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Datenblatt-Suchmaschine für elektronische Bauteile |
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Datenblatt-Suchmaschine für elektronische Bauteile |
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CS5208-1 Datenblatt(PDF) 5 Page - Cherry Semiconductor Corporation |
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CS5208-1 Datenblatt(HTML) 5 Page - Cherry Semiconductor Corporation |
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5 / 7 page  Application Notes: continued 5 Figure 1. While not required, a bypass capacitor connected between the adjust pin and ground will improve transient response and ripple rejection. A 0.1µF tantalum capacitor is recom- mended for Òfirst cutÓ design. Value and type may be var- ied to optimize performance vs price. The CS5208-1 linear regulator has an absolute maximum specification of 6V for the voltage difference between VIN and VOUT. However, the IC may be used to regulate volt- ages in excess of 6V. The main considerations in such a design are power-up 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 mil- liseconds, while the regulator responds in less than one microsecond. In this case, the linear regulator begins charg- ing the output capacitor as soon as the VIN to VOUT differ- ential is large enough that the pass transistor conducts cur- rent. VOUT is essentially at ground, and VIN is on the order of several hundred millivolts, so the pass transistor is in dropout. As VIN increases, the pass transistor will remain in dropout, and current is passed to the load until VOUT is in regulation. Further increase in VIN brings the pass tran- sistor 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 6V is not exceeded. However, the maximum ratings of the IC will be exceeded in a short circuit condition. Short circuit conditions will result in the immediate operation of the pass transistor outside of its safe operating area. Over-voltage stresses will then cause destruction of the pass transistor before over- current or thermal shutdown circuitry can become active. Additional circuitry may be required to clamp VIN to VOUT differential to less than 6V if failsafe operation is required. One possible clamp circuit is illustrated below; however, the design of clamp circuitry must be done on an applica- tion 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 cir- cuit conditions indefinitely while protecting the IC. Figure 2. The output compensation capacitor helps determine three main characteristics of a linear regulator: start-up 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 capaci- tor with almost zero ESR can cause instability. The alu- minum electrolytic capacitor is the least expensive solu- tion. However, when the circuit operates at low tempera- tures, both the value and ESR of the capacitor will vary considerably. The capacitor manufacturerÕs data sheet pro- vides this information. A 300µF tantalum capacitor will work for most applica- tions, but with high current regulators such as the CS5208-1 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 instanta- neous load current. The ESR of the output capacitor causes an immediate drop in output voltage given by: ÆV = ÆI ´ 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 transient load conditions. The output capacitor net- work should be as close to the load as possible for the best results. Protection Diodes When large external capacitors are used with a linear regu- lator it is sometimes necessary to add protection diodes. If the input voltage of the regulator gets shorted, the output capacitor will discharge into the output of the regulator. The discharge current depends on the value of the capaci- tor, the output voltage, and the rate at which VIN drops. In the CS5208-1 regulator, the discharge path is through a large junction and protection diodes are not usually need- ed. If the regulator is used with large values of output capacitance and the input voltage is instantaneously short- Stability Considerations VIN VOUT VAdj EXTERNAL SUPPLY Other Adjustable Operation Considerations V OUT V IN CS5208-1 R1 Adj R2 |
Ähnliche Teilenummer - CS5208-1 |
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Ähnliche Beschreibung - CS5208-1 |
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