since it features an equivalent performance of a battery. 3.1. Super Capacitor Keep in mind: Supercaps are polarized! Parameter: Large range of capacities available 0.022 F to 70 F (and larger) Connection in parallel or serial possible to enlarge capacity or voltage range Nominal voltage 5.5 V Temperature range up to 85°C Available for reflow and wave soldering Three package types: coin-cell, stacked coin-cell and fitted with wire leads Advantages: Ideal as battery replacement Fast charging and discharging possible Unlimited number of charge / discharge cycles No chemical leaking or outgassing GREEN product, RoHS compliant , no recycling limitations No safety measures during charging necessary Operation and full performance also at very low temperature (sub-freezing) Maintenance free Short coming: Linear discharge voltage characteristic (i = constant) prevents to use the full energy stored Comparison of Supercap with Battery: Eco friendly Number of cycles for charging / discharging Temperature range Capacity Supercap Good Battery Bad Unlimited Very limited Full range Equivalent Limited Good 5/21 Micro Crystal Temperature compensated Real-Time Clock with back-up power supply White Paper 3.1.1. Supercaps tested When selecting Supercaps, check for devices with low leakage current. Supercaps for high current applications typically have high leakage currents and must therefore be avoided. Evaluated Supercaps from Panasonic (Gold Capacitors): 0.1 F EECS0HD104H (5.5 V, Series SD) 0.47 F EECS5R5H474 (5.5 V, Series SG) 1.0 F EECS5R5V105 (5.5 V, Series SG) Tolerance Unit cost in RESR Typ. RISO Size of Package $ @ 1kHz @ +25°C (L x B x H) (25+) capacity(*) 0.080 to 11.5 x 10.5 0.1 F ≤ 75 Ω 32 MΩ horizontal 1.30 0.180 F x 5.5 -25°C to 0.376 to 20.5 x 19.5 5.5 V 0.47 F ≤ 30 Ω 24 MΩ horizontal 1.99 70°C 1.41 F x 6.5 0.80 to 19.0 x 5.5 x 1.0 F ≤ 30 Ω 13 MΩ vertical 2.01 1.80 F 21.0 (*) The capacity tolerances are typical (-20/+80%). The listed 0.47 F-type, however, is specified with (-20/+300%)! Temperature Max. operating voltage Capacity Characteristics at low temperatures Capacity at -25°C: ±30% of the nominal value referenced at +20°C Internal resistance RESR at -25°C: ≤5x the nominal value referenced at +20°C Performance after 1000 hours 5.5 V, +70°C Capacity change: ±30% RESR: ≤ 4x larger Storage capability after 1000 hours at +70°C not charged The capacitor maintains the specified performance 3.1.2. Life time calculation According to the equation from Arrhenius (doubling the lifetime for lowering the temperature by 10K): LX = LSpec * 2 T0 - TA 10 Example: Life time of the capacitor at +30°C and charged to 5.5V: 70 – 30 10 L30 = 1000 * 2 → 16‘000 h * 1 day 24 h = 16‘ 000 hours = 667 days(*) Parameter Comments LSpec = Life time as specified 1000 h at 5.5 V, +70°C(*) LX = Life time target T0 = Max ambient temperature +70°C TA = Ambient temperature of capacitor (*) The lower the supply voltage the longer the life time: (e.g. 3000 h at 4 V, +70°C) 6/21 Micro Crystal Temperature compensated Real-Time Clock with back-up power supply White Paper 3.1.3. Self discharge With short charging times (e.g. 10 minutes) the capacitor is not fully charged due to variations of the internal isolation resistance and leakage currents*. Therefore the inital voltage drop is larger. Self discharge: Function of discharge versus charging time, start condition 5 V: *) Source: Panasonic “Gold Capacitors ABC0000PE103_TechnGuide_Oct 1st 2014” 7/21 Micro Crystal Temperature compensated Real-Time Clock with back-up power supply White Paper 3.2. Application diagram During VDD-Operation the Supercap C1 will be charged through the Schottky-diode D1. The I²C-bus interface (SDA, ̅̅̅̅̅) are accessible by the MCU. SCL) and the interrupt signal (INT In VBACKUP-Operation, VDD = 0 V, the RTC Module is only supplied by the Capacitor C1. The I²C interface and interrupt are not accessible, since the MCU is in power down mode. The RTC Module is fully functioning as long as VBACKUP is ≥ 1.5 V. For longest autonomy the CLKOUT signal must be switched off. (CLKOE = 0 V). Power consumption derives now only from the operating current of the RTC Module, and the leakage currents of C1 and D1. As soon as VDD is switched on again the RTC can be accessed and the Supercap C1 gets recharged. Circuit diagram of the RTC Module RV-8803-C7 with back-up Supercap: D1 BAS70 VBACKUP C1 EECS0HD104H R1 100 Ω 0.1 F C2 100 nF R2 10 kΩ VDD_8803 RTC RV-8803-C7 C1 D1 R1 C2 R2, R3, R4 CLKOUT VSS R3 10 kΩ R4 10 kΩ INT INT SDA SDA SCL SCL CLKOUT VDD VDD MCU PIC 18LF4620 CLKOE EVI VDD nc VSS Supercap e.g. EECS0HD104H (0.1 F, 5.5 V) Schottky-Diode BAS70 100 Ω Resistor to limit the initial charging current 100 nF capacitor for decoupling close to the RTC package The I²C-bus connections SCL and SDA, and the ̅̅̅̅̅ INT output have open drain configuration and require a pull-up resistor of typically 10 kΩ (active only during VDD-Operation) Not used to minimize the current consumption (CLKOE connected to VSS) 8/21 Micro Crystal Temperature compensated Real-Time Clock with back-up power supply White Paper 3.3. VDD-Operation The voltages VI and VO at the inputs and outputs of the RV-8803-C7 must not exceed VBACKUP at pin VDD_8803 by more than 0.3 V. Therefore a Schottky-diode with a low forward voltage drop of VF = 0.3 V at 200 µA and +25°C is used. 3.3.1. Maximal inrush current The Supercap does not per se require a dropping resistor. The current is just limited by the internal resistance R ESR of the capacitor. The determined conditions for the largest current IC1max (Worst-Case) are: RESR of the Supercap, at maximum Voltage VDD = 5.5 V and maximum ambient temperature T A = 70°C. If needed R1 can be used for further limiting the inrush current IC1max (RESR + R1). Resistor R1 may be necessary to limit the current: To protect the Schottky-diode. The max current for the BAS70 (D1) Schottky-Diode IFmax = 70 mA If the DC/DC-Converter or voltage regulator of the main supply is not capable of delivering sufficient current. 3.3.2. Charging current of the Supercap The charging current depends on the maximum possible voltage VBACKUP and the forward voltage VF of the Schottky-diode. The charging current is the sum of the current for charging the ideal capacitor and its leaking current though the isolation resistance RISO. The current will therefore never be zero. The charging current after 24 hours of a 0.1 F Supercap at 25°C will be in the order of IC1= 0.9 µA, the VF will drop to 0.2 V. Expected charging current IC1 after long time charging with VDD = 5.5 V, 20°C: C1 0.1 F 0.47 F 1.0 F Charging current IC1 After 24 hours 0.9 µA 1.8 µA 3 µA After 100 hours 0.3 µA 0.5 µA 0.8 µA 9/21 Micro Crystal Temperature compensated Real-Time Clock with back-up power supply White Paper 3.3.3. Internal resistance of the Supercap The internal resistance of the Supercap influences: The inrush current The charging and discharging time of C1 after VDD is turned on The voltage drop during VBACKUP-Operation The estimation of the max inrush current depends on the smallest internal resistance RESR of the fresh (not aged) Supercap. Suppliers specify it as ESR = Equivalent Series Resistance, measured at 1 kHz. Smallest internal resistor RESR: RESR TA = -20°C TA = +25°C 0.1 F 110 Ω 30 Ω 0.47 F 40 Ω 10 Ω 1.0 F 40 Ω 10 Ω Source: Panasonic “Gold Capacitors ABC0000PE103_TechnGuide_Oct 1st 2014” C1 TA = +70°C 25 Ω 9Ω 9Ω To estimate the longest charging time of the uncharged Supercap C1 after VDD is applied, the internal DCresistance RDC ≈ RESR and the subsequently calculated serial resistor R1 are relevant. The shortest discharge time is not of importance since it would only apply if VBACKUP is shorted. For the time constant T: T = (RDC + R1) * C1 Longest charging time t expressed as a factor of T: C1 RDC R1 0.1 F 0.47 F 1.0 F 75 Ω 30 Ω 30 Ω 100 Ω 100 Ω 100 Ω t=T (VBACKUP ≈ 63%) 18 s 61 s 130 s t = 5*T (VBACKUP > 99%) 88 s 306 s 650 s The largest voltage drop across the Supercap depends on its internal resistance and the limiting resistor R1 during VBACKUP-Operation. Relevant are also the largest load current and largest internal resistance RDC ≈ RESR, the max current of the RTC Module IDD_8803max and the maximum leakage current ID1_Lmax of the Schottky-diode. Largest voltage drop VC1max: VC1max = (Imax ) * (RDCmax + R1) VC1max = (IDD_8803max + ID1_max ) * (RDCmax + R1) With C1 = 0.1 F: VC1max = (350 nA + 110 nA) * (75 Ω + 100 Ω) = 0.08 mV With C1 = 0.47 F and 1.0 F: VC1max = (350 nA + 110 nA) * (30 Ω + 100 Ω) = 0.06 mV This shows the max voltage drop VC1max is negligible and therefore will not be considered in the following calculations (VC1 = 0 V). 10/21 Micro Crystal Temperature compensated Real-Time Clock with back-up power supply White Paper 3.3.4. Dimensioning of R1 Worst-case: VDD = 5.5 V, TA = 70°C Schottky BAS70: At IFmax = 70 mA and TA = 70°C: VF = 0.9 V Max inrush current: IC1max = 70 mA IC1max = VDD – VF RESR + R1 Necessary limiting resistor R1: R1 = VDD – VF − RESR IC1max EECS0HD104H (0.1 F, 5.5 V, Series SD): 5.5 V – 0.9 V − 25 Ω = 41 Ω 70 mA R1 = R1 selected = 100 Ω EECS5R5H474 (0.47 F, 5.5V, Series SG) and EECS5R5V105 (1.0 F, 5.5 V, Series SG): R1 = 5.5 V – 0.9 V − 9 Ω = 57 Ω 70 mA R1 selected, also = 100 Ω 3.3.5. Schottky-Diode BAS70 is a Schottky-diode with very small leakage current and the preferred small forward voltage drop VF. When for example the charging current for the 0.1F Supercap has dropped after 24 hours to 0.9 µA, the resulting VF is then reduced to just 0.2 V. Schottky-Diode BAS70 VF vs. IF: BAS70 10000 IF in uA 1000 100 VF @ +70°C VF @ +25°C 10 VF @ -20°C 1 0 0.1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 VF in V 11/21 Micro Crystal Temperature compensated Real-Time Clock with back-up power supply White Paper 3.4. VBACKUP-Operation As soon as VDD is switched off (goes to 0 V) the Supercap is taking over automatically and supplies the power to the RTC Module RV-8803-C7. The starting value of VBACKUP depends on the originally applied voltage VDD and the voltage drop across the Schottky-diode VF. VBACKUP however depends also on the charging level of the capacitor and the ambient temperature. During VBACKUP-Operation the Supercap is discharged by the sum of the 3 currents: operating current of the RTC Module RV-8803-C7 and the two leakage currents of the capacitor and of the Schottky-diode. To monitor the discharge voltage it is advised to use a high impedance meter such as an Agilent 3458A multimeter. At the 10V range it features an internal resistance of >10 GΩ. This adds a negligible discharge current of <0.55 nA at 5.5 V. VBACKUP-Operation: Agilent 3458A > 10 GΩ V C1 EECS0HD104H R1 100 Ω 0.1 F C2 100 nF RDC RESR RISO R2 10 kΩ VDD_8803 RTC RV-8803-C7 VSS R3 10 kΩ INT SDA SDA SCL SCL CLKOUT VDD = 0V R4 10 kΩ INT VDD VDD MCU PIC 18LF4620 CLKOE EVI Hauptspeisung Main supply ausgeschaltet switched off D1 BAS70 VBACKUP nc VSS 12/21 Micro Crystal Temperature compensated Real-Time Clock with back-up power supply White Paper 3.4.1. Verification of the time tracking accuracy The verification of the correct time of the RTC Module cannot be tested with the above circuit during VBACKUPOperation (time keeping). VDD must be switched on to activate the MCU for I²C-bus communication and interrupt. Procedure: 1. VDD-Operation: a. Charging the Supercap b. Start measurement of VBACKUP with Agilent 3458A (>10 GΩ) and recording with e.g. VEE Pro software. (VBACKUP and reference-time) c. RTC initialization via I²C-bus interface (time, date and setting all flags to 0) 2. VDD is switched off: a. Circuitry is in VBACKUP-Operation (time keeping) 3. If e.g. VBACKUP ≤ 1.5 V: a. Turn on VDD b. Read RTC-time and the flags F1V and F2V via I²C-bus c. If the flags F1V and F2V are still at 0, the RTC Module was operating continuously during the backup time. The read RTC-time can now be compared with the reference time. Hint: The circuit can be adapted such that the RTC-time can be monitored with little impact to the back-up time. 1. Increase pull-up resistor R2 to 100 kΩ. Connect it between the RTC-Pin INT and the cathode of the Schottky-diode (Pull-up to VBACKUP) 2. Cut the INT connection to the PIC, since the input impedance of the PIC pin is only 25kΩ 3. Program One-minute-interrupt (pulse duration 15.6 ms) 12 4. With the help of a high impedance probe (e.g. HIP101, input impedance: 10 Ω 0.1 pF input capacitance typ. 0.3 pA, max. 1 pA) the interrupt signal can be used to measure the period with a Timer/Counter. The average current consumption of this circuitry with V BACKUP = 3 V and 25°C is increased by only 9 nA (derived from the leakage current and the 15.6 ms pulse on the RTC-pin INT). 13/21 Micro Crystal Temperature compensated Real-Time Clock with back-up power supply White Paper 3.4.2. VBACKUP discharge characteristic The application circuit includes also the test setup for measuring the discharging characteristic of the capacitor at VBACKUP. VBACKUP discharging behavior with 0.1 F, 25°C, charging condition VDD = 3.0 V, charging times 10 min and 24 hours: After a long charging time (24 h) a very small charging current of 1 µA is present. (This is almost independent of the size of C1 and VDD). Across the Schottky-diode we reach the low VF. After a short charging time (10 min) the Supercap is not fully charged (inhomogeneity of double layer capacitor structure) a significantly larger charging current is flowing through the Schottky-diode (up to some mA). The increased forward voltage VF and the missing part of the charge voltage of C1 can be replaced by a constant term in the equation. In addition to the forward voltage VF the correction voltage VK must be subtracted from VDD. For the starting voltage V0 in VBACKUP-Operation results: V0 = VDD – VF – VK Forward voltage VF in function of the charging time: Charging time 24 hours 10 minutes TA = -20°C VF TA = +25°C TA = +70°C 0.25 V 0.2 V 0.1 V Correction voltage VK in function of the charging time: Charging time VK 24 hours 10 minutes 0V 0.13 V 14/21 Micro Crystal Temperature compensated Real-Time Clock with back-up power supply White Paper 3.4.3. Leakage current of Supercaps It is beneficial to determine the isolation resistance RISO of the Supercap. This way the average leakage current IC1_L can be calculated for any average back-up supply voltage ∅VBACKUP. The isolation resistance RISO decreases with rising temperature. It can be calculated with the help of the suppliers’ specified discharge characteristics, or based on bench tested parameters: t2 - t1 RISO = U C1 * ln ( 2 ) U1 Procedure with help of the measured voltage discharge characteristics: 1. Determine the capacity of C1 by discharging the capacitor with a constant current ICONST (1 mA/F as reference): t2 - t1 C1 = ICONST * U1 - U2 2. Acquire data for the discharge characteristics (no load) 3. Calculate RISO The average leakage current IC1_L is now calculated based on RISO and the average back-up voltage ∅VBACKUP = (V0 + V1) / 2 IC1_L = ∅VBACKUP RISO Example: Charging conditions: - TA = +25°C - VC1 = 5 V, 24 hours with C1 = 0.1 F, - t1 = 0 s, t2 = 100 hours = 360‘000 s, U1 = 5 V, U2 = 4.47 V: 360'000 s - 0 s = 32'129 kΩ 4.47 V 0.1 F * ln ( ) 5V The average leakage current is calculated for the average back up voltage RISO = - ∅VBACKUP = (2.8 V + 1.5 V) / 2 = 2.15 V: 2.15 V IC1_L = = 67 nA 32'129 kΩ Isolation resistance RISO: RISO TA = -20°C TA = +25°C 0.1 F 32‘100 kΩ 178‘000 kΩ (*) (*) VC1 = 5 V, 24 hours 0.47 F 133‘084 kΩ 24‘000 kΩ (*) (*) 1.0 F 13‘400 kΩ 74‘305 kΩ Data from Panasonic “Gold Capacitors ABC0000PE103_TechnGuide_Oct 1st 2014” (*) Missing values were calculated by linear interpolation or extrapolation Charging conditions C1 TA = +70°C 4‘310 kΩ 3‘222 kΩ (*) 1‘799 kΩ (*) Average leakage currents IC1_L at different average backup voltages ∅VBACKUP: ∅VBACKUP (2.8 V + 1.5 V) / 2 = 2.15 V (VDD = 3.0 V, 24 hours) (5.3 V + 1.5 V) / 2 = 3.4 V (VDD = 5.5 V, 24 hours) C1 0.1 F 0.47 F 1.0 F 0.1 F 0.47 F 1.0 F TA = -20°C 12 nA 16 nA 29 nA 19 nA 26 nA 46 nA IC1_L TA = +25°C 67 nA 90 nA 160 nA 106 nA 142 nA 254 nA TA = +70°C 499 nA 667 nA 1195 nA 789 nA 1055 nA 1890 nA 15/21 Micro Crystal Temperature compensated Real-Time Clock with back-up power supply White Paper 3.4.4. Current consumption RV-8803-C7 To be considered: The RTC Module RV-8803-C7 performs one temperature measurement every second. For a short pulse of 1.3 ms the supply current IDD_8803_PULS will be increased to some 19 µA. To take an accurate measurement the current must be integrated over one second. Typical current consumption IDD_8803 of the RTC Module RV-8803-C7: IDD_8803 ∅VBACKUP TA = -20°C TA = +25°C 1.5 V 195 nA 235 nA 2.15 V 200 nA 240 nA 3V 200 nA 240 nA 3.4 V 200 nA 245 nA 5.5 V 210 nA 250 nA TA = +70°C 330 nA 335 nA 345 nA 350 nA 360 nA 3.4.5. Leakage current of the Schottky-Diode Typical leakage current ID1_L of the Schottky-Diode BAS70: ∅VBACKUP 2.15 V 3.4 V 5V TA = -20°C 0.03 nA 0.05 nA 0.07 nA ID1_L TA = +25°C 1.3 nA 2 nA 3 nA TA = +70°C 47 nA 75 nA 110 nA 16/21 Micro Crystal Temperature compensated Real-Time Clock with back-up power supply White Paper 3.4.6. Back-up time calculation The standard formula to calculate the discharge time of a capacitor assumes a constant current is applied. The total current is the sum of the operating current of the RTC plus the leakage currents of the Schottky-diode and the Supercap. The average back-up voltage is ∅VBACKUP = (V1 + V0)/2. Example: Supercap C1 = 0.1 F (EECS0HD104H), VDD = 3.0 V, TA = 25°C, after charging it for 24 hours. t= t= t= C1 * (V0 - V1 - VC1 ) I C1 * (VDD - VF - VK - V1 - VC1 ) IC1_L + IDD_8803 + ID1_L 0.1 F * (3.0 V - 0.2 V - 0 V - 1.5 V - 0 V) = 421'711 seconds 67 nA + 240 nA + 1.3 nA = 117 hours = 4.9 days Parameter t = back-up time in seconds, target value to calculate C1 = capacity in Farad TA = ambient temperature VDD = main power supply V0 = starting voltage , VBACKUP = V0 VF = forward voltage of the Schottky-Diode VK = correction voltage V1 = final voltage after back-up time t, VBACKUP = V1 VC1 = voltage drop across internal resistance RDC (ca. RESR) and current limiting resistor R1. Negligible. ∅VBACKUP = average back-up voltage V I = constant discharging current in A IC1_L = average leakage current of the Supercap Details and example values C1 = 0.1 F TA = 25°C VDD = 3.0 V V0 = VDD – VF – VK after charging for 24 hours VK = 0 V V1 = 1.5 V (VDD_MIN of RV-8803-C7) VC1 = (IDD_8803 + ID1_L) * (RDC + R1) VC1 = 0 V ∅VBACKUP = (V0 + V1)/2 (used to calculate the currents) I = IC1_L + IDD_8803 + ID1_L IC1_L = ∅VBACKUP / RISO IDD_8803 = average power consumption of the RTC Module RV-8803-C7 ID1_L = average leakage current of the Schottky-diode 17/21 Micro Crystal Temperature compensated Real-Time Clock with back-up power supply White Paper 3.4.7. Buffered back-up time Back-up time as function of capacitor size, C1 (0.1 F, 0.47 F, 1 F), charge time of the Supercap (10 minutes or 24 hours), ambient temperature (-20°C, 25°C, 70°C) and the operating voltage VDD (3.0 V, 5.5 V). Backup-time in days: TA -20°C +25°C +70°C C1 1.0 F 0.47 F 0.1 F 1.0 F 0.47 F 0.1 F 1.0 F 0.47 F 0.1 F VDD = 3.0 V Charged 10 min Charged 24 h 57 63 28 31 6.1 6.8 34 37 19 21 4.4 4.9 9 10 6.6 7.3 1.7 1.8 VDD = 5.5 V Charged 10 min Charged 24 h 170 177 87 90 19 20 85 88 51 53 12 12 19 20 14 14 3.6 3.7 Graphic chart back-up time in function of size of the Supercap (VDD = 3.0 V): 18/21 Micro Crystal Temperature compensated Real-Time Clock with back-up power supply White Paper Graphic chart back-up time in function of size of the Supercap (VDD = 5.5 V): 19/21 Micro Crystal Temperature compensated Real-Time Clock with back-up power supply White Paper 4. Conclusion It is the exceptionally low power consumption of the Micro Crystal RTC Module RV-8803-C7 that allows the industry, for the first time, to implement user friendly Supercaps as the back-up power supply. The resulting prolonged back-up times that are realized by implementing the proposed solution are very beneficial for many different applications. Since the impact of the key factors, leakage currents, ambient temperatures and voltage ranges can be quantified, the desired minimum time of available power back-up can be determined by selecting the correctly sized Supercap. All the tests and measurements discussed in this paper were conducted using actual hardware. The goal was to identify the different leakage currents and to make sure the calculated back-up times for the selected capacitor sizes corresponded with the actual results. Low cost RTC back-up solutions utilizing minimal PC-board area and requiring little BOM impact, can now be designed-in simply by using a Supercap, a Schottky-diode, and the RTC Module RV-8803-C7 manufactured by Micro Crystal. 20/21 Micro Crystal Temperature compensated Real-Time Clock with back-up power supply White Paper 5. Document version Date Version # Changes Juni 2015 0.90 Juni 2015 0.91 August 2015 0.92 September 2015 0.93 December 2015 1.0E Erster Entwurf in English Geändert Wort Anwender Entfernt Kapazitätsbereich Ergänzt erste Schaltung Ergänzt IC1_Lmax und IDD_8803max Ergänzt Schottky BAS70 Vereinfacht zweite Schaltung Separiert Beispiel Leckstrom des Superkondensators Ergänzt Stromverbrauch RV-8803-C7 Vereinfacht Berechnung der Backup-Zeit Vereinfacht Backup-Zeiten Geändert, „der RTC“ zu „die Echtzeituhr“ und „das RTC-Modul“ Kleine Änderungen in Satzstellungen Hinzugefügt: Autor Kleine Textänderungen Ergänzt Verwendete Superkondensatoren Ergänzt Überwachung der RTC-Zeit mit abgeänderter Schaltung Geändert Darstellung der Backup-Zeiten (neu: 3.0 V / 5.5 V) English version Information furnished is believed to be accurate and reliable. However, Micro Crystal assumes no responsibility for the consequences of the use of such information or for any infringement of patents or other rights of third parties which may result from its use. In accordance with our policy of continuous development and improvement, Micro Crystal reserves the right to modify specifications mentioned in this publication without prior notice. This product is not authorized for use as critical component in life support devices or systems. 21/21