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CS52015-1 데이터시트(PDF) 5 Page - Cherry Semiconductor Corporation |
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CS52015-1 데이터시트(HTML) 5 Page - Cherry Semiconductor Corporation |
5 / 7 page 5 Applications Information: continued Figure 1. Resistor divider scheme. The CS52015-1 linear regulator has an absolute maximum specification of 7V for the voltage difference between VIN and VOUT. However, the IC may be used to regulate volt- ages in excess of 7V. 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 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 millivolts, with the result that the pass transistor is in dropout. As the sup- ply 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 fol- lows 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 differen- tial of 7V 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. Over- voltage stresses will then cause destruction of the pass transistor before overcurrent or thermal shutdown circuit- ry can become active. Additional circuitry may be required to clamp the VIN to VOUT differential to less than 7V if fail- safe operation is required. One possible clamp circuit is illustrated in figure 2; however, the design of clamp cir- cuitry 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 2: Short Circuit Protection Circuit for High Voltage Application. The output or 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 manufacturersÕ data sheet pro- vides this information. A 22µF tantalum capacitor will work for most applications, but with high current regulators such as the CS52015-1 the transient response and stability improve with higher val- ues of capacitance. The majority of applications for this regulator involve large changes in load current so the out- put capacitor must supply the instantaneous 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 load transient conditions. The output capacitor net- work should be as close as possible to the load for the best results. 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 CS52015-1 linear regulator, the discharge path is through a large junction and protection diodes are not usu- ally needed. If the regulator is used with large values of output capacitance and the input voltage is instantaneous- ly shorted to ground, damage can occur. In this case, a diode connected as shown in Figure 2 is recommended. Protection Diodes Stability Considerations VIN VOUT VAdj EXTERNAL SUPPLY VOUT VOUT VIN CS52015-1 VIN Adj R1 R2 C1 CAdj VOUT C2 VREF IAdj |
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