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Zakładka z wyszukiwarką danych komponentów |
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Zakładka z wyszukiwarką danych komponentów |
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FM3164 Arkusz danych(PDF) 7 Page - Cypress Semiconductor |
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FM3164 Arkusz danych(HTML) 7 Page - Cypress Semiconductor |
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7 / 27 page  FM31256/3164 Processor Companion Document Number: 001-86391 Rev. *B Page 7 of 27 Backup Power The real-time clock/calendar is intended to be permanently powered. When the primary system power fails, the voltage on the VDD pin will drop. When VDD is less 2.5V, the RTC (and event counters) will switch to the backup power supply on VBAK. The clock operates at extremely low current in order to maximize battery or capacitor life. However, an advantage of combining a clock function with FRAM memory is that data is not lost regardless of the backup power source. The IBAK current varies with temperature and voltage (see DC parametric table). The following graph shows IBAK as a function of VBAK. These curves are useful for calculating backup time when a capacitor is used as the VBAK source. Figure 7. IBAK vs. VBAK Voltage The minimum VBAK voltage varies linearly with temperature. The user can expect the minimum VBAK voltage to be 1.23V at +85°C and 1.90V at -40°C. The tested limit is 1.55V at +25°C. The minimum VBAK voltage has been characterized at -40°C and +85°C but is not 100% tested. Figure 8. VBAK (min.) vs. Temperature Trickle Charger To facilitate capacitor backup the VBAK pin can optionally provide a trickle charge current. When the VBC bit, register 0Bh bit 2, is set to 1 the VBAK pin will source approximately 15 µA until VBAK reaches VDD or 3.75V whichever is less. In 3V systems, this charges the capacitor to VDD without an external diode and resistor charger. In 5V systems, it provides the same convenience and also prevents the user from exceeding the VBAK maximum voltage specification. In the case where no battery is used, the VBAK pin should be tied according to the following conditions: For 3.3V systems, VBAK should be tied to VDD. This assumes VDD does not exceed 3.75V. For 5V systems, attach a 1 µF capacitor to VBAK and turn the trickle charger on. The VBAK pin will charge to the internal backup voltage which regulates itself to about 3.6V. VBAK should not be tied to 5V since the VBAK (max) specification will be exceeded. A 1 µF capacitor will keep the companion functions working for about 1.5 second. Although VBAK may be connected to VSS, this is not recommended if the companion is used. None of the companion functions will operate below about 2.5V Note: systems using lithium batteries should clear the VBC bit to 0 to prevent battery charging. The VBAK circuitry includes an internal 1 K series resistor as a safety element. |
Podobny numer części - FM3164 |
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Podobny opis - FM3164 |
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