Datasheet LTC3873 (Analog Devices) - 7
| Hersteller | Analog Devices |
| Beschreibung | No RSENSE Constant Frequency Current Mode Boost/Flyback/SEPIC DC/DC Controller |
| Seiten / Seite | 18 / 7 — OPERATION. Main Control Loop. Shunt Regulator. Start-Up/Shutdown. Figure … |
| Dateiformat / Größe | PDF / 267 Kb |
| Dokumentensprache | Englisch |
OPERATION. Main Control Loop. Shunt Regulator. Start-Up/Shutdown. Figure 1. SW Pin (Internal Sense Pin)
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LTC3873
OPERATION Main Control Loop
The LTC3873 can be used either by sensing the voltage The LTC3873 is a general purpose N-channel switching drop across the power MOSFET or by connecting the SW DC/DC converter for boost, flyback 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 efficiency, increases power density and reduces the cost of the MOSFET maximizes converter efficiency 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 on when the oscillator sets the PWM latch and is turned A built-in shunt regulator from the VCC pin to GND limits off when the current comparator resets the latch. The the voltage on the VCC pin to approximately 9.3V as long as divided-down output voltage is compared to an internal the shunt regulator is not forced to sink more than 25mA. 1.2V reference by the error amplifier, which outputs an The shunt regulator permits the use of a wide variety of error signal at the I powering schemes that exceed the LTC3873’s absolute TH pin. The voltage on the ITH pin sets the current comparator input threshold. When the maximum ratings. Further details on powering schemes load current increases, a fall in the V are described in the Application Information section. FB 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 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 transitions into and out of shutdown according to the state diagram shown in Figure 3. L D VIN VOUT The undervoltage lockout (UVLO) mechanism prevents the VCC + SW LTC3873 from trying to drive a MOSFET with insufficient COUT LTC3873 VSW voltage. The voltage at the VCC pin must exceed VTURNON NGATE GND GND 3873 F01 LTC3873
Figure 1. SW Pin (Internal Sense Pin)
SHUT DOWN
Connection for Maximum Efficiency
L D V VRUN/SS > VSHDN IN < VTURNOFF VRUN/SS < VSHDN V AND V IN VOUT (NOMINALLY 4V) (NOMINALLY 0.8V) IN > VTURNON (NOMINALLY 8.4V) V V SW CC + NGATE COUT LTC3873 SW LTC3873 GND RSENSE ENABLED GND 3873 F02 3873 F03
Figure 2. SW Pin (Internal Sense Pin) Connection for Sensing Resistor Figure 3. Start-Up/Shutdown State Diagram
3873fb For more information www.linear.com/LTC3873 7
OPERATION Main Control Loop
The LTC3873 can be used either by sensing the voltage The LTC3873 is a general purpose N-channel switching drop across the power MOSFET or by connecting the SW DC/DC converter for boost, flyback 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 efficiency, increases power density and reduces the cost of the MOSFET maximizes converter efficiency 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 on when the oscillator sets the PWM latch and is turned A built-in shunt regulator from the VCC pin to GND limits off when the current comparator resets the latch. The the voltage on the VCC pin to approximately 9.3V as long as divided-down output voltage is compared to an internal the shunt regulator is not forced to sink more than 25mA. 1.2V reference by the error amplifier, which outputs an The shunt regulator permits the use of a wide variety of error signal at the I powering schemes that exceed the LTC3873’s absolute TH pin. The voltage on the ITH pin sets the current comparator input threshold. When the maximum ratings. Further details on powering schemes load current increases, a fall in the V are described in the Application Information section. FB 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 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 transitions into and out of shutdown according to the state diagram shown in Figure 3. L D VIN VOUT The undervoltage lockout (UVLO) mechanism prevents the VCC + SW LTC3873 from trying to drive a MOSFET with insufficient COUT LTC3873 VSW voltage. The voltage at the VCC pin must exceed VTURNON NGATE GND GND 3873 F01 LTC3873
Figure 1. SW Pin (Internal Sense Pin)
SHUT DOWN
Connection for Maximum Efficiency
L D V VRUN/SS > VSHDN IN < VTURNOFF VRUN/SS < VSHDN V AND V IN VOUT (NOMINALLY 4V) (NOMINALLY 0.8V) IN > VTURNON (NOMINALLY 8.4V) V V SW CC + NGATE COUT LTC3873 SW LTC3873 GND RSENSE ENABLED GND 3873 F02 3873 F03
Figure 2. SW Pin (Internal Sense Pin) Connection for Sensing Resistor Figure 3. Start-Up/Shutdown State Diagram
3873fb For more information www.linear.com/LTC3873 7