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LTC3784 Datasheet(PDF) 11 Page - Linear Technology |
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LTC3784 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 28 page  LTC7820 11 7820fc For more information www.linear.com/LTC7820 OPERATION OPERATION programmed window around half of VHIGH_SENSE voltage or the VHIGH_SENSE and VLOW_SENSE voltages must be lower than 1V and 0.5V respectively. To clear the current fault, the voltage drop from ISENSE+ pin to ISENSE– pin has to be lower than 50mV. To clear temperature faults, the IC temperature has to be lower than 165°C. The FAULT pin may be pulled up by external resistors to voltages up to 80V. It can be used to control an external disconnect FET to isolate the input and output during fault conditions. High Side Current Sensing For over current protection, the LTC7820 uses a sense resistorRSENSEtomonitorthecurrent.Thesensingresistor has to be placed at the drain of the very top MOSFET M1. For voltage divider and inverter applications, the current flows into the drain of the MOSFET M1, so the ISENSE+ pin should be connected to the sensing resistor then to the drain of the MOSFET M1. For voltage doubler applications, the current flows out of the drain of the MOSFET M1, so the ISENSE+ pin should be connected directly to the drain of the MOSFET M1. See Typical Applications section for examples. In most applications, the current through the senseresistorisapulsecurrentandthepeakvalueismuch higher than the average load current. A RC filter on the ISENSE– pin, with a time constant lower than the switching frequency,maybeusedtosettheprecisionaveragecurrent protection. If overcurrent protection is not desired, short the ISENSE+ and ISENSE– pins together and connect them to the drain of the top MOSFET M1 directly. Frequency Selection Theselectionofswitchingfrequencyisatrade-offbetween efficiency and component size. Low frequency operation increasesefficiencybyreducingMOSFETswitchinglosses, but requires larger capacitance to maintain low output ripplevoltageandlowoutputimpedance.TheFREQpincan be used to program the controller’s operating frequency from 100kHz to 1MHz. There is a precision 10µA current flowing out of the FREQ pin, so the user can program the controller’s switching frequency with a single resistor to GND.ThevoltageontheFREQpinisequaltotheresistance multiplied by 10μA current (e.g. the voltage is 1V with a 100k resistor from the FREQ pin to GND). In the linear region, the switching frequency, fS, can be estimated based on the equation: fS (kHz) = RFREQ(kΩ) • 8 – 317kHz Figure 2 also shows the relationship between the voltage on the FREQ pin and switching frequency. Power Good and UV (PGOOD and UV pins) When the UV pin voltage is lower than 1V, the PGOOD pin is pulled low. The PGOOD pin is also pulled low when the RUN pin is low or when the LTC7820 is starting up. The PGOOD pin is released only when the LTC7820 is switching and UV pin is higher than 1V. The PGOOD pin will flag power bad immediately when the UV pin is low. However, there is an internal 20μs power good mask and 120mV hysteresis when UV goes higher than 1V. The PGOOD pin may be pulled up by external resistors to sources up to 45V. PGOOD signal can be used to enable or disable the output loads. If the loads are switching mode converters or LDOs with ENABLE/RUN pins, this allows easy for interfacing. With proper setup on the UV pin, PGOOD can enable the loads at the output when the output voltage is above a certain value. PGOOD can also be used to control the RUN pin of another LTC7820 if two or more parts are cascaded to achieve higher step-down ratios. Figure 2. Relationship Between Switching Frequency and Voltage at the FREQ Pin FREQ PIN VOLTAGE (V) 0 1400 1200 800 400 1000 600 200 0 2 1 7820 F02 2.5 1.5 0.5 |
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