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LP3982ILD-2.77 Datasheet(PDF) 6 Page - National Semiconductor (TI) |
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LP3982ILD-2.77 Datasheet(HTML) 6 Page - National Semiconductor (TI) |
6 / 12 page Typical Performance Characteristics Unless otherwise specified, V IN =VO + 0.5V, CIN =COUT = 2.2µF, C CC = 33nF, TJ = 25˚C, VSHDN =VIN. (Continued) Power-Up Response Power-Down Response 20036911 20036912 Application Information General Information LP3982 is package, pin and performance compatible with Maxim’s MAX8860 excluding reverse battery protection and Dual Mode™ function (fixed and adjustable combined). Figure 1 shows the functional block diagram for the LP3982. A 1.25V bandgap reference, an error amplifier and a PMOS pass transistor perform voltage regulation while being sup- ported by shutdown, fault, and the usual Temperature and current protection circuitry The regulator’s topology is the classic type with negative feedback from the output to one of the inputs of the error amplifier. Feedback resistors R 1 and R2 are either internal or external to the IC, depending on whether it is the fixed voltage version or the adjustable version. The negative feed- back and high open loop gain of the error amplifier cause the two inputs of the error amplifier to be virtually equal in voltage. If the output voltage changes due to load changes, the error amplifier provides the appropriate drive to the pass transistor to maintain the error amplifier’s inputs as virtually equal. In short, the error amplifier keeps the output voltage constant in order to keep its inputs equal. Output Voltage Setting (ADJ version only) The output voltage is set according to the amount of nega- tive feedback (Note that the pass transistor inverts the feed- back signal.) Figure 2 simplifies the topology of the LP3982. This type of regulator can be represented as an op amp configured as non-inverting amplifier and a fixed DC Voltage (V REF) for its input signal. The special characteristic of this op amp is its extra-large output transistor that only sources current. In terms of its non-inverting configuration, the output voltage equals V REF times the closed loop gain: Utilize the following equation for adjusting the output to a particular voltage: Choose R 2 = 100k to optimize accuracy, power supply re- jection, noise and power consumption. 20036913 FIGURE 1. Functional Block Diagram for the LP3982 www.national.com 6 |
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