LTC3873-5 OPERATIONMain Control Loop The LTC3873-5 can be used either by sensing the voltage The LTC3873-5 is a general purpose N-channel switching drop across the power MOSFET or by connecting the SW DC/DC converter for boost, fl yback and SEPIC applications. pin to a conventional sensing resistor in the source of the Its No R power MOSFET. Sensing the voltage across the power SENSE sensing technique improves effi ciency, increases power density and reduces the cost of the MOSFET maximizes converter effi ciency and minimizes the overall solution. component count; the maximum rating for this pin, 60V, allows MOSFET sensing in a wide output voltage range. For circuit operation, please refer to the Functional Diagram of the IC and the Typical Application on the front page. Shunt Regulator During normal operation, the power MOSFET is turned A built-in shunt regulator from the V on when the oscillator sets the PWM latch and is turned CC pin to GND limits the voltage on the V off when the current comparator resets the latch. The CC pin to approximately 9.3V as long as the shunt regulator is not forced to sink more than 25mA. divided-down output voltage is compared to an internal The shunt regulator permits the use of a wide variety of 1.2V reference by the error amplifi er, which outputs an powering schemes that exceed the LTC3873-5’s absolute error signal at the ITH pin. The voltage on the ITH pin maximum ratings. Further details on powering schemes sets the current comparator input threshold. When the are described in the Application Information section. load current increases, a fall in the VFB voltage relative to the reference voltage causes the ITH pin to rise, causing Start-Up/Shutdown the current comparator to trip at a higher peak inductor current value. The average inductor current will therefore The LTC3873-5 has two shutdown mechanisms to disable rise until it equals the load current, thereby maintaining and enable operation: an undervoltage lockout on the output regulation. VCC supply pin voltage and a threshold RUN/SS pin. The LTC3873-5 transitions into and out of shutdown according L D to the state diagram shown in Figure 3. V V IN OUT The undervoltage lockout (UVLO) mechanism prevents VCC + SW C the LTC3873-5 from trying to drive a MOSFET with in- OUT V LTC3873-5 SW suffi cient VGS. The voltage at the VCC pin must exceed NGATE GND GND 38735 F01 LTC3873-5 Figure 1. SW Pin (Internal Sense Pin) SHUT DOWN Connection for Maximum Effi ciency V L D V V RUN/SS > VSHDN IN < VTURNOFF RUN/SS < VSHDN AND V V V IN > VTURNON IN OUT (NOMINALLY 2.9V) (NOMINALLY 0.7V) (NOMINALLY 4.1V) V V SW CC + NGATE COUT LTC3873-5 RSL SW LTC3873-5 GND RSENSE ENABLED GND 38735 F02 38735 F03 Figure 2. SW Pin (Internal Sense Pin) Connection for Sensing ResistorFigure 3. Start-Up/Shut Down State Diagram 38735fb 7
