Datasheet LTC3803 (Analog Devices) - 8
| Manufacturer | Analog Devices |
| Description | Constant Frequency Current Mode Flyback DC/DC Controller in ThinSOT |
| Pages / Page | 16 / 8 — OPERATION. Powering the LTC3803. Internal Soft-Start. Adjustable Slope … |
| File Format / Size | PDF / 173 Kb |
| Document Language | English |
OPERATION. Powering the LTC3803. Internal Soft-Start. Adjustable Slope Compensation
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LTC3803
OPERATION
The undervoltage lockout (UVLO) mechanism prevents the
Powering the LTC3803
LTC3803 from trying to drive a MOSFET with insuffi cient In the simplest case, the LTC3803 can be powered from VGS. The voltage at the VCC pin must exceed VTURNON a high voltage supply through a resistor. A built-in shunt (nominally 8.7V) at least momentarily to enable LTC3803 regulator from the V operation. The V CC pin to GND will draw as much CC voltage is then allowed to fall to VTURNOFF current as needed through this resistor to regulate the (nominally 5.7V) before undervoltage lockout disables the V LTC3803. This wide UVLO hysteresis range supports the CC voltage to around 9.5V as long as the VCC pin is not forced to sink more than 25mA. This shunt regulator is use of a bias winding on the fl yback transformer to power always active, even when the LTC3803 is in shutdown, the LTC3803—see the section Powering the LTC3803. since it serves the vital function of protecting the VCC pin The ITH/RUN pin can be driven below VITHSHDN (nominally from seeing too much voltage. 0.28V) to force the LTC3803 into shutdown. An internal For higher effi ciency or for wide V 0.3μA current source always tries to pull this pin towards IN range applications, fl yback controllers are typically powered through a separate VCC. When the ITH/RUN pin voltage is allowed to exceed bias winding on the fl yback transformer. The LTC3803 has VITHSHDN, and VCC exceeds VTURNON, the LTC3803 begins the wide UVLO hysteresis and small V to operate and an internal clamp immediately pulls the CC supply current draw that is needed to support such bootstrapped hysteretic ITH/RUN pin up to about 0.7V. In operation, the ITH/RUN start-up schemes. pin voltage will vary from roughly 0.7V to 1.9V to represent current comparator thresholds from zero to maximum. The VCC pin must be bypassed to ground immediately adjacent to the IC pins with a minimum of a 10μF ceramic
Internal Soft-Start
or tantalum capacitor. Proper supply bypassing is neces- sary to supply the high transient currents required by the An internal soft-start feature is enabled whenever the MOSFET gate driver. LTC3803 comes out of shutdown. Specifi cally, the ITH/ RUN voltage is clamped and is prevented from reaching
Adjustable Slope Compensation
maximum until roughly 1.4ms has passed. This allows the input and output currents of LTC3803-based power The LTC3803 injects a 5μA peak current ramp out through supplies to rise in a smooth and controlled manner on its SENSE pin which can be used for slope compensation in start-up. designs that require it. This current ramp is approximately linear and begins at zero current at 6% duty cycle, reach- ing peak current at 80% duty cycle. Additional details are provided in the Applications Information section. 3803fc 8 Document Outline FEATURES DESCRIPTION APPLICATIONS TYPICAL APPLICATION ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION ORDER INFORMATION ELECTRICAL CHARACTERISTICS TYPICAL PERFORMANCE CHARACTERISTICS PIN FUNCTIONS BLOCK DIAGRAM OPERATION APPLICATIONS INFORMATION TYPICAL APPLICATIONS PACKAGE DESCRIPTION REVISION HISTORY TYPICAL APPLICATIONS RELATED PARTS
OPERATION
The undervoltage lockout (UVLO) mechanism prevents the
Powering the LTC3803
LTC3803 from trying to drive a MOSFET with insuffi cient In the simplest case, the LTC3803 can be powered from VGS. The voltage at the VCC pin must exceed VTURNON a high voltage supply through a resistor. A built-in shunt (nominally 8.7V) at least momentarily to enable LTC3803 regulator from the V operation. The V CC pin to GND will draw as much CC voltage is then allowed to fall to VTURNOFF current as needed through this resistor to regulate the (nominally 5.7V) before undervoltage lockout disables the V LTC3803. This wide UVLO hysteresis range supports the CC voltage to around 9.5V as long as the VCC pin is not forced to sink more than 25mA. This shunt regulator is use of a bias winding on the fl yback transformer to power always active, even when the LTC3803 is in shutdown, the LTC3803—see the section Powering the LTC3803. since it serves the vital function of protecting the VCC pin The ITH/RUN pin can be driven below VITHSHDN (nominally from seeing too much voltage. 0.28V) to force the LTC3803 into shutdown. An internal For higher effi ciency or for wide V 0.3μA current source always tries to pull this pin towards IN range applications, fl yback controllers are typically powered through a separate VCC. When the ITH/RUN pin voltage is allowed to exceed bias winding on the fl yback transformer. The LTC3803 has VITHSHDN, and VCC exceeds VTURNON, the LTC3803 begins the wide UVLO hysteresis and small V to operate and an internal clamp immediately pulls the CC supply current draw that is needed to support such bootstrapped hysteretic ITH/RUN pin up to about 0.7V. In operation, the ITH/RUN start-up schemes. pin voltage will vary from roughly 0.7V to 1.9V to represent current comparator thresholds from zero to maximum. The VCC pin must be bypassed to ground immediately adjacent to the IC pins with a minimum of a 10μF ceramic
Internal Soft-Start
or tantalum capacitor. Proper supply bypassing is neces- sary to supply the high transient currents required by the An internal soft-start feature is enabled whenever the MOSFET gate driver. LTC3803 comes out of shutdown. Specifi cally, the ITH/ RUN voltage is clamped and is prevented from reaching
Adjustable Slope Compensation
maximum until roughly 1.4ms has passed. This allows the input and output currents of LTC3803-based power The LTC3803 injects a 5μA peak current ramp out through supplies to rise in a smooth and controlled manner on its SENSE pin which can be used for slope compensation in start-up. designs that require it. This current ramp is approximately linear and begins at zero current at 6% duty cycle, reach- ing peak current at 80% duty cycle. Additional details are provided in the Applications Information section. 3803fc 8 Document Outline FEATURES DESCRIPTION APPLICATIONS TYPICAL APPLICATION ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION ORDER INFORMATION ELECTRICAL CHARACTERISTICS TYPICAL PERFORMANCE CHARACTERISTICS PIN FUNCTIONS BLOCK DIAGRAM OPERATION APPLICATIONS INFORMATION TYPICAL APPLICATIONS PACKAGE DESCRIPTION REVISION HISTORY TYPICAL APPLICATIONS RELATED PARTS
