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NCP1086D2T-ADJ Datasheet(PDF) 6 Page - ON Semiconductor |
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NCP1086D2T-ADJ Datasheet(HTML) 6 Page - ON Semiconductor |
6 / 14 page NCP1086 http://onsemi.com 6 CAdj IAdj Figure 15. Resistor Divider Scheme VREF R2 R1 C2 VOUT VIN C1 VIN VOUT Adj NCP1086 The adjustable output linear regulator has an absolute maximum specification of 7.0 V for the voltage difference between VIN and VOUT. However, the IC may be used to regulate voltages in excess of 7.0 V. The main considerations in such a design are powerup and short circuit capability. In most applications, ramp−up of the power supply to VIN is fairly slow, typically on the order of several tens of milliseconds, while the regulator responds in less than one microsecond. In this case, the linear regulator begins charging the load as soon as the VIN to VOUT differential is large enough that the pass transistor conducts current. The load at this point is essentially at ground, and the supply voltage is on the order of several hundred mV, with the result that the pass transistor is in dropout. As the supply to VIN increases, the pass transistor will remain in dropout, and current is passed to the load until VOUT reaches the point at which the IC is in regulation. Further increase in the supply voltage brings the pass transistor out of dropout. The result is that the output voltage follows the power supply ramp−up, staying in dropout until the regulation point is reached. In this manner, any output voltage may be regulated. There is no theoretical limit to the regulated voltage as long as the VIN to VOUT differential of 7.0 V is not exceeded. However, the possibility of destroying the IC in a short circuit condition is very real for this type of design. Short circuit conditions will result in the immediate operation of the pass transistor outside of its safe operating area. Overvoltage stresses will then cause destruction of the pass transistor before overcurrent or thermal shutdown circuitry can become active. Additional circuitry may be required to clamp the VIN to VOUT differential to less than 7.0 V if fail−safe operation is required. One possible clamp circuit is illustrated in Figure 16; however, the design of clamp circuitry must be done on an application by application basis. Care must be taken to ensure the clamp actually protects the design. Components used in the clamp design must be able to withstand the short circuit condition indefinitely while protecting the IC. Figure 16. Short Circuit Protection Circuit for High Voltage Application VIN VOUT Adj NCP1086 VOUT EXTERNAL SUPPLY Stability Considerations The output or compensation capacitor helps determine three main characteristics of a linear regulator: startup delay, load transient response and loop stability. The capacitor value and type is based on cost, availability, size and temperature constraints. A tantalum or aluminum electrolytic capacitor is best, since a film or ceramic capacitor with almost zero ESR can cause instability. The aluminum electrolytic capacitor is the least expensive solution. However, when the circuit operates at low temperatures, both the value and ESR of the capacitor will vary considerably. The capacitor manufacturers’ data sheet provides this information. A 22 mF tantalum capacitor will work for most applications, but with high current regulators such as the NCP1086 series the transient response and stability improve with higher values of capacitance. The majority of applications for this regulator involve large changes in load current, so the output capacitor must supply the instantaneous load current. The ESR of the output capacitor causes an immediate drop in output voltage given by: DV + DI ESR For microprocessor applications it is customary to use an output capacitor network consisting of several tantalum and ceramic capacitors in parallel. This reduces the overall ESR and reduces the instantaneous output voltage drop under load transient conditions. The output capacitor network should be as close as possible to the load for the best results. |
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