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LTC692 Datasheet(PDF) 10 Page - Linear Technology |
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LTC692 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  LTC692/LTC693 10 0692fb The LTC692/LTC693 are protected for safe area operation with a short-circuit limit. Output current is limited to ap- proximately 200mA. If the device is overloaded for long periods of time, thermal shutdown turns the power switch off until the device cools down. The threshold temperature for thermal shutdown is approximately 155°C with about 10°C of hysteresis which prevents the device from oscil- lating 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 collected 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 current. Since no current goes to the substrate, the current collected by the VBATT pin is strictly junction leakage. A 125Ω 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 bat- tery backup in CMOS RAM and other low power CMOS circuitry. The supply current in battery backup mode is 1μA 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 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 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 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 1μA maximum over temperature and the external resistor required to hold VBATT below VCC is: R V – 50mV 1A CC ≤ μ With VCC = 4.25V, a 3.9M resistor will work. With a 3V battery, this resistor will draw only 0.77μA from the bat- tery, which is negligible in most cases. APPLICATIONS INFORMATION 5V 3V 0.1μF 0.1μF VBATT VCC LTC693 VOUT GND 4 3 1 2 5 ANY PNP POWER TRANSISTOR 692_3 • F02 BATT ON 5V 3V 0.1μF 0.1μF 692_3 • F03 VOUT – VBATT R I = R VBATT VCC LTC692 LTC693 VOUT GND Figure 2. Using BATT ON to Drive External PNP Transistor Figure 3. Charging External Battery Through VOUT |
Similar Part No. - LTC692_15 |
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Similar Description - LTC692_15 |
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