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LM1086ILDX-5.0 Scheda tecnica(PDF) 8 Page - National Semiconductor (TI) |
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LM1086ILDX-5.0 Scheda tecnica(HTML) 8 Page - National Semiconductor (TI) |
8 / 15 page Typical Performance Characteristics (Continued) Line Transient Response Load Transient Response 10094847 10094848 Application Note GENERAL Figure 1 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 guaran- teed to provide a minimum dropout voltage 1.5V over tem- perature, at full load. OUTPUT VOLTAGE The LM1086 adjustable version develops at 1.25V reference voltage, (V REF), 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 I ADJ 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. 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 in- crease phase margin and thus increase stability. The equiva- lent series resistance (ESR) of solid tantalum or aluminum electrolytic capacitors is used to provide the appropriate zero (approximately 500 kHz). The Aluminum electrolytic are less expensive than tantal- ums, but their ESR varies exponentially at cold tempera- tures; therefore requiring close examination when choosing the desired transient response over temperature. Tantalums are a convenient choice because their ESR varies less than 2:1 over temperature. The recommended load/decoupling capacitance is a 10uF tantalum or a 50uF aluminum. These values will assure stability for the majority of applications. The adjustable versions allows an additional capacitor to be used at the ADJ pin to increase ripple rejection. If this is done the output capacitor should be increased to 22uF for tantal- ums or to 150uF for aluminum. 10094865 FIGURE 1. Basic Functional Diagram for the LM1086, excluding Protection circuitry 10094817 FIGURE 2. Basic Adjustable Regulator www.national.com 8 |
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