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LTC693CS Datasheet(PDF) 9 Page - Linear Technology |
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LTC693CS Datasheet(HTML) 9 Page - Linear Technology |
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9 / 16 page  9 LTC692/LTC693 memory backup instead of a battery. The charging resistor for the rechargeable batteries should be connected to VOUT since this eliminates the discharge path that exists when the resistor is connected to VCC (Figure 3). Replacing the Backup Battery When changing the backup battery with system power on, spurious resets can occur while the battery is re- moved due to battery standby current. Although battery standby current is only a tiny leakage current, it can still charge up the stray capacitance on the VBATT pin. The oscillation cycle is as follows: When VBATT reaches within 50mV of VCC, the LTC692/LTC693 switch to battery backup. VOUT pulls VBATT low and the devices go back to normal operation. The leakage current then charges up the VBATT pin again and the cycle repeats. If spurious resets during battery replacement pose no problems, then no action is required. Otherwise, a resistor from VBATT to GND will hold the pin low while changing the battery. For example, the battery standby current is 1 µA maximum over temperature and the external resistor required to hold VBATT below VCC is: With VCC = 4.25V, a 3.9M resistor will work. With a 3V battery, this resistor will draw only 0.77 µA from the battery, which is negligible in most cases. S APPLICATI I FOR ATIO The LTC692/LTC693 are protected for safe area operation with a short circuit limit. Output current is limited to approximately 200mA. If the device is overloaded for long periods of time, thermal shutdown turns the power switch off until the device cools down. The threshhold tempera- ture for thermal shutdown is approximately 155 °C with about 10 °C of hysteresis which prevents the device from oscillating in and out of shutdown. The PNP switch used in competitive devices was not chosen for the internal power switch because it injects unwanted current into the substrate. This current is col- lected by the VBATT pin in competitive devices and adds to the charging current of the battery which can damage lithium batteries. The LTC692/LTC693 use a charge pumped NMOS power switch to eliminate unwanted charging current while achieving low dropout and low supply cur- rent. Since no current goes to the substrate, the current collected by the VBATT pin is strictly junction leakage. A 125 Ω PMOS switch connects the VBATT input to VOUT in battery backup mode. The switch is designed for very low dropout voltage (input-to-output differential). This feature is advantageous for low current applications such as battery backup in CMOS RAM and other low power CMOS circuitry. The supply current in battery backup mode is 1 µA maximum. The operating voltage at the VBATT pin ranges from 2.0V to 4.0V. High value capacitors, such as electrolytic or farad- size double layer capacitors, can be used for short term Figure 2. Using BATT ON to Drive External PNP Transistor 5V 3V 0.1µF 0.1µF VBATT VCC LTC693 VOUT GND 4 3 1 2 5 ANY PNP POWER TRANSISTOR LTC692/3 • F02 BATT ON Figure 3. Charging External Battery Through VOUT 5V 3V 0.1µF 0.1µF LTC692/3 • F03 VOUT – VBATT R I = R VBATT VCC LTC692 LTC693 VOUT GND R V – 50mV 1A CC ≤ µ |
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