Datasheet LT3509 (Analog Devices) - 10

LT3509
applicaTions inForMaTion External Synchronization
the input voltage during the switch on time. Depending The external synchronization provides a trigger to the on the input and output voltages the boost supply can be internal oscillator. As such, it can only raise the frequency provided by the input voltage, one of the regulated outputs above the free-run value. To allow for device and or an independent supply such as an LDO. component tolerances, the free run frequency should be
Input Voltage Range
set to at least 12% lower than the lowest supplied external synchronization reference. The oscillator and hence the Firstly, the LT3509 imposes some hard limits due to the switching frequency can then pushed up from 12% above undervoltage lock-out and the overvoltage protection. A the free-run frequency, set by the selected RT. For example, given application will also have a reduced, normal operating if the minimum external clock is 300kHz, the RT should range over which maximum efficiency and lowest ripple be chosen for 264kHz. are obtained. This usually requires that the device is operating at a fixed frequency without skipping pulses. The SYNC input has a threshold of 1.0V nominal so it is There may also be zones above and below the normal compatible with most logic levels. The duty cycle is not range where regulation is maintained but efficiency and critical provided the high or low pulse width is at least ripple may be compromised. At the low end, insufficient 80ns. If not used, the SYNC input should be tied low with input voltage will cause loss of regulation and increased 10kΩ less to avoid noise pickup. ripple—this is the dropout range. At the high end if the
Design Procedure
duty cycle becomes too low this will cause pulse skipping and excessive ripple. This is the pulse-skip region. Both Before starting detailed design a number of key design situations also lead to higher noise at frequencies other than parameters should be established as these may affect the chosen switching frequency. Occasional excursions design decisions and component choices along the way. into pulse-skip mode, during surges for example, may be One of the main things to determine apart from the desired tolerable. Pulse skipping will also occur at light loads even output voltages is the input voltage range. Both the normal within the normal operating range but ripple is usually not operating range and the extreme conditions of surges degraded because at light load the output capacitor can and/or dips or brown-outs need to be known. Then the hold the voltage steady between pulses. operating frequency should be considered and if there are particular requirements to avoid interference. If there For input voltages greater than 30V, there are restrictions are very specific frequencies that need to be avoided then on the inductor value. See the Inductor Selection section external synchronization may be needed. This could also be for details. desirable if multiple switchers are used as low frequency To ensure the regulator is operating in continuous mode beating between similar devices can be undesirable. For it is necessary to calculate the duty cycle for the required efficient operation this converter requires a boost supply so output voltage over the full input voltage range. This must that the base of the output transistor can be pumped above then be compared with minimum and maximum practical duty cycles. 3509fd 10 For more information www.linear.com/LT3509 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Related Parts