Datasheet LT3999 (Analog Devices) - 7
| Manufacturer | Analog Devices |
| Description | Low Noise, 1A, 1MHz Push-Pull DC/DC Driver with Duty Cycle Control |
| Pages / Page | 16 / 7 — OPERATION Overview. Current Limit and Soft-Start. Push-Pull Topology. … |
| File Format / Size | PDF / 310 Kb |
| Document Language | English |
OPERATION Overview. Current Limit and Soft-Start. Push-Pull Topology. Duty Cycle Control. Other Features
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LT3999
OPERATION Overview Current Limit and Soft-Start
The LT3999 is a monolithic isolated push-pull DC trans- The LT3999 ILIM/SS pin programs the cycle-by-cycle former driver. It includes functions such as duty cycle switch current limit and the soft-start time. A resistor on control, soft-start and protection features. the ILIM/SS pin sets the current limit. A capacitor on the pin in conjunction with the resistor sets the soft-start time.
Push-Pull Topology
When the programmed current limit is reached the switch In a push-pull topology, a pair of switches operating out is immediately turned off and remains off for the remainder of phase generate a square wave voltage pulse on the of the cycle. Leaving the ILIM/SS pin unconnected will primary side of a center tapped transformer. The diodes disable the programmable current limit and the LT3999 on the secondary side rectify the voltage and generate will default to its internal current limit. the output voltage. This voltage is simply VIN times the transformer turns ratio. The soft-start function ramps the maximum switch current over the programmed soft-start time. The purpose of the
Duty Cycle Control
soft-start is to reduce inrush current from the input supply. The LT3999 duty cycle control provides, to a degree, line
Other Features
regulation. The duty cycle is programmed by a resistor on the RDC pin and the OVLO/DC voltage. By making the The LT3999 protection features include overvoltage lock- OVLO/DC voltage a function of V out (OVLO), undervoltage lockout (UVLO) and thermal IN the duty cycle will adjust with varying V shutdown. IN thereby keeping VOUT constant. This feature is useful in cases where an LDO is used to post The OVLO function is programmed with the OVLO/DC pin. regulate the output of the LT3999. By pseudo regulating Switching is disabled during an OVLO event. An internal the output with the duty cycle control the power dissipa- overvoltage lockout on the VIN pin is also provided to tion in the LDO is minimized. protect the LT3999. Leaving the RDC pin floating or connecting it to the OVLO/ The UVLO function is programmed with the UVLO pin. DC pin disables the duty cycle function and the LT3999 Switching is disabled during a UVLO event. The UVLO operates at close to 50% duty cycle. pin is also used to put the LT3999 into a low quiescent shutdown state. At a junction temperature above the operating tempera- ture range the thermal shutdown function turns off both switches. 3999fa For more information www.linear.com/LT3999 7 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Package Description Typical Application Related Parts
OPERATION Overview Current Limit and Soft-Start
The LT3999 is a monolithic isolated push-pull DC trans- The LT3999 ILIM/SS pin programs the cycle-by-cycle former driver. It includes functions such as duty cycle switch current limit and the soft-start time. A resistor on control, soft-start and protection features. the ILIM/SS pin sets the current limit. A capacitor on the pin in conjunction with the resistor sets the soft-start time.
Push-Pull Topology
When the programmed current limit is reached the switch In a push-pull topology, a pair of switches operating out is immediately turned off and remains off for the remainder of phase generate a square wave voltage pulse on the of the cycle. Leaving the ILIM/SS pin unconnected will primary side of a center tapped transformer. The diodes disable the programmable current limit and the LT3999 on the secondary side rectify the voltage and generate will default to its internal current limit. the output voltage. This voltage is simply VIN times the transformer turns ratio. The soft-start function ramps the maximum switch current over the programmed soft-start time. The purpose of the
Duty Cycle Control
soft-start is to reduce inrush current from the input supply. The LT3999 duty cycle control provides, to a degree, line
Other Features
regulation. The duty cycle is programmed by a resistor on the RDC pin and the OVLO/DC voltage. By making the The LT3999 protection features include overvoltage lock- OVLO/DC voltage a function of V out (OVLO), undervoltage lockout (UVLO) and thermal IN the duty cycle will adjust with varying V shutdown. IN thereby keeping VOUT constant. This feature is useful in cases where an LDO is used to post The OVLO function is programmed with the OVLO/DC pin. regulate the output of the LT3999. By pseudo regulating Switching is disabled during an OVLO event. An internal the output with the duty cycle control the power dissipa- overvoltage lockout on the VIN pin is also provided to tion in the LDO is minimized. protect the LT3999. Leaving the RDC pin floating or connecting it to the OVLO/ The UVLO function is programmed with the UVLO pin. DC pin disables the duty cycle function and the LT3999 Switching is disabled during a UVLO event. The UVLO operates at close to 50% duty cycle. pin is also used to put the LT3999 into a low quiescent shutdown state. At a junction temperature above the operating tempera- ture range the thermal shutdown function turns off both switches. 3999fa For more information www.linear.com/LT3999 7 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Package Description Typical Application Related Parts
