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SC483ITSTRT Datasheet(PDF) 9 Page - Semtech Corporation |
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SC483ITSTRT Datasheet(HTML) 9 Page - Semtech Corporation |
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9 / 27 page  9 2005 Semtech Corp. www.semtech.com SC483 POWER MANAGEMENT ensure, since this is a soft discharge, that there are no dangerous negative voltage excursions to be concerned about. In order for the soft discharge circuitry to function correctly, the chip supply must be present. Output Voltage Selection The output voltage (OUT2 shown) is set by the feedback resistors R9 & R13 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: 5 . 0 13 R 9 R 1 V OUT • + = 0402 0402 C15 56p 0402 VOUT2 VSSA2 R9 20k0 R13 14k3 VOUT EN/PSV2 8 TON2 9 VOUT2 10 VCCA2 11 FB2 12 PGD2 13 VSSA2 14 PGND2 15 DL2 16 VDDP2 17 ILIM2 18 LX2 19 DH2 20 BST2 21 U1 SC483 OUT2 Figure 2: Setting The Output Voltage Current Limit Circuit Current limiting of the SC483 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 SC483 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 Undervoltage 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-to-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 16) R ILIM is R10 and R SENSE is the RDS(ON) of Q4. For resistor sensing, a sense resistor is placed between the source of Q4 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 drop across R ILIM, a positive over-current exists and the high side MOSFET will not be allowed to turn on. When using an external sense resistor R SENSE is the resistance of the sense resistor. |
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