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TPS61059DRCR Datasheet(PDF) 12 Page - Texas Instruments |
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TPS61059DRCR Datasheet(HTML) 12 Page - Texas Instruments |
12 / 30 page www.ti.com Softstart Overvoltage Protection (OVP) Efficiency and Sense Voltage Thermal Shutdown TPS61058 TPS61059 SLVS572B – APRIL 2005 – REVISED DECEMBER 2005 DETAILED DESCRIPTION (continued) To avoid high inrush current during start-up, special care is taken to control the inrush current. When the device is first enabled, the output capacitor is charged with a constant pre-charge current of 115mA (typ) until either the output voltage is typically 0.1V below the input voltage or the feedback voltage is 500mV (typ). The rectifying switch is current limited during the pre-charge phase. This also limits the output current under short circuit conditions at the output. The fixed pre-charge current during start-up allows the device to start up without problems when driving single die white LEDs as long as the LED start-up current is set to a value lower than the pre-charge current (84 mA min.). Refer to the application section for more details. When the device has finished start-up and is ready for high current operation, the device forces IOK output to ground, starts switching and regulates the LED current to the desired value (e.g. torch or flashlight current level). As with any current source, the output voltage rises when the load becomes high impedance or gets disconnected. To prevent the output voltage exceeding the maximum switch voltage rating (7 V) of the main switch, an overvoltage protection circuit is integrated. As soon as the output voltage exceeds the OVP threshold, the converter stops switching and the output voltage decreases. When the output voltage falls below the OVP threshold, the converter continues operation until the output voltage exceeds the OVP threshold again. The voltage across the sense resistor (RS) has a direct effect of the converter efficiency. Because the sense voltage does not contribute to the output power (PLED), the lower this voltage the higher the efficiency. It is therefore recommended to operate with a sense voltage of approximately 0.75V at maximum LED current. An internal thermal shutdown is implemented and turns off the internal MOSFETs when the typical junction temperature of 140 °C is exceeded. The thermal shutdown has a hysteresis of typically 20°C. Refer to the Thermal Information section. 12 |
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