Texas Instruments LM2585S-3.3

Overview

LM2585S-3.3 from Texas Instruments (formerly National Semiconductor) is a monolithic 3A flyback/boost regulator IC with fixed 3.3V output, 100 kHz fixed-frequency oscillator, 4V–40V input range, and integrated 3.0A/65V NPN power switch-designed for isolated DC-DC conversion in industrial power supplies and embedded systems requiring compact, low-component-count regulation.

For engineers reviewing the LM2585S-3.3 datasheet, LM2585S-3.3 pinout, LM2585S-3.3 application, or LM2585S-3.3 equivalent, key selection considerations include its TO-263-5 package thermal performance (θ_JA = 26°C/W with 7.4× copper), ±4% system output voltage tolerance over line/load, soft-start current of 11.0 μA, and compatibility with standard flyback transformers like Coilcraft S6000-A for single-output 3.3V designs.

Technical Context

The LM2585S-3.3 implements current-mode control with cycle-by-cycle current limiting, enabling stable operation under wide input (4–40V) and load transients. Its internal 1.23V reference feeds a transconductance error amplifier (G_M = 1.193 mmho typical) that compares feedback voltage against V_REF = 3.3V to regulate output.

Switching is governed by a 100 kHz oscillator with duty cycle up to 98%, driving an NPN transistor capable of 3.0A peak current and 65V standoff. Protection includes undervoltage lockout (3.30V threshold), thermal shutdown, and internal current limiting-critical for reliable flyback operation without external sensing.

Key Specifications

Pinout & Package

LM2585S-3.3 is housed in a 5-lead TO-263 (D²PAK) surface-mount package (NSC TS5B), featuring exposed thermal pad for enhanced heat dissipation. The package supports horizontal mounting on PCB copper areas ranging from 0.136 in² (θ_JA = 56°C/W) to 1.0064 in² (θ_JA = 26°C/W).

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar flyback/boost regulator applications.

Compared with LM2585T-3.3, the LM2585S-3.3 offers superior thermal performance in surface-mount layouts, while LM2577S-3.3 trades flyback flexibility for boost-specific optimization-making LM2585S-3.3 the only choice for isolated 3.3V generation in space-constrained industrial PCBs.

Availability

LM2585S-3.3 is available at Aetrix Electronics and suitable for industrial power supplies, isolated sensor interfaces, compact boost converters, and telecom auxiliary rails requiring stable component supply across extended temperature ranges and multi-year production cycles.

Supply support for LM2585S-3.3 includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

Texas Instruments acquired National Semiconductor in 2011 and maintains full technical and manufacturing continuity for the SIMPLE SWITCHER® product line, including legacy regulators like the LM2585 series.

The LM2585S-3.3 belongs to TI's SIMPLE SWITCHER® family-designed specifically for cost-sensitive, low-component-count DC-DC conversion in industrial, automotive, and embedded applications where reliability and ease of design are prioritized over ultra-high efficiency.

FAQ

What is the maximum continuous output current supported by the LM2585S-3.3 in a flyback configuration?

The LM2585S-3.3 supports up to 1.4A continuous output current in flyback topology when paired with the Coilcraft S6000-A transformer (Figure 7), as confirmed in the datasheet's Transformer Selection Table. This rating assumes proper thermal management using ≥0.4896 in² copper area (θ_JA = 35°C/W) and operation within the −40°C to +125°C junction temperature range. Higher loads require heatsinking or forced air cooling.

Does the LM2585S-3.3 require external feedback resistors to set its output voltage?

No-the LM2585S-3.3 has an internally trimmed 3.3V reference connected directly to the FB pin, eliminating the need for external resistors. Unlike the adjustable LM2585-ADJ version, the LM2585S-3.3 integrates the feedback divider, simplifying layout and improving regulation accuracy by removing resistor tolerance and temperature drift variables from the control loop.

Can the LM2585S-3.3 be used in boost (step-up) topology, and what are the key limitations?

Yes, the LM2585S-3.3 supports boost operation per Figure 30 in the datasheet, but output current must be externally limited to ≤3.0A since the internal current limit does not protect the entire boost path during short circuits. Also, efficiency drops significantly below 5V input due to fixed 100 kHz frequency and NPN saturation losses-designs should verify thermal rise at minimum input voltage and full load.

What is the purpose of the COMP pin on the LM2585S-3.3, and how is it typically configured?

The COMP pin is the error amplifier output and serves as the compensation node for the control loop. In standard flyback designs (Figure 2), it connects to a series RC network (e.g., 2 kΩ + 0.47 μF) between COMP and GND to stabilize phase margin. For the LM2585S-3.3, this network is mandatory to prevent oscillation-TI's Switchers Made Simple® software provides optimized values based on transformer and output capacitor selection.

How does the LM2585S-3.3 handle short-circuit conditions in flyback versus boost topologies?

In flyback mode, the LM2585S-3.3 survives output shorts by dropping switching frequency to 25 kHz (per Application Hints), allowing full energy release from the transformer before next cycle-enabling inherent current limiting. In boost mode, however, the short bypasses the switch entirely; external current limiting (e.g., input fuse or active current limiter) is mandatory to prevent damage, as the internal 3.0A limit does not engage.