Datasheet LT1120 (Analog Devices) - 7

LT1120
U U W U APPLICATIO S I FOR ATIO Reference Overload Protection
Internal to the LT1120 is a 2.5V trimmed class B output The main regulator in the LT1120 is current limited at reference. The reference was designed to be able to source approximately 250mA. The current limit is stable with both or sink current so it could be used in supply splitting input voltage and temperature. applications as well as a general purpose reference for Like most other IC regulators, a minimum load is required external circuitry. The design of the reference allows it to on the output of the LT1120 to maintain regulation. For source typically 4mA or 5mA and sink 2mA. The available most standard regulators this is normally specified at source and sink current decreases as temperature in- 5mA. Of course, for a micropower regulator this would be creases. It is sometimes desirable to decrease the AC a tremendously large current. The output must be large output impedance by placing an output capacitor on them. enough to absorb all the leakage current of the pass The reference in the LT1120 becomes unstable with large transistor at the maximum operating temperature. It also capacitive loads placed directly on it. When using an effects the transient response; low output currents have output capacitor, about 20Ω should be used to isolate the long recovery times from load transients. At high operat- capacitor from the reference pin. This 20Ω resistor can be ing temperatures the minimum load current increases and placed directly in series with the capacitor or alternatively having too low of a load current may cause the output to the reference line can have 20Ω placed in series with it and go unregulated. Devices are tested for minimum load then a capacitor to ground. This is shown in Figure 1. Other current at high temperature. The output voltage setting than placing large capacitive loads on the reference, no resistors to the feedback terminal can usually be used to other precautions are necessary and the reference is provide the minimum load current. stable with nominal stray capacitances.
Frequency Compensation
The LT1120 is frequency compensated by a dominant pole on the output. An output capacitor of 10μF is usually large REF REF 6 6 enough to provide good stability. Increasing the output OUTPUT 20Ω capacitor above 10μF further improves stability. In order 20Ω OR OUTPUT to ensure stability, a feedback capacitor is needed between + + the output pin and the feedback pin. This is because stray 10μF 10μF capacitance can form another pole with the large value of 1120 F01 feedback resistors used with the LT1120. Also, a feedback
Figure 1. Bypassing Reference
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. 1120fd 7