CARACTERÍSTICAS

2A Internal Power Switch

Adjustable Switching Frequency

Single Feedback Resistor Sets VOUT

Synchronizable to External Clock

High Gain SHDN Pin Accepts Slowly Varying Input Signals

Wide Input Voltage Range: 2.5V to 32V

Low VCESAT Switch: 300mV at 1.5A (Typical)

Integrated Soft-Start Function

Easily Configurable as a Boost or Inverting Converter

User Configurable Undervoltage Lockout (UVLO)

Tiny 8-Lead 3mm × 3mm DFN and 8-Lead MSOP Packages

 

APLICACIONES

VFD Bias Supplies

TFT-LCD Bias Supplies

GPS Receivers

DSL Modems

Local Power Supply

 

DESCRIPCIÓN

The LT®3580 is a PWM DC/DC converter containing an internal 2A, 42V switch. The LT3580 can be configured as either a boost, SEPIC or inverting converter. Capable of generating 12V at 550mA or –12V at 350mA from a 5V input, the LT3580 is ideal for many local power supply designs.

The LT3580 has an adjustable oscillator, set by a resistor from the RT pin to ground. Additionally, the LT3580 can be synchronized to an external clock. The free running or synchronized switching frequency range of the part can be set between 200kHz and 2.5MHz.

The LT3580 also features innovative SHDN pin circuitry that allows for slowly varying input signals and an adjust able undervoltage lockout function.

Additional features such as frequency foldback and soft-start are integrated. The LT3580 is available in tiny 3mm × 3mm 8-lead DFN and 8-lead MSOP packages.

 

OPERACIÓN

The LT3580 uses a constant-frequency, current mode con trol scheme to provide excellent line and load regulation. Refer to the Block Diagram which shows the LT3580 in a boost configuration. At the start of each oscillator cycle, the SR latch (SR1) is set, which turns on the power switch, Q1. The switch current flows through the internal current sense resistor generating a voltage proportional to the switch current. This voltage (amplified by A4) is added to a stabilizing ramp and the resulting sum is fed into the positive terminal of the PWM comparator A3. When this voltage exceeds the level at the negative input of A3, the SR latch is reset, turning off the power switch. The level at the negative input of A3 (VC pin) is set by the error amplifier A1 (or A2) and is simply an amplified version of the difference between the feedback voltage (FB pin) and the reference voltage (1.215V or 5mV depending on the configuration). In this manner, the error amplifier sets the correct peak current level to keep the output in regulation.

The LT3580 has a novel FB pin architecture that can be used for either boost or inverting configurations. When configured as a boost converter, the FB pin is pulled up to the internal bias voltage of 1.215V by the RFB resistor connected from VOUT to FB. Comparator A2 becomes inactive and comparator A1 performs the inverting amplification from FB to VC. When the LT3580 is in an inverting configuration, the FB pin is pulled down to 5mV by the RFB resistor connected from VOUT to FB. Comparator A1 becomes inactive and comparator A2 performs the non inverting amplification from FB to VC.

SEPIC Topology

The LT3580 can be configured as a SEPIC (single-ended primary inductance converter). This topology allows for the input to be higher, equal, or lower then the desired output voltage. Output disconnect is inherently built into the SEPIC topology, meaning no DC path exists between the input and output. This is useful for applications requiring the output to be disconnected from the input source when the circuit is in shutdown.

Inverting Topology

The LT3580 can also work in a dual inductor inverting topology. The part’s unique feedback pin allows for the inverting topology to be built by simply changing the connection of external components. This solution results in very low output voltage ripple due to inductor L2 in series with the output. Abrupt changes in output capacitor current are eliminated because the output inductor delivers current to the output during both the off-time and the on-time of the LT3580 switch.