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G924-285T1UF Datasheet(PDF) 7 Page - Global Mixed-mode Technology Inc |
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G924-285T1UF Datasheet(HTML) 7 Page - Global Mixed-mode Technology Inc |
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7 / 9 page  Ver: 0.2 Preliminary Oct 01, 2004 TEL: 886-3-5788833 http://www.gmt.com.tw 7 G924 Global Mixed-mode Technology Inc. Over Current Protection The G924 uses a current mirror to monitor the output current. A small portion of the PMOS output transis- tor’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 1.20V 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 circuit 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 G924 is set to 550mA. Note that the input bypass ca- pacitor 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 ac- tual voltage at the IN pin may exceed the absolute maximum rating. Over Temperature Protection To prevent abnormal temperature from occurring, the G924 has a built-in temperature monitoring circuit. When it detects the temperature is above 145oC, the output transistor is turned off. When the IC is cooled down to below 120oC, the output is turned on again. In this way, the G924 will be protected against abnormal junction temperature during operation. Shutdown Mode When the SHDN pin is connected a logic low voltage, the G924 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 dis- charged to ground through the internal resistor voltage divider. Operating Region and Power Dissipation Since the G924 is a linear regulator, its power dissipa- tion 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 the G924 die and the ambient air, θJA, is the thermal re- sistance of the chosen package to the ambient air. For surface mount device, heat sinking is accomplished by using the heat spreading capabilities of the PC board and its copper traces. In the case of a SOT-23-5 package, the thermal resistance is typically 240oC/Watt. (See Recommended Minimum Footprint). Refer to “Safe Operating Area” of the Typical Per- formance Characteristics is the G924 valid operating region & refer to “Maximum Power Dissipation vs. Temperature” is the maximum power dissipation of SOT-23-5. The die attachment area of the G924’s lead frame is connected to pin 2, which is the GND pin. Therefore, the GND pin of G924 can carry away the heat of the G924 die very effectively. To improve the power dis- sipation, 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 G924. Larger input ca- pacitor 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 lo- cated several inches from the power source. Power-Supply Rejection and Operation from Sources Other than Batteries The G924 is designed to deliver low dropout voltages and low quiescent currents in battery powered sys- tems. Power-supply rejection is 53dB at low frequen- cies as the frequency increases above 20kHz; the output capacitor is the major contributor to the rejec- tion of power-supply noise. When operating from sources other than batteries, improve supply-noise rejection and transient response by increasing the values of the input and output ca- pacitors, and using passive filtering techniques. Load Transient Considerations The G924 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 10mA to 300mA is 30mV. Increasing the output capacitor's value and decreasing its ESR attenuates 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. Be- cause the G924 use a P-channel MOSFET pass tran- sistor, their dropout voltage is a function of RDS(ON) multiplied by the load current cause the G924 use a P-channel MOSFET pass transistor, their dropout voltage is a function of RDS(ON) multiplied by the load current. |
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