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LM1086IS-3.3 Scheda tecnica(PDF) 8 Page - Texas Instruments |
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LM1086IS-3.3 Scheda tecnica(HTML) 8 Page - Texas Instruments |
8 / 24 page LM1086 SNVS039H – JUNE 2000 – REVISED MAY 2013 www.ti.com APPLICATION NOTE GENERAL Figure 18 shows a basic functional diagram for the LM1086-Adj (excluding protection circuitry) . The topology is basically that of the LM317 except for the pass transistor. Instead of a Darlingtion NPN with its two diode voltage drop, the LM1086 uses a single NPN. This results in a lower dropout voltage. The structure of the pass transistor is also known as a quasi LDO. The advantage a quasi LDO over a PNP LDO is its inherently lower quiescent current. The LM1086 is specified to provide a minimum dropout voltage 1.5V over temperature, at full load. Figure 18. Basic Functional Diagram for the LM1086, excluding Protection circuitry OUTPUT VOLTAGE The LM1086 adjustable version develops at 1.25V reference voltage, (VREF), between the output and the adjust terminal. As shown in figure 2, this voltage is applied across resistor R1 to generate a constant current I1. This constant current then flows through R2. The resulting voltage drop across R2 adds to the reference voltage to sets the desired output voltage. The current IADJ from the adjustment terminal introduces an output error . But since it is small (120uA max), it becomes negligible when R1 is in the 100 Ω range. For fixed voltage devices, R1 and R2 are integrated inside the devices. Figure 19. Basic Adjustable Regulator STABILITY CONSIDERATION Stability consideration primarily concern the phase response of the feedback loop. In order for stable operation, the loop must maintain negative feedback. The LM1086 requires a certain amount series resistance with capacitive loads. This series resistance introduces a zero within the loop to increase phase margin and thus increase stability. The equivalent series resistance (ESR) of solid tantalum or aluminum electrolytic capacitors is used to provide the appropriate zero (approximately 500 kHz). 8 Submit Documentation Feedback Copyright © 2000–2013, Texas Instruments Incorporated Product Folder Links: LM1086 |
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