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RT2855ALGQW Datasheet(PDF) 7 Page - Richtek Technology Corporation |
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RT2855ALGQW Datasheet(HTML) 7 Page - Richtek Technology Corporation |
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7 / 16 page  RT2855A/B 7 DS2855A/B-01 September 2015 www.richtek.com © Copyright 2015 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation. Soft-Start (SS) The RT2855A/B soft-start uses an external pin (SS) to clamp the output voltage and allow it to slowly rise. After VEN is high and VREG5 exceeds its UVLO threshold, the IC begins to source 6 μA from the SS pin. An external capacitor at SS is used to adjust the soft-start timing. The available capacitance range is from 2.7nF to 220nF. Do not leave SS unconnected. During start-up, while the SS capacitor charges, the RT2855A/B operate in discontinuous mode with very small pulses. This prevents negative inductor currents and keeps the circuit from sinking current. Therefore, the output voltage may be pre-biased to some positive level before start-up. Once the VSS ramp charges enough to raise the internal reference above the feedback voltage, switching will begin and the output voltage will smoothly rise from the pre-biased level to its regulated level. After VSS rises above about 2.2V output over-and under-voltage protections are enabled and the RT2855B begins continuous-switching operation. An internal linear regulator (VREG5) produces a 5.1V supply from VIN that powers the internal gate drivers, PWM logic, reference, analog circuitry, and other blocks. If VIN is 6V or greater, VREG5 is guaranteed to provide significant power for external loads. PGOOD Comparator PGOODis an open drain output controlled by a comparator connected to the feedback signal. If FB exceeds 90% of the internal reference voltage, PGOOD will be high impedance. Otherwise, the PGOOD output is connected to PGND. External Bootstrap Capacitor Connect a 0.1 μF low ESR ceramic capacitor between BOOT and SW. This bootstrap capacitor provides the gate driver supply voltage for the high side N-channel MOSFET switch. Over Temperature Protection The RT2855A/B includes an over temperature protection (OTP) circuitry to prevent overheating due to excessive power dissipation. The OTP will shut down switching operation when the junction temperature exceeds 150 °C. Once the junction temperature cools down by approximately 20 °C the IC will resume normal operation with a complete soft-start. For continuous operation, provide adequate cooling so that the junction temperature does not exceed 150 °C. Output Discharge Control When EN pin is low, the RT2855A/B will discharge the output with an internal 50 Ω MOSFET connected between VOUT to GND pin. OVP/UVP Protection The RT2855A/B detects over and under voltage conditions by monitoring the feedback voltage on FB pin. The two functions are enabled after approximately 1.7 times the soft-start time. When the feedback voltage becomes higher than 120% of the target voltage, the OVP comparator will go high to turn off both internal high side and low side MOSFETs. When the feedback voltage is lower than 70% of the target voltage for 250 μs, the UVP comparator will go high to turn off both internal high side and low side MOSFETs. |
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