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LTC694CN8 Datasheet(PDF) 9 Page - Linear Technology |
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LTC694CN8 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 16 page  9 LTC690/LTC691 LTC694/LTC695 S APPLICATI I FOR ATIO size double layer capacitors, can be used for short term memory back-up instead of a battery. The charging resis- tor for both capacitors and rechargeable batteries should be connected to VOUT since this eliminates the discharge path that exists when the resistor is connected to VCC (Figure 3). of external PNP transistor (Figure 2). If higher currents are needed with the LTC690 and LTC694, a high current Schottky diode can be connected from the VCC pin to the VOUT pin to supply the extra current. Figure 2. Using BATT ON to Drive External PNP Transistor The LTC690 family is protected for safe area operation with short-circuit limit. Output current is limited to ap- proximately 200mA. If the device is overloaded for long period 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 LTC690 family uses 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 VBATT pin is strictly junction leakage. A 125 Ω PMOS switch connects the VBATT input to VOUT in battery back-up 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 back-up in CMOS RAM and other low power CMOS circuitry. The supply current in battery back-up mode is 1 µA maximum. The operating voltage at the VBATT pin ranges from 2.0V to 4.25V. High value capacitors, such as electrolytic or farad- Replacing the Back-Up Battery When changing the back-up battery with system power on, spurious resets can occur while battery is removed 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 LTC690 switches to battery back-up. VOUT pulls VBATT low and the device goes 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 = 4.5V, a 4.3M resistor will work. With a 3V battery, this resistor will draw only 0.7 µA from the battery, which is negligible in most cases. R V – 50mV 1A CC ≤ µ 5V 3V 0.1 µF 0.1 µF VBATT VCC LTC691 LTC695 VOUT GND 4 3 1 2 5 ANY PNP POWER TRANSISTOR 690 F02 BATT ON 5V 3V 0.1 µF 0.1 µF VBATT VCC LTC690 LTC691 LTC694 LTC695 VOUT GND 690 F03 VOUT – VBATT R I = R Figure 3. Charging External Battery Through VOUT |
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