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LTC3725EMSE Datasheet(PDF) 6 Page - Linear Technology |
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LTC3725EMSE Datasheet(HTML) 6 Page - Linear Technology |
6 / 20 page LTC3725 6 3725f PI FU CTIO S UVLO (Pin 1): Undervoltage Lockout. Connect to a resis- tive voltage divider to monitor input voltage VIN. Enables converter operation for VUVLO > 1.242V. Hysteresis is a fixed 16mV hysteresis voltage with a 4.9 µA hysteresis current that combines with the Thevenin resistance of the divider to set the total UVLO hysteresis voltage. This input also senses VIN for voltage feedforward. Finally, this pin can be used for external run/stop control. SSFLT (Pin 2): Combination Soft-Start and Fault Indica- tor. A capacitor to GND sets the duty cycle ramp-up rate during start-up. To indicate a fault, the SSFLT pin is momentarily pulled up to within 1.3V of VCC. NDRV (Pin 3): Drive for the External NMOS Linear Regu- lator. Connect to the gate of the NMOS and connect a pull up resistor to the input voltage (VIN). Optionally, to create a trickle charger omit the NMOS device and connect NDRV to VCC. FB/IN+ (Pin 4): This pin has several functions. One wind- ing of a pulse transformer is connected to the FB/IN+ and FS/IN– pins. The other pulse transformer winding is con- nected to the LTC3706. The LTC3725 automatically de- tects when the LTC3706 applies a pulse-encoded signal to the FB/IN+ and FS/IN– pins and decodes duty cycle infor- mation for control of the primary-side gate drive (see Operation below). In secondary-side control, primary- side gate drive bias power is also extracted from the FB/IN+ and FS/IN– pins using an on-chip full-wave rectifier. For primary-side control connect this pin to an optoisolator for feedback control of converter output voltage using an internal optoisolator biasing network. FS/IN– (Pin 5): This pin has several functions. Place a resistor from this pin to GND to set the oscillator fre- quency. For secondary-side control with the LTC3706, connect one winding of the pulse transformer for opera- tion as described for the FB/IN+ pin above. PGND (Pin 6): Supply Return for the Bottom Gate Driver and the On-Chip Bridge Rectifier. GATE (Pin 7): Gate Drive. Connect to the gate of the external MOSFET. VCC (Pin 8): Main VCC Power for All Driver and Control Circuitry. VSLMT (Pin 9): Volt-Second Limit. Form an R-C integrator by connecting a resistor from VIN to VSLMT and a capacitor from VSLMT to ground. The gate drive is turned off when the voltage on the VSLMT pin exceeds 1.26V. IS (Pin 10): Input to the Overcurrent Comparator. Connect to the positive terminal of a current-sense resistor in series with the source of the ground-referenced bottom MOSFET. GND (Pin 11): Signal Ground. |
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