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LT1121AIS8-5 Datasheet(PDF) 10 Page - Linear Technology |
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LT1121AIS8-5 Datasheet(HTML) 10 Page - Linear Technology |
10 / 12 page 10 LT1121/LT1121-3.3/LT1121-5 S APPLICATI I FOR ATIO that the ESR of the capacitor is low (ceramic) the sug- gested series resistor is shown in Table 5. The LT1121 is a micropower device and output transient response will be a function of output capacitance. See the Transient Re- sponse curves in the Typical Performance Characteristics. Larger values of output capacitance will decrease the peak deviations and provide improved output transient re- sponse. Bypass capacitors, used to decouple individual components powered by the LT1121, will increase the effective value of the output capacitor. Table 5. SUGGESTED SERIES OUTPUT CAPACITANCE RESISTOR 0.33 µF2Ω 0.47 µF1Ω 0.68 µF1Ω >1 µF None Needed Protection Features The LT1121 incorporates several protection features which make it ideal for use in battery-powered circuits. In addi- tion to the normal protection features associated with monolithic regulators, such as current limiting and ther- mal limiting, the device is protected against reverse input voltages, reverse output voltages, and reverse voltages from output to input. Current limit protection and thermal overload protection are intended to protect the device against current overload conditions at the output of the device. For normal opera- tion, the junction temperature should not exceed 125 °C. The input of the device will withstand reverse voltages of 30V. Current flow into the device will be limited to less than 1mA (typically less than 100 µA) and no negative voltage will appear at the output. The device will protect both itself and the load. This provides protection against batteries that can be plugged in backwards. For fixed voltage versions of the device, the output can be pulled below ground without damaging the device. If the input is open circuit or grounded the output can be pulled below ground by 20V. The output will act like an open circuit, no current will flow out of the pin. If the input is powered by a voltage source, the output will source the short-circuit current of the device and will protect itself by thermal limiting. For the adjustable version of the device, the output pin is internally clamped at one diode drop below ground. Reverse current for the adjustable device must be limited to 5mA. In circuits where a backup battery is required, several different input/output conditions can occur. The output voltage may be held up while the input is either pulled to ground, pulled to some intermediate voltage, or is left open circuit. Current flow back into the output will vary depending on the conditions. Many battery-powered cir- cuits incorporate some form of power management. The following information will help optimize battery life. Table 6 summarizes the following information. The reverse output current will follow the curve in Figure 2 when the input pin is pulled to ground. This current flows through the output pin to ground. The state of the shut- down pin will have no effect on output current when the input pin is pulled to ground. In some applications it may be necessary to leave the input to the LT1121 unconnected when the output is held high. This can happen when the LT1121 is powered from a rectified AC source. If the AC source is removed, then the input of the LT1121 is effectively left floating. The reverse output current also follows the curve in Figure 2 if the input pin is left open. The state of the shutdown pin will have no effect on the reverse output current when the input pin is floating. OUTPUT VOLTAGE (V) 0 100 90 80 70 60 50 40 30 20 10 0 8 1121• F02 2 4 6 10 13 57 9 TJ = 25°C VIN < VOUT CURRENT FLOWS INTO OUTPUT PIN TO GROUND LT1121-3.3 LT1121 (VOUT = VADJ) LT1121-5 Figure 2. Reverse Output Current |
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