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ILC7081AIM5ADJ датащи(PDF) 7 Page - Fairchild Semiconductor |
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ILC7081AIM5ADJ датащи(HTML) 7 Page - Fairchild Semiconductor |
7 / 16 page ILC7080/81 REV. 1.0.7 4/3/03 7 The Effects of ESR (Equivalent Series Resistance) The ESR of a capacitor is a measure of the resistance due to the leads and the internal connections of the component. Typically measured in m Ω (milli-ohms) it can increase to ohms in some cases. Wherever there is a combination of resistance and current, volt- ages will be present. The control functions of LDOs use two voltages in order to maintain the output precisely; VOUT and VREF. With reference to the block diagram in figure 2, VOUT is fed back to the error amplifier and is used as the supply voltage for the internal components of the 7080/81. So any change in VOUT will cause the error amplifier to try to compensate to maintain VOUT at the set level and noise on VOUT will be reflected into the supply of each internal circuit. The reference voltage, VREF, is influenced by the CNOISE pin. Noise into this pin will add to the reference voltage and be fed through the circuit. These fac- tors will not cause a problem if some simple steps are taken. Figure 5 shows where these added ESR resistances are present in the typical LDO circuit. Figure 5. ESR in COUT and CNOISE With this in mind low ESR components will offer better perfor- mance as LDOs may be exposed to large transients of output voltage, and current flows through the capacitors in order to fil- ter these transient swings. ESR is less of a problem with CIN as the voltage fluctuations at the input will be filtered by the LDO. However, being aware of these current flows, there is also another potential source of induced voltage noise from the resis- tance inherent in the PCB trace. Figure 6 shows where the addi- tive resistance of the PCB can manifest itself. Again these resistances may be very small, but a summation of several cur- rents can develop detectable voltage ripple and will be amplified by the LDO. Particularly the accumulation of current flows in the ground plane can develop significant voltages unless care is taken. With a degree of care, the ILC7080/81 will yield outstanding performance. Printed Circuit Board Layout Guidelines As was mentioned in the previous section, to take full advan- tage of any high performance LDO regulator requires paying careful attention to grounding and printed circuit board (PCB) layout. Figure 6. Inherent PCB resistance Figure 7 shows the effects of poor grounding and PCB layout caused by the ESR and PCB resistances and the accumulation of current flows. Note particularly that during high output load current, the LDO regulator’s ground pin and the ground return for COUT and CNOISE are not at the same potential as the system ground. This is due to high frequency impedance caused by PCB’s trace inductance and DC resistance. The current loop between COUT, CNOISE and the LDO regulator’s ground pin will degrade performance of the LDO. Figure 7. Effects of poor circuit layout Figure 8 shows an optimum schematic. In this schematic, high output surge current has little effect on the ground cur- rent and noise bypass current return of the LDO regulator. Note that the key difference here is that COUT and CNOISE are directly connected to the LDO regulator’s ground pin. The LDO is then separately connected to the main ground plane and returned to a single point system ground. ILC7080 ILC7081 V IN SOT-23-5 C IN ON OFF 12 3 4 5 C NOISE RF LDOTM Regulator R* R* R C V OUT C OUT I C I OUT ILC7080 ILC7081 SOT-23-5 ON OFF 12 3 4 5 C NOISE R PCB ESR R PCB V IN V IN I OUT C OUT I 1 I 2 V OUT R PCB R PCB R PCB C OUT C IN ON/OFF V OUT V IN True GND (0V) GND1 GND2 GND3 ILC7080/81 5 23 1 4 SOT23-5 ILOAD ILOAD +IC OUT +IC NOISE +IGND |
Аналогичный номер детали - ILC7081AIM5ADJ |
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Аналогичное описание - ILC7081AIM5ADJ |
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