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LTC3873ETS8-PBF Datasheet(PDF) 7 Page - Linear Technology |
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LTC3873ETS8-PBF Datasheet(HTML) 7 Page - Linear Technology |
7 / 16 page LTC3873 7 3873fa OPERATION Main Control Loop The LTC3873 is a general purpose N-channel switching DC/DC converter for boost, flyback and SEPIC applications. Its No RSENSE sensing technique improves efficiency, increases power density and reduces the cost of the overall solution. For circuit operation, please refer to the Functional Diagram of the IC and the Typical Application on the front page. During normal operation, the power MOSFET is turned on when the oscillator sets the PWM latch and is turned off when the current comparator resets the latch. The divided-down output voltage is compared to an internal 1.2V reference by the error amplifier, which outputs an error signal at the ITH pin. The voltage on the ITH pin sets the current comparator input threshold. When the load current increases, a fall in the VFB voltage relative to the reference voltage causes the ITH pin to rise, causing the current comparator to trip at a higher peak inductor current value. The average inductor current will therefore rise until it equals the load current, thereby maintaining output regulation. The LTC3873 can be used either by sensing the voltage drop across the power MOSFET or by connecting the SW pin to a conventional sensing resistor in the source of the power MOSFET. Sensing the voltage across the power MOSFET maximizes converter efficiency and minimizes the component count; the maximum rating for this pin, 60V, allows MOSFET sensing in a wide output voltage range. Shunt Regulator A built-in shunt regulator from the VCC pin to GND limits the voltage on the VCC pin to approximately 9.3V as long as the shunt regulator is not forced to sink more than 25mA. The shunt regulator permits the use of a wide variety of powering schemes that exceed the LTC3873’s absolute maximum ratings. Further details on powering schemes are described in the Application Information section. Start-Up/Shutdown The LTC3873 has two shutdown mechanisms to disable and enable operation: an undervoltage lockout on the VCC supply pin voltage and a threshold RUN/SS pin. The LTC3873 transitions into and out of shutdown according to the state diagram shown in Figure 3. The undervoltage lockout (UVLO) mechanism prevents the LTC3873 from trying to drive a MOSFET with insufficient voltage. The voltage at the VCC pin must exceed VTURNON Figure 1. SW Pin (Internal Sense Pin) Connection for Maximum Efficiency Figure 2. SW Pin (Internal Sense Pin) Connection for Sensing Resistor Figure 3. Start-Up/Shutdown State Diagram COUT VSW VOUT VIN GND L D + NGATE GND VCC SW 3873 F01 LTC3873 COUT VSW RSENSE VOUT VIN GND L D + NGATE GND VCC SW 3873 F02 LTC3873 LTC3873 SHUT DOWN VRUN/SS < VSHDN (NOMINALLY 0.8V) 3873 F03 VIN < VTURNOFF (NOMINALLY 4V) VRUN/SS > VSHDN AND VIN > VTURNON (NOMINALLY 8.4V) LTC3873 ENABLED |
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