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LT1020CN Datasheet(PDF) 8 Page - Linear Technology |
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LT1020CN Datasheet(HTML) 8 Page - Linear Technology |
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8 / 16 page  8 LT1020 sn1020 1020fcs Like most other IC regulators, a minimum load is required on the output of the LT1020 to maintain regulation. For most standard regulators this is normally specified at 5mA. Of course, for a micropower regulator this would be a tremendously large current. The output current must be large enough to absorb all the leakage current of the pass transistor at the maximum operating temperature. It also affects the transient response; low output currents have long recovery times from load transients. At high operat- ing temperatures the minimum load current increases and having too low of a load current may cause the output to go unregulated. Devices are tested for minimum load current at high temperature. The output voltage setting resistors to the feedback terminal can usually be used to provide the minimum load current. Frequency Compensation The LT1020 is frequency compensated by a dominant pole on the output. An output capacitor of 10 µF is usually large enough to provide good stability. Increasing the output capacitor above 10 µF further improves stability. In order to insure stability, a feedback capacitor is needed between the output pin and the feedback pin. This is because stray capacitance can form another pole with the large value of feedback resistors used with the LT1020. Also, a feedback capacitor minimizes noise pickup and improves ripple rejection. With the large dynamic operating range of the output current, 10000:1, frequency response changes widely. Low AC impedance capacitors are needed to insure stabil- ity. While solid tantalum are best, aluminum electrolytics can be used but larger capacitor values may be needed. The CURRENT LIMIT pin allows one of the internal nodes to be rolled off with a 0.05 µF capacitor to ground. With this capacitor, lower values of regulator output capacitance can be used (down to 1 µF) for low (<20mA) output currents. Values of capacitance greater than 0.05 µF de- grade the transient response, so are not recommended. If the CURRENT LIMIT pin is connected to GND, the current limit is decreased and only a 1 µF output capacitor is needed. When bypassing the reference, a 20 Ω resistor must be connected in series with the capacitor. than placing large capacitive loads on the reference, no other precautions are necessary and the reference is stable with nominal stray capacitances. Figure 2. Minimizing Oscillation in Thermal Shutdown Overload Protection The main regulator in the LT1020 is current limited at approximately 350mA. The current limit is stable with both input voltage and temperature. A current limit pin, when strapped to ground, decreases the output current. This allows the output current to be set to a lower value than 250mA. The output current available with the current limit pin strapped to ground is not well controlled so if precise current limiting is desired it should be provided externally as is shown in some of the application circuits. If the device is overloaded for long periods of time, thermal shutdown turns the output off. In thermal shutdown, there may be some oscillations which can disturb external circuitry. A diode connected between the reference and feedback terminal provides hysteresis under thermal shut- down, so that the device turns on and off with about a 5 second period and there are no higher frequency oscilla- tions. This is shown in Figure 2. This diode is recom- mended for most applications. Thermal shutdown temperature is set at approximately 145 °C. Figure 1. Bypassing Reference + + REF REF OUTPUT OUTPUT 20 Ω 20 Ω 10 µF 10 µF OR 1020 F01 4 4 + 10 µF 0.001 µF * 2 11 4 VOUT FB REF LT1020 1020 F02 * DIODE ADDS FEEDBACK APPLICATIO S I FOR ATIO |
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