Datasheet LTC3803-5 (Analog Devices) - 8
| Hersteller | Analog Devices |
| Beschreibung | Constant Frequency Current Mode Flyback DC/DC Controller in ThinSOT |
| Seiten / Seite | 16 / 8 — OPERATION. Powering the LTC3803-5. Adjustable Slope Compensation. … |
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OPERATION. Powering the LTC3803-5. Adjustable Slope Compensation. Internal Soft-Start
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LTC3803-5
OPERATION
The undervoltage lockout (UVLO) mechanism prevents
Powering the LTC3803-5
the LTC3803-5 from trying to drive a MOSFET with in- In the simplest case, the LTC3803-5 can be powered from suffi cient VGS. The voltage at the VCC pin must exceed a high voltage supply through a resistor. A built-in shunt VTURNON (nominally 4.8V) at least momentarily to enable regulator from the V LTC3803-5 operation. The V CC pin to GND will draw as much CC voltage is then allowed to current as needed through this resistor to regulate the fall to VTURNOFF (nominally 4V) before undervoltage lockout V disables the LTC3803-5. CC voltage to around 8.1V as long as the VCC pin is not forced to sink more than 25mA. This shunt regulator is The ITH/RUN pin can be driven below VITHSHDN (nominally always active, even when the LTC3803-5 is in shutdown, 0.28V) to force the LTC3803-5 into shutdown. An internal since it serves the vital function of protecting the VCC pin 0.3μA current source always tries to pull this pin towards from seeing too much voltage. VCC. When the ITH/RUN pin voltage is allowed to exceed The V V CC pin must be bypassed to ground immediately ad- ITHSHDN, and VCC exceeds VTURNON, the LTC3803-5 jacent to the IC pins with a ceramic or tantalum capacitor. begins to operate and an internal clamp immediately Proper supply bypassing is necessary to supply the high pulls the ITH/RUN pin up to about 0.7V. In operation, the transient currents required by the MOSFET gate driver. ITH/RUN pin voltage will vary from roughly 0.7V to 1.9V 10μF is a good starting point. to represent current comparator thresholds from zero to maximum.
Adjustable Slope Compensation Internal Soft-Start
The LTC3803-5 injects a 5μA peak current ramp out through its SENSE pin which can be used for slope compensation in An internal soft-start feature is enabled whenever the designs that require it. This current ramp is approximately LTC3803-5 comes out of shutdown. Specifi cally, the ITH/ linear and begins at zero current at 6.5% duty cycle, reach- RUN voltage is clamped and is prevented from reaching ing peak current at 80% duty cycle. Additional details are maximum until roughly 0.7ms has passed. This allows provided in the Applications Information section. the input and output currents of LTC3803-5-based power supplies to rise in a smooth and controlled manner on start-up. 38035fd 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 APPLICATION RELATED PARTS
OPERATION
The undervoltage lockout (UVLO) mechanism prevents
Powering the LTC3803-5
the LTC3803-5 from trying to drive a MOSFET with in- In the simplest case, the LTC3803-5 can be powered from suffi cient VGS. The voltage at the VCC pin must exceed a high voltage supply through a resistor. A built-in shunt VTURNON (nominally 4.8V) at least momentarily to enable regulator from the V LTC3803-5 operation. The V CC pin to GND will draw as much CC voltage is then allowed to current as needed through this resistor to regulate the fall to VTURNOFF (nominally 4V) before undervoltage lockout V disables the LTC3803-5. CC voltage to around 8.1V as long as the VCC pin is not forced to sink more than 25mA. This shunt regulator is The ITH/RUN pin can be driven below VITHSHDN (nominally always active, even when the LTC3803-5 is in shutdown, 0.28V) to force the LTC3803-5 into shutdown. An internal since it serves the vital function of protecting the VCC pin 0.3μA current source always tries to pull this pin towards from seeing too much voltage. VCC. When the ITH/RUN pin voltage is allowed to exceed The V V CC pin must be bypassed to ground immediately ad- ITHSHDN, and VCC exceeds VTURNON, the LTC3803-5 jacent to the IC pins with a ceramic or tantalum capacitor. begins to operate and an internal clamp immediately Proper supply bypassing is necessary to supply the high pulls the ITH/RUN pin up to about 0.7V. In operation, the transient currents required by the MOSFET gate driver. ITH/RUN pin voltage will vary from roughly 0.7V to 1.9V 10μF is a good starting point. to represent current comparator thresholds from zero to maximum.
Adjustable Slope Compensation Internal Soft-Start
The LTC3803-5 injects a 5μA peak current ramp out through its SENSE pin which can be used for slope compensation in An internal soft-start feature is enabled whenever the designs that require it. This current ramp is approximately LTC3803-5 comes out of shutdown. Specifi cally, the ITH/ linear and begins at zero current at 6.5% duty cycle, reach- RUN voltage is clamped and is prevented from reaching ing peak current at 80% duty cycle. Additional details are maximum until roughly 0.7ms has passed. This allows provided in the Applications Information section. the input and output currents of LTC3803-5-based power supplies to rise in a smooth and controlled manner on start-up. 38035fd 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 APPLICATION RELATED PARTS