LT3759 DC/DC controller design

RF cable can be customized for other specifications

Overview:
Introduction Many of today's electronic devices require a negative output or positive output converter, and sometimes both. In addition, they must be operated with a variety of power sources, including USB, wall adapters, alkaline batteries, and lithium batteries. In order to produce outputs of different polarities from variable input voltages, power supply designers often use multiple regulator ICs, resulting in an increase in the number of inventory components.
The LT3759 operates from an input voltage range of 1.6V to 42V and uses the same feedback pin to control the positive or negative output, reducing inventory and simplifying design. The device also integrates many popular features such as soft start, adjustable frequency and synchronization into a small footprint. The LT3759 is available in a 5mm x 4mm 12-lead MSE package and can be used in a variety of configurations such as boost, SEPIC, flyback and Cuk topologies.

This article describes the design based on the LT3759 DC/DC controller.

Wide input voltage range and internal LDO
The LT3759's wide input range simplifies power supply designs that must be compatible with a wide range of input power supplies. Because the LT3759 includes two internal low dropout (LDO) voltage regulators that are powered by the VIN and DRIVE pins, there is no need to add an external voltage regulator or a slow charge hysteresis startup scheme for simple startup and bias. Set. The LT3759's internal INTVCC current limit function prevents the IC from generating excessive on-chip power dissipation.

Output voltage detection becomes easy
The LT3759 features a novel FBX pin architecture that simplifies the design of negative output and positive output converters. It contains two internal error amplifiers (one detects the positive output and the other detects the negative output) and allows the FBX pin to be connected directly from the positive or negative output to the voltage divider, eliminating the positive or negative output Detect any confusion related and simplify board layout. You only need to determine the output polarity and topology, and the rest of the work can be done to the LT3759.

Adjustable/synchronous switching frequency converters typically need to operate at a specific frequency, especially in RF communications applications, which are sensitive to spectral noise in certain frequency bands. Moreover, if the area available for the converter is limited, operating at a higher frequency allows for the use of smaller components, thereby reducing the board space and output ripple required. If power loss is an indicator of concern, performing switching operations at a lower frequency reduces switching losses and improves efficiency. The switching frequency ranges from 100kHz to 1MHz and can be set with a single resistor connected between the RT pin and ground. In addition, the device can be synchronized to an external clock via the SYNC pin.

Precision UVLO and Soft Start An input supply undervoltage lockout (UVLO) for sequencing or initiating overcurrent protection is easily implemented by driving a UVLO with a resistor divider from the VIN supply. When VIN is at the desired UVLO rising threshold voltage, the divider output will produce 1.22V on the UVLO pin. The UVLO pin has an adjustable input hysteresis that allows the IC to ignore a configurable input supply voltage drop before deactivating the converter. During the UVLO process, the IC is deactivated and the VIN quiescent current drops to 1μA or less.
The soft start function is available through the SS pin, which reduces peak input current when starting or recovering from a fault condition and avoids output voltage overshoot. The SS pin suppresses inrush current by reducing the peak current of the switch. In this way, the soft-start function allows the output capacitor to be gradually charged to its final value.
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