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RT8101AGS Datasheet(PDF) 9 Page - Richtek Technology Corporation |
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RT8101AGS Datasheet(HTML) 9 Page - Richtek Technology Corporation |
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9 / 15 page  9 RT8101/A Preliminary DS8101/A-01 March 2007 www.richtek.com Application Information Power On Reset The RT8101/A automatically initializes upon applying of input power VCC. The power on reset function (POR) continually monitors the input bias supply voltage at the VCC pin. The POR trip level is typically 9.6V at VCC rising. VIN Detection After POR the RT8101/A continuously generates a 10kHz pulse train with 1 μs pulse width to turn on the upper MOSFET for detecting the existence of VIN. RT8101/A keeps monitoring PHASE pin voltage during the detection period. As soon as the PHASE voltage crosses 1.5V two times, VIN existence is recognized and the RT8101/A initiates its soft start cycle as described in next section. Figure 1 Soft Start A built-in soft-start is used to prevent surge current from VIN to VOUT during power on. After the existence of VIN is detected, soft-start (SS) begins automatically. The feedback voltage (VFB) is clamped by internal linear ramping up SS voltage, causing PWM pulse width increasing slowly and thus inducing little surge current. Soft-start completes when SS voltage exceeds internal reference voltage (0.8V), the time duration is about 3.2ms. Over Current Protection The RT8101/A senses the current flowing through lower MOSFET for over current protection (OCP) by sensing the PHASE pin voltage as shown in the Functional Block Diagram. + - VIN POR_H PHASE UGATE PHASE_M 1.5V Internal Counter will count (VPHASE > 1.5V) two times (rising & falling) to recognize when VIN is ready. 1st 2nd PHASE waveform A 30 μA current source flows through the internal resistor 21.6k Ω to PHASE pin causing 0.65V voltage drop across the resistor. OCP is triggered if the voltage at PHASE pin (drop of lower MOSFET VDS) is lower than −0.25V when low side MOSFET conducting. Accordingly inductor current threshold for OCP is a function of conducting resistance of lower MOSFET RDS(ON) as : OCSET DS(ON) 30 A 21.6k-0.4V I R μ × = If MOSFET with RDS(ON) = 10m Ω is used, the OCP threshold current is about 25A. Once OCP is triggered, the RT8101/A enters hiccup mode and re-soft starts again. The RT8101/A shuts down after OCP hiccups twice. OCP Time (2.5ms/Div) UGATE (10V/Div) IOUT (10A/Div) OCP Time (2.5ms/Div) UGATE (10V/Div) IOUT (10A/Div) Figure 3. Power On then Shorted Figure 4. Shorted then Power On |
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