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SC470ITSTRT Datasheet(PDF) 9 Page - Semtech Corporation |
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SC470ITSTRT Datasheet(HTML) 9 Page - Semtech Corporation |
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9 / 31 page  9 2005 Semtech Corp. www.semtech.com SC470 POWER MANAGEMENT Application Information (Cont.) Output Voltage Selection The output voltage is set by the feedback resistors R3 & R7 of Figure 2 below. The internal reference is 1.5V, so the voltage at the feedback pin is multiplied by three to match the 1.5V reference. Therefore, the output can be set to a minimum of 0.5V. The equation for setting the output voltage is: Figure 2: Setting The Output Voltage Current Limit Circuit Current limiting of the SC470 can be accomplished in two ways. The on-state resistance of the low-side MOSFET can be used as the current sensing element or sense resistors in series with the low-side source can be used if greater accuracy is desired. R DS(ON) sensing is more efficient and less expensive. In both cases, the R ILIM resistor between the ILIM pin and LX pin set the over current threshold. This resistor R ILIM is connected to a 10µA current source within the SC470 which is turned on when the low side MOSFET turns on. When the voltage drop across the sense resistor or low side MOSFET equals the voltage across the RILIM resistor, positive current limit will activate. The high-side MOSFET will not be turned on until the voltage drop across the sense element (resistor or MOSFET) falls below the voltage across the R ILIM resistor. In an extreme over- current situation, the top MOSFET will never turn back on and eventually the part will latch off due to output undervoltage (see Output Under-voltage Protection). The current sensing circuit actually regulates the inductor valley current (see Figure 3). This means that if the current limit is set to 10A, the peak current through the inductor would be 10A plus the peak ripple current, and the average current through the inductor would be 10A plus 1/2 the peak-to-peak ripple current. The equations for setting the valley current and calculating the average current through the inductor are shown below: ILIMIT ILOAD IPEAK TIME Valley Current-Limit Threshold Point Figure 3: Valley Current Limiting The equation for the current limit threshold is as follows: A R R 10e I SENSE ILIM 6 - LIMIT • = Where (referring to Figure 8 on Page 17) R ILIM is R4 and R SENSE is the RDS(ON) of Q2. For resistor sensing, a sense resistor is placed between the source of Q2 and PGND. The current through the source sense resistor develops a voltage that opposes the voltage developed across R ILIM. When the voltage developed across the R SENSE resistor reaches the voltage R3 20k0 R7 20k0 EN/PSV TON VOUT VCCA FB PGD VSSA PGND DL VDDP ILIM LX DH BST U1 SC470 VOUT 0402 0402 C5 56p 0402 5 . 0 7 R 3 R 1 V OUT • + = |
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