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ILC7280AR2830X Fiches technique(PDF) 7 Page - Fairchild Semiconductor |
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ILC7280AR2830X Fiches technique(HTML) 7 Page - Fairchild Semiconductor |
7 / 9 page PRODUCT SPECIFICATION ILC7280 REV. 1.0.5 6/3/02 7 Application Information VIN A and B These pins are connected internally through a galvanic connection. For maximum power from each regulator, both VINA and VINB must be connected externally to V+. Enable/Shutdown Forcing ENA and/or ENB to a voltage greater than 2V, enables the regulator(s). These inputs are CMOS logic compatible gates. If this feature is not required, connect ENAand/or ENB to VIN. Note that VINA and VINB are connected internally. To minimize the effect of imbalanced current sharing and possible noise, both VINA and VINB should also be connected externally. Input Capacitor A 1µF capacitor should be placed from VINA/B to GND if there is more than 10 inches of wire between the input and the ac filter capacitor or if a battery is used as the input. Reference Bypass Capacitor CNOISE (the reference voltage bypass capacitor) may be connected to the internal VREF which is common to regula- tor’s A and B. For low noise applications use of 1nF CNOISE is recom- mended. Value higher than 1nF will lead to minimum improvement of output noise, but it will substantially increase the start-up time. Lower value of CNOISE results in faster start –up. If a slow or delayed start up time is desired, a larger value of CNOISE is used. Conversely, faster start up times or instant-on applications will require smaller values of CNOISE or its omission with the pin left open. The trade- off of noise to response time should be considered. Output Capacitor An output capacitor is required from VOUTA and VOUTB to GND to prevent oscillation and minimize the effect of load transient currents. The minimum size of the output capaci- tor(s) is dependent on the usage of CNOISE and its value. Without CNOISE, a minimum of 0.47µF is recommended. For CNOISE = 1nF, a minimum of 2.2µF is recommended. Larger values of output capacitance will slightly slow the regulator’s response during power up. The ILC7280 remains stable even with ESR values as low as 10m Ω. If the system design calls for smaller load currents, lower capacitance may be used. Below 10mA the capacitance may be reduced to 0.33µF. No-load Stability The ILC7280 will remain stable and in regulation with no load current. These are desirable performance features for applications such as keep-alive modes in CMOS systems. Split-Supply Operation When using the ILC7280 in a system requiring that the load be returned to the negative voltage source, the output(s) must be diode clamped to inhibit significant voltage excursions below ground. A simple external diode clamp to ground will protect the device from damage. Thermal Considerations In order to minimize thermal resistance ( θJA), the device mounted on conventional FR4 PCB material should be surrounded as much ground copper ground plane as possible. In a worst case application with minimum trace widths and no ground plane, the MSOP-8 package exhibits a thermal resistance of 200 °C/W. The maximum allowable power dissipation is calculated in the following examples. Thermal Evaluation Examples The maximum allowed package power dissipation is: PD(max) =(TJmax–TA) / θJA, where TJmax is the maximum junction temperature and TA is the ambient temperature. For an ambient temperature of 50°C PD(max) = (150°C - 50°C) / 200°C/W PD(max) = 500mW If the intent is to operate from a 4V power source with a 150mA load current from both outputs at a 50°C ambient temperature, the expected power dissipation is found in the following calculation: PD (each regulator) = (VIN – VOUT) * IOUT + (VIN * IGND) PD (each regulator) = (4V – 3V) * 150mA + (4V * 0.12mA) PD (each regulator) = 150mW PD (both regulators) = 2 * 150mW PD (both regulators) = 300mW In this example the total power dissipated is 300mW which is below the 500mW maximum package consideration and therefore safe to operate. It should be noted that it is not always possible to operate both regulators at the maximum output current. |
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