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LT1120CH Datasheet(PDF) 7 Page - Linear Technology |
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LT1120CH Datasheet(HTML) 7 Page - Linear Technology |
7 / 12 page 7 LT1120 APPLICATIO S I FOR ATIO Reference Internal to the LT1120 is a 2.5V trimmed class B output reference. The reference was designed to be able to source or sink current so it could be used in supply splitting applications as well as a general purpose reference for external circuitry. The design of the reference allows it to source typically 4mA or 5mA and sink 2mA. The available source and sink current decreases as temperature in- creases. It is sometimes desirable to decrease the AC output impedance by placing an output capacitor on them. The reference in the LT1120 becomes unstable with large capacitive loads placed directly on it. When using an output capacitor, about 20 Ω should be used to isolate the capacitor from the reference pin. This 20 Ω resistor can be placed directly in series with the capacitor or alternatively the reference line can have 20 Ω placed in series with it and then a capacitor to ground. This is shown in Figure 1. Other than placing large capacitive loads on the reference, no other precautions are necessary and the reference is stable with nominal stray capacitances. Overload Protection The main regulator in the LT1120 is current limited at approximately 250mA. The current limit is stable with both input voltage and temperature. Like most other IC regulators, a minimum load is required on the output of the LT1120 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 must be large enough to absorb all the leakage current of the pass transistor at the maximum operating temperature. It also effects 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 LT1120 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 ensure 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 LT1120. 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. + REF OUTPUT OR 20 Ω 10 µF 1120 F01 6 + REF OUTPUT 20 Ω 10 µF 6 Figure 1. Bypassing Reference |
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