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FAN2558S33X Fiches technique(PDF) 7 Page - Fairchild Semiconductor |
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FAN2558S33X Fiches technique(HTML) 7 Page - Fairchild Semiconductor |
7 / 15 page PRODUCT SPECIFICATIONS FAN2558/FAN2559 REV. 1.0.4 3/15/04 7 Control Functions Enable Pin Connecting 2V or greater to the Enable pin will enable the output, while 0.4V or less will disable it while reducing the quiescent current consumption to less than 1µA. If this shut- down function is not needed, the pin can simply be con- nected permanently to the VIN pin. Allowing this pin to float will cause erratic operation. Error Flag (Power Good) Fault conditions such as input voltage dropout (low VIN), overheating, or overloading (excessive output current), will set an error flag. The PG pin which is an open-drain output, will go LOW when VOUT is less than 95% or the specified output voltage. When the voltage at VOUT is greater than 95% of the specified output voltage, the PG pin is HIGH. A logic pull-up resistor of 47K Ω is recommended at this out- put. The pin can be left disconnected if unused. Thermal Protection The FAN2558/FAN2559 is designed to supply high peak output currents for brief periods, however sustained exces- sive output load at high input - output voltage difference will increase the device’s temperature and exceed maximum rat- ings due to power dissipation. During output overload condi- tions, when the die temperature exceeds the shutdown limit temperature of 150°C, an onboard thermal protection will disable the output until the temperature drops approximately 10°C below the limit, at which point the output is re-enabled. During a thermal shutdown, the user may assert the power- down function at the Enable pin, reducing power consump- tion to a minimum. Thermal Characteristics The FAN2558/FAN2559 is designed to supply 180mA at the specified output voltage with an operating die (junction) temperature of up to 125°C. Once the power dissipation and thermal resistance is known, the maximum junction tempera- ture of the device can be calculated. While the power dissipa- tion is calculated from known electrical parameters, the actual thermal resistance depends on the thermal characteris- tics of the SOT23-5 surface-mount package and the sur- rounding PC board copper to which it is mounted. The power dissipation is equal to the product of the input-to- output voltage differential and the output current plus the ground current multiplied by the input voltage, or: The ground pin current IGND can be found in the charts provided in the Electrical Characteristics section. The relationship describing the thermal behavior of the package is: where TJ(max) is the maximum allowable junction tempera- ture of the die, which is 125°C, and TA is the ambient operat- ing temperature. θJA is dependent on the surrounding PC board layout and can be empirically obtained. While the θJC (junction-to-case) of the SOT23-5 package is specified at 130°C /W, the θJA of the minimum PWB footprint will be at least 235°C /W. This can be improved by providing a heat sink of surrounding copper ground on the PWB. Depending on the size of the copper area, the resulting θJA can range from approximately 180°C /W for one square inch to nearly 130°C /W for 4 square inches. The addition of backside cop- per with through-holes, stiffeners, and other enhancements can also aid in reducing thermal resistance. The heat contrib- uted by the dissipation of other devices located nearby must be included in the design considerations. Once the limiting parameters in these two relationships have been determined, the design can be modified to ensure that the device remains within specified operating conditions. If overload conditions are not considered, it is possible for the device to enter a thermal cycling loop, in which the circuit enters a shutdown condition, cools, re-enables, and then again overheats and shuts down repeatedly due to an unmanaged fault condition. Adjustable Version The FAN2558ADJ includes an input pin ADJ which allows the user to select an output voltage ranging from 1V to near VIN, using an external resistor divider. The voltage VADJ pre- sented to the ADJ pin is fed to the onboard error amplifier which adjusts the output voltage until VADJ is equal to the onboard bandgap reference voltage of 1.00V(typ). The equa- tion is: Since the bandgap reference voltage is trimmed, 1% initial accuracy can be achieved. The total value of the resistor chain should not exceed 250KOhm total to keep the error amplifier biased during no-load conditions. Programming output voltages very near VIN need to allow for the magni- tude and variation of the dropout voltage VDO over load, sup- ply, and temperature variations. Note that the low-leakage FET input to the CMOS error amplifier induces no bias current error to the calculation. PD VIN VOUT – () I OUT VIN IGND × + × = PD max () TJ max () TA – θ JA ------------------------------- = VOUT 0.59V 1 R1 R2 ------ + × = |
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Description similaire - FAN2558S33X |
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