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LTC3603 Datasheet(PDF) 8 Page - Linear Technology |
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LTC3603 Datasheet(HTML) 8 Page - Linear Technology |
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8 / 16 page  LTC3621/LTC3621-2 8 3621f For more information www.linear.com/LTC3621 operaTion VIN Overvoltage Protection In order to protect the internal power MOSFET devices against transient voltage spikes, the LTC3621 constantly monitors the VIN pin for an overvoltage condition. When VIN rises above 19V, the regulator suspends operation by shutting off both power MOSFETs. Once VIN drops below 18.7V, the regulator immediately resumes normal opera- tion. The regulator executes its soft-start function when exiting an overvoltage condition. Low Supply Operation The LTC3621 incorporates an undervoltage lockout circuit which shuts down the part when the input voltage drops Output Voltage Programming For non-fixed output voltage parts, the output voltage is set by external resistive divider according to the following equation: VOUT =0.6V • 1+ R2 R1 The resistive divider allows the FB pin to sense a fraction of the output voltage as shown in Figure 1. IRMS ≅IOUT(MAX) VOUT VIN VIN VOUT –1 This formula has a maximum at VIN = 2VOUT, where: IRMS ≅ IOUT 2 This simple worst-case condition is commonly used for design because even significant deviations do not offer much relief. Note that ripple current ratings from capacitor manufacturers are often based on only 2000 hours of life which makes it advisable to further derate the capacitor, or choose a capacitor rated at a higher temperature than required.Severalcapacitorsmayalsobeparalleledtomeet size or height requirements in the design. For low input voltage applications, sufficient bulk input capacitance is needed to minimize transient effects during output load changes. Output Capacitor (COUT) Selection The selection of COUT is determined by the effective series resistance(ESR)thatisrequiredtominimizevoltageripple and load step transients as well as the amount of bulk capacitance that is necessary to ensure that the control below 2.7V. As the input voltage rises slightly above the undervoltage threshold, the switcher will begin its basic operation. However, the RDS(ON) of the top and bottom switch will be slightly higher than that specified in the electrical characteristics due to lack of gate drive. Refer to graph of RDS(ON) versus VIN for more details. Soft-Start TheLTC3621hasaninternal800µssoft-startramp.During start-up soft-start operation, the switcher will operate in pulse-skipping mode. applicaTions inForMaTion VOUT R2 R1 3621 F01 CFF LTC3621 SGND FB Figure 1. Setting the Output Voltage Input Capacitor (CIN) Selection The input capacitance, CIN, is needed to filter the square wave current at the drain of the top power MOSFET. To prevent large voltage transients from occurring, a low ESR input capacitor sized for the maximum RMS current should be used. The maximum RMS current is given by: |
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