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FAN2534SX Datasheet(PDF) 5 Page - Fairchild Semiconductor |
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FAN2534SX Datasheet(HTML) 5 Page - Fairchild Semiconductor |
5 / 11 page REV. 1.0.2 8/26/03 5 PRODUCT SPECIFICATION FAN2534/FAN2535 Functional Description Utilizing CMOS technology, the FAN2534/FAN2535 product family is optimized for use in compact battery powered systems, offering a unique combination of high ripple rejection, low noise, low power consumption, extremely low dropout voltages, high tolerance for a variety of output capacitors, and less than 1µA "OFF" current. In the circuit, a differential amplifier controls a series-pass P-Channel MOSFET, and a separate error amplifier com- pares the load voltage at the output with an onboard trimmed low voltage bandgap reference. The series resistance of the pass P-Channel MOSFET is approximately 1 Ohm, yielding an unusually low dropout voltage even under high load conditions. Thermal shutdown and current limit circuits protect the device under extreme conditions. When the device tempera- ture reaches 155°C, the output is disabled, until the device cooles down by 15°C, then re-enabled. The user can to shut down the device using the Enable control pin at any time. The current limit circuit is trimmed, which leads to consis- tent power on /enable delays, and provides safe short circuit current densities even in narrow traces of the PCB. A carefully optimized control loop accommodates a wide range of ESR values in the output bypass capacitor, allowing the user to optimize space, cost, and performance require- ments. An Enable pin shuts down the regulator output to conserve power, reducing supply current to less than 1µA. The output can then be re-Enabled within 150µSec, fulfilling the fast power-cycling needs of CDMA applications. Depending on the device type selected, other control and status functions are available at pin 4. The fixed-voltage versions are available with either a noise-bypass pin or an Error flag pin option. The error flag can be used as a diagnos- tic function to indicate that the output voltage has dropped more than 5% below the nominal value. The adjustable-voltage versions utilize pin 4 to connect to an external voltage divider which feeds back to the regulator error amplifier, thus setting the output voltage to the desired value. Applications Information External Capacitors – Selection The FAN2534/FAN2535 allows the user to utilize a wide variety of capacitors compared to other LDO products. An innovative design approach offers significantly reduced sensitivity to ESR (Equivalent Series Resistance), which degrades regulator loop stability in older designs. While the improvements featured in the FAN2534/FAN2535 family greatly simplify the design task, capacitor quality still must be considered if the designer is to achieve optimal circuit performance. In general, ceramic capacitors offer superior ESR performance, and a smaller case size than tantalums. Input Capacitor An input capacitor of 2.2µF (nominal value) or greater, connected between the Input pin and Ground, placed in close proximity to the device, will improve transient response and ripple rejection. Higher values will further improve ripple rejection and transient response. An input capacitor is recommended when the input source, either a battery or a regulated AC voltage, is located far from the device. Any good quality ceramic, tantalum, or metal film capacitor will give acceptable performance, however in extreme cases capacitor surge current ratings may have to be considered. Output Capacitor An output capacitor is required to maintain regulator loop stability. Stable operation will be achieved with a wide variety of capacitors with ESR values ranging from 0m Ω up to 400m Ω. Multilayer ceramic, tantalum or aluminum electrolytic capacitors may be used. A nominal value of at least 1µF is recommended. Note that the choice of output capacitor effects load transient response, ripple rejection, and it has a slight effect on noise performance as well. Bypass Capacitor (FAN2534-XX Only) In the fixed-voltage configuration, connecting a capacitor between the bypass pin and ground can significantly reduce output noise. Values ranging from 0pF to 47nF can be used, depending on the sensitivity to output noise in the application. At the high-impedance Bypass pin, care must be taken in the PCB layout to minimize noise pickup, and capacitors must be selected to minimize current loading (leakage). Noise pickup from external sources can be considerable. Leakage currents into the Bypass pin will directly affect regulator accuracy and should be kept as low as possible; thus, high- quality ceramic and film types are recommended for their low leakage characteristics. Cost-sensitive applications not concerned with noise can omit this capacitor. |
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