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FAN1540DX Fiches technique(PDF) 11 Page - Fairchild Semiconductor |
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FAN1540DX Fiches technique(HTML) 11 Page - Fairchild Semiconductor |
11 / 15 page 11 www.fairchildsemi.com FAN1539/FAN1540 Rev. 1.1.1 Applications Information General Circuit Description The FAN1539/FAN1540 is an advanced low dropout volt- age regulator, specially designed for applications in por- table computers, where high performance and low quiescent current are required. The device has an inter- nal trimmed bandgap voltage reference and an internal output voltage sense divider. These two signals form the input to the error amplifier which regulates the output voltage. The FAN1539/FAN1540 has a complete set of internal protection circuitry including thermal shutdown, short cir- cuit current limit and electrostatic discharge protection. Low ESR ceramic capacitors are needed for input as well as output pins to maintain the circuit stability. Short Circuit Current Limit The device has internal over-current limit and short cir- cuit protection. Under over-current conditions the device current is determined by the current limit threshold. Once the device is released from short circuit conditions, the normal level of current limit is gradually re-established as the device output voltage reaches normal levels. Special circuitry has been added to ensure that recovery from short circuit current conditions does not lead to exces- sive overshoot of the output voltage — a phenomenon often encountered in conventional regulators. Thermal Protection The FAN1539/FAN1540 is designed to supply at least 1A/1.3A output currents. Excessive output load at high input-output voltage difference will cause the device tem- perature to increase and exceed maximum ratings due to power dissipation. During output overload conditions, when the die temperature exceeds the shutdown limit temperature of 160°C, an onboard thermal protection will disable the output until the temperature drops approxi- mately 15°C below the limit, at which point the output is re-enabled. Thermal Characteristics The FAN1539/FAN1540 is designed to supply at least 1A/1.3A 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 temperature of the device can be cal- culated. While the power dissipation is calculated from known electrical parameters, the actual thermal resis- tance depends on the thermal characteristics of the cho- sen package and the surrounding 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 temper- ature of the die, which is 150°C, and TA is the ambient operating temperature. θJA is dependent on the sur- rounding PC board layout and can be empirically obtained. While the θJC (junction-to-case) of the 6-lead MLP package is specified at 8°C /W, the θJA for a mini- mum PWB footprint will be in substantially higher. This can be improved upon by providing a heat sink of sur- rounding copper ground on the PWB. Depending on the size of the copper area, and the thickness of the copper layer, the resulting θJA can vary over a wide range. The addition of backside copper with through-holes, stiffen- ers, and other enhancements can also aid in reducing thermal resistance. Thermal simulations performed on a thermally optimized board layout indicate that θJA as low as 20°C /W can be achieved. For example, the heat contributed by the dissipation of other devices located nearby must be included in the design consider- ations. Overload conditions also need to be 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 a persistent fault condition. Capacitor ESR and Printed Circuit Board Layout The FAN1539/FAN1540 has been optimized to accom- modate low ESR bypass capacitors down to 0 m Ω. For best results it is important to place both input and output bypass capacitors as near to the input and output pins as possible. Use of X7R types such as Murata’s GRM31CR70J106KA01B (10 µF) and GRM43ER71A226KE01B (22 µF) or similar compo- nent from TDK. The capacitors should connect directly to the ground plane. Use of ground plane on the top and the bottom side of the PCB is recommended. As many vias as possible should be used to minimize ground plane resistance. P D V IN V OUT – ()I OUT V INIGND + = P D max () T J max () TA – θ JA -------------------------------- = |
Numéro de pièce similaire - FAN1540DX |
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Description similaire - FAN1540DX |
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