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SS8014-33GTR Folha de dados(PDF) 8 Page - Silicon Standard Corp. |
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SS8014-33GTR Folha de dados(HTML) 8 Page - Silicon Standard Corp. |
8 / 10 page www.SiliconStandard.com 8 of 10 SS8014-xxG Over Current Protection The SS8014 uses a current mirror to monitor the output current. A small portion of the PMOS output transistor’s current is mirrored onto a resistor such that the voltage across this resistor is proportional to the output current. This voltage is compared against the 1.25V reference. Once the output current exceeds the limit, the PMOS output transistor is turned off. Once the output transistor is turned off, the current monitoring voltage decreases to zero, and the output PMOS is turned on again. If the over current condition persist, the over current protection cir- cuit will be triggered again. Thus, when the output is shorted to ground, the output current will be alternating between 0 and the over current limit. The typical over current limit of the SS8014 is set to 350mA. Note that the input bypass capacitor of 1µF must be used in this case to filter out the input voltage spike caused by the surge current due to the inductive effect of the package pin and the printed circuit board’s routing wire. Otherwise, the actual voltage at the IN pin may exceed the absolute maximum rating. Over Temperature Protection To prevent abnormal temperature from occurring, the SS8014 has a built-in temperature monitoring circuit. When it detects the temperature is above 150 oC, the output transistor is turned off. When the IC is cooled down to below 135 oC, the output is turned on again. In this way, the SS8014 will be protected against abnor- mal junction temperature during operation. Shutdown Mode When the SHDN pin is connected a logic low voltage, the SS8014 enters shutdown mode. All the analog cir- cuits are turned off completely, which reduces the current consumption to only the leakage current. The output is disconnected from the input. When the output has no load at all, the output voltage will be discharged to ground through the internal resistor voltage divider. Operating Region and Power Dissipation Since the SS8014 is a linear regulator, its power dissi- pation is always given by P = IOUT (VIN – VOUT). The maximum power dissipation is given by: PDMAX = (TJ – TA)/ ΘJA = (150-25) / 240 = 520mW where (TJ – TA) is the temperature difference between the SS8014 die and the ambient air, and θ JA, is the thermal resistance of the chosen package to the ambient air. For surface mount devices, heat sinking is accomplished by using the heat spreading capabilities of the PC board and its copper traces. In the case of a SOT23-5 package, the thermal resistance is typically 240 oC/Watt. (See Rec- ommended Minimum Footprint) [Figure 2]. Refer to Fig- ure 3 for the SS8014 valid operating region (Safe Op- erating Area) & refer to Figure 4 for the maximum power dissipation of the SOT-23-5. The die attachment area of the SS8014’s lead frame is connected to pin 2, which is the GND pin. Therefore, the GND pin of SS8014 can carry away the heat of the SS8014 die very effectively. To improve the power dissipation, connect the GND pin to ground using a large ground plane near the GND pin. Applications Information Capacitor Selection and Regulator Stability Normally, use a 1µF capacitor on the input and a 1µF capacitor on the output of the SS8014. Larger input capacitor values and lower ESR provide better sup- ply-noise rejection and transient response. A higher- value input capacitor (10µF) may be necessary if large, fast transients are anticipated and the device is located several inches from the power source. For stable opera- tion over the full temperature range, with load currents up to 120mA, a minimum of 1µF is recommended. Power-Supply Rejection and Operation from Sources Other than Batteries The SS8014 is designed to deliver low dropout volt- ages and low quiescent currents in battery powered sys- tems. Power-supply rejection is 57dB at low frequencies as the frequency increases above 20 kHz; the output capacitor is the major contributor to the rejection of power-supply noise. When operating from sources other than batteries, im- prove supply-noise rejection and transient response by increasing the values of the input and output capacitors, and using passive filtering techniques. Load Transient Considerations The SS8014 load-transient response graphs show two components of the output response: a DC shift of the output voltage due to the different load currents, and the transient response. Typical overshoot for step changes in the load current from 0mA to 100mA is 12mV. Increasing the output capacitor's value and decreasing its ESR at- tenuates transient spikes. Input-Output (Dropout) Voltage A regulator's minimum input-output voltage differential (or dropout voltage) determines the lowest usable supply voltage. In battery-powered systems, this will determine the useful end-of-life battery voltage. Because the SS8014 uses a P-channel MOSFET pass transistor, the dropout voltage is a function of RDS(ON) multiplied by the load current. 1/12/2005 Rev.2.10 |
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