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FAN25800 Datasheet(PDF) 8 Page - ON Semiconductor |
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FAN25800 Datasheet(HTML) 8 Page - ON Semiconductor |
8 / 11 page © 2014 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN25800 • Rev. 1.5 7 Circuit Description The FAN25800 is a linear low-dropout (LDO) regulator that has high PSRR and low output noise. The enable control pin can be used to shut down the device and disconnect the output load from the input. During shutdown, the supply current drops below 1 µA. The LDO is designed to be stable with space-saving ceramic capacitors as small as 0201 case size. Enable and Soft-Start When EN is LOW, all circuits are off and the IC draws <550 nA of current. The EN pin does not have an internal pull-down resistor and must not be left floating. When EN is HIGH and VIN is above the UVLO threshold, the regulator begins a soft-start cycle for the output. The soft-start cycle controls inrush current, limiting it to the ILIM peak current limit. Short-Circuit and Thermal Protection The output current is short-circuit protected. When an output fault occurs, the output current is automatically limited to ILIM and VOUT drops. The resultant VOUT is equal to ILIM multiplied by the fault impedance. Short-circuit fault or output overload may cause the die temperature to increase and exceed the maximum rating due to power dissipation. In such cases (depending upon the ambient temperature; the VIN, load current, and thermal resistance (θJB) of the mounted die), the device may enter thermal shutdown. If the die temperature exceeds the thermal shutdown temperature threshold, the onboard thermal protection disables the output until the temperature drops below its hysteresis value. At that point, the output is re-enabled and a new soft-start sequence occurs. Thermal Considerations For best performance, the die temperature and the power dissipated should be kept at moderate values. The maximum power dissipated can be evaluated based on the following relationship: JA A J D T T P (max) (max) (1) where TJ(max) is the maximum allowable junction temperature of the die; TA is the ambient operating temperature; and θJA is dependent on the surrounding PCB layout and can be improved by providing a heat sink of surrounding copper ground. The addition of backside copper with through-holes, stiffeners, and other enhancements can help red uce θJA. The heat contributed by the dissipation of devices nearby must be included in design considerations. Capacitor Selection An output capacitor with an effective capacitance between 400 nF and 15 µF is required for loop stability. The ESR value should be within 3 to 100 m . DC bias characteristics of the capacitors must be considered when selecting the voltage rating and the case size of the capacitor. Figure 16 is a typical derating curve for a 0201 case size, 1.5 µF, 6.3 V, X5R capacitor. Figure 16. Capacitor DC Bias Characteristics Typical Application for Post Regulation Due to its high PSRR and low output noise, the FAN25800 can be used as a post-DC-DC regulator to reduce output ripple and output noise at high efficiency for noise-sensitive applications. Figure 17 shows a post- DC-DC regulation of the LDO with a buck converter. The capacitor on the output of the buck converter can be shared by the LDO as its input capacitor. DC-DC Converter FAN25800 EN EN L C2 C1 C3 VIN VOUT 3.6 V 3.3 V 5.0 V VIN SW GND GND Figure 17. LDO as Post DC-DC Regulator PCB Layout Recommendations Capacitors should be placed as close to the IC as possible. All power and ground pins should be routed to their capacitors using top copper. The copper area connecting to the IC should be maximized to improve thermal performance. Figure 18. Recommended Layout |
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