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CS5257A-1 Datasheet(PDF) 8 Page - ON Semiconductor |
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CS5257A-1 Datasheet(HTML) 8 Page - ON Semiconductor |
8 / 11 page CS5257A−1 http://onsemi.com 8 solution. However, when the circuit operates at low temperatures, both the value and ESR of the capacitor will vary considerably. The capacitor manufacturer’s data sheet provides this information. A 300 mF tantalum capacitor will work for most applications, but with high current regulators such as the CS5257A−1 the transient response and stability improve with higher values of capacitor. 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 transient load conditions. The output capacitor network should be as close to the load as possible for the best results. Protection Diodes When large external capacitors are used with a linear regulator 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 capacitor, the output voltage, and the rate at which VCONTROL drops. In the CS5257A−1 regulator, the discharge path is through a large junction and protection diodes are not usually needed. If the regulator is used with large values of output capacitance and the input voltage is instantaneously shorted to ground, damage can occur. In this case, a diode connected as shown in Figure 18 is recommended. Figure 18. Diode Protection Against VCONTROL Short Circuit Conditions CS5257A−1 VCONTROL VPOWER VOUT VSENSE Adjust Use of the diode has the added benefit of bleeding VOUT to ground if VCONTROL is shorted. This prevents an unregulated output from causing system damage. A rule of thumb useful in determining if a protection diode is required is to solve for current I + C V T where: I is the current flow out of the load capacitance when VCONTROL is shorted, C is the value of load capacitance, V is the output voltage, and T is the time duration required for VCONTROL to transition from high to being shorted. If the calculated current is greater than or equal to the typical short circuit current value provided in the specifications, serious thought should be given to the use of a protection diode. Current Limit The internal current limit circuit limits the output current under excessive load conditions. Short Circuit Protection The device includes short circuit protection circuitry that clamps the output current at approximately two amperes less than its current limit value. This provides for a current foldback function, which reduces power dissipation under a direct shorted load. Thermal Shutdown The thermal shutdown circuitry is guaranteed by design to activate above a die junction temperature of approximately 150°C and to shut down the regulator output. This circuitry has 25°C of typical hysteresis, thereby allowing the regulator to recover from a thermal fault automatically. Calculating Power Dissipation and Heat Sink Requirements High power regulators such as the CS5257A−1 usually operate at high junction temperatures. Therefore, it is important to calculate the power dissipation and junction temperatures accurately to ensure that an adequate heat sink is used. Since the package tab is connected to VOUT on the CS5257A−1, electrical isolation may be required for some applications. Also, as with all high power packages, thermal compound in necessary to ensure proper heat flow. For added safety, this high current LDO includes an internal thermal shutdown circuit. The thermal characteristics of an IC depend on the following four factors: junction temperature, ambient temperature, die power dissipation, and the thermal resistance from the die junction to ambient air. The maximum junction temperature can be determined by: TJ(max) + TA(max) ) PD(max) RQJA The maximum ambient temperature and the power dissipation are determined by the design while the maximum junction temperature and the thermal resistance depend on the manufacturer and the package type. The maximum power dissipation for a regulator is: |
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