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LT3999 Datasheet(PDF) 7 Page - Linear Technology

Part No. LT3999
Description  Low Noise, 1A, 1MHz Push-Pull DC/DC Driver with Duty Cycle Control
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Maker  LINER [Linear Technology]
Homepage  http://www.linear.com
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LT3999 Datasheet(HTML) 7 Page - Linear Technology

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LT3999
7
3999fa
For more information www.linear.com/LT3999
OPERATION
Overview
The LT3999 is a monolithic isolated push-pull DC trans-
former driver. It includes functions such as duty cycle
control, soft-start and protection features.
Push-Pull Topology
In a push-pull topology, a pair of switches operating out
of phase generate a square wave voltage pulse on the
primary side of a center tapped transformer. The diodes
on the secondary side rectify the voltage and generate
the output voltage. This voltage is simply VIN times the
transformer turns ratio.
Duty Cycle Control
The LT3999 duty cycle control provides, to a degree, line
regulation. The duty cycle is programmed by a resistor
on the RDC pin and the OVLO/DC voltage. By making the
OVLO/DC voltage a function of VIN the duty cycle will
adjust with varying VIN thereby keeping VOUT constant.
This feature is useful in cases where an LDO is used to post
regulate the output of the LT3999. By pseudo regulating
the output with the duty cycle control the power dissipa-
tion in the LDO is minimized.
Leaving the RDC pin floating or connecting it to the OVLO/
DC pin disables the duty cycle function and the LT3999
operates at close to 50% duty cycle.
Current Limit and Soft-Start
The LT3999 ILIM/SS pin programs the cycle-by-cycle
switch current limit and the soft-start time. A resistor on
the ILIM/SS pin sets the current limit. A capacitor on the
pin in conjunction with the resistor sets the soft-start time.
When the programmed current limit is reached the switch
isimmediatelyturnedoffandremainsofffortheremainder
of the cycle. Leaving the ILIM/SS pin unconnected will
disable the programmable current limit and the LT3999
will default to its internal current limit.
Thesoft-startfunctionrampsthemaximumswitchcurrent
over the programmed soft-start time. The purpose of the
soft-startistoreduceinrushcurrentfromtheinputsupply.
Other Features
The LT3999 protection features include overvoltage lock-
out (OVLO), undervoltage lockout (UVLO) and thermal
shutdown.
The OVLO function is programmed with the OVLO/DC pin.
Switching is disabled during an OVLO event. An internal
overvoltage lockout on the VIN pin is also provided to
protect the LT3999.
The UVLO function is programmed with the UVLO pin.
Switching is disabled during a UVLO event. The UVLO
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.


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