Datasheet LTC3803-3 (Analog Devices) - 8

LTC3803-3
OPERATION
The undervoltage lockout (UVLO) mechanism prevents regulator from the VCC pin to GND will draw as much the LTC3803-3 from trying to drive a MOSFET with in- current as needed through this resistor to regulate the suffi cient VGS. The voltage at the VCC pin must exceed VCC voltage to around 9.5V as long as the VCC pin is not VTURNON (nominally 8.7V) at least momentarily to enable forced to sink more than 25mA. This shunt regulator is LTC3803-3 operation. The VCC voltage is then allowed always active, even when the LTC3803-3 is in shutdown, to fall to VTURNOFF (nominally 5.7V) before undervoltage since it serves the vital function of protecting the VCC pin lockout disables the LTC3803-3. This wide UVLO hysteresis from seeing too much voltage. range supports the use of a bias winding on the fl yback For higher effi ciency or for wide V transformer to power the LTC3803-3—see the section IN range applications, fl yback controllers are typically powered through a separate Powering the LTC3803-3. bias winding on the fl yback transformer. The LTC3803-3 The ITH/RUN pin can be driven below the shutdown has the wide UVLO hysteresis (1V min) and small VCC threshold (VITHSHDN) to force the LTC3803-3 into shut- supply current draw (<90μA when VCC < VTURNON) that is down. An internal 0.3μA current source always tries to needed to support such bootstrapped hysteretic start-up pull this pin towards VCC. When the ITH/RUN pin voltage schemes. is allowed to exceed VITHSHDN, and VCC exceeds VTURNON, The V the LTC3803-3 begins to operate and an internal clamp CC pin must be bypassed to ground immediately adjacent to the IC pins with a minimum of a 1μF ceramic immediately pulls the ITH/RUN pin up to about 0.7V. In or tantalum capacitor. Proper supply bypassing is neces- operation, the ITH/RUN pin voltage will vary from roughly sary to supply the high transient currents required by the 0.7V to 1.9V to represent current comparator thresholds MOSFET gate driver. from zero to maximum.
Adjustable Slope Compensation Internal Soft-Start
The LTC3803-3 injects a 5μA peak current ramp out through An internal soft-start feature is enabled whenever the its SENSE pin which can be used for slope compensation in LTC3803-3 comes out of shutdown. Specifi cally, the ITH/ designs that require it. This current ramp is approximately RUN voltage is clamped and is prevented from reaching linear and begins at zero current at 8% duty cycle, reach- maximum until roughly 1.4ms has passed. This allows ing peak current at 80% duty cycle. Additional details are the input and output currents of LTC3803-3-based power provided in the Applications Information section. supplies to rise in a smooth and controlled manner on start-up.
Powering the LTC3803-3
In the simplest case, the LTC3803-3 can be powered from a high voltage supply through a resistor. A built-in shunt 38033fd 8 Document Outline FEATURES DESCRIPTION APPLICATIONS TYPICAL APPLICATION ABSOLUTE MAXIMUM RATINGS PACKAGE/ORDER INFORMATION ORDER INFORMATION ELECTRICAL CHARACTERISTICS TYPICAL PERFORMANCE CHARACTERISTICS PIN FUNCTIONS BLOCK DIAGRAM OPERATION APPLICATIONS INFORMATION TYPICAL APPLICATIONS PACKAGE DESCRIPTION REVISION HISTORY TYPICAL APPLICATION RELATED PARTS