Texas Instruments LM2585S-5.0

Overview

LM2585S-5.0 from Texas Instruments (formerly National Semiconductor) is a monolithic 3A flyback/boost switching regulator IC with fixed 5.0V output, 100 kHz fixed-frequency oscillator, 4V–40V input range, and integrated 3.0A/65V NPN power switch. It delivers ±4% output voltage tolerance over line and load, supports soft-start, current-mode control, and thermal/UVLO protection - used in isolated DC-DC converters for industrial power supplies.

For engineers reviewing the LM2585S-5.0 datasheet, LM2585S-5.0 pinout, LM2585S-5.0 application, or LM2585S-5.0 equivalent, key selection criteria include its TO-263-5 package thermal performance (θ_JA = 26°C/W with 7.4× copper), fixed-output flyback topology suitability, 3.0A switch current limit, and compatibility with standard off-the-shelf flyback transformers (e.g., Coilcraft S6057-A, Renco RL-5751).

Technical Context

The LM2585S-5.0 implements current-mode PWM control using an internal 100 kHz oscillator and cycle-by-cycle current limiting. Its error amplifier compares feedback voltage against a trimmed 5.0V reference (4.913–5.088 V), driving a comparator that terminates switch conduction when sensed current matches the error amp output.

It operates in flyback or boost topologies with external transformer/inductor, diode, and capacitors. Protection includes undervoltage lockout (3.05–3.75 V), thermal shutdown, and internal current limit (4.0–7.0 A peak). The 5-pin TO-263 package integrates the switch emitter as common ground, enabling direct PCB heat sinking.

Key Specifications

Pinout & Package

LM2585S-5.0 is housed in a 5-lead TO-263 (D²PAK) surface-mount package (NSC TS5B), with the exposed tab electrically connected to Pin 2 (Emitter/Ground) and thermally coupled to PCB copper for enhanced heat dissipation.

Key Features

Applications

Equivalent & Alternatives

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

Compared with LM2585T-5.0, the LM2585S-5.0 provides superior thermal performance in SMT layouts, while LM2577-5.0 trades switching frequency, current capability, and regulation accuracy for lower system cost - making LM2585S-5.0 optimal for thermally demanding, isolated 5 V flyback designs.

Availability

LM2585S-5.0 is available at Aetrix Electronics and suitable for industrial power supplies, automotive auxiliary converters, and telecom adapters requiring stable component supply, long-term lifecycle support, and traceable sourcing.

Supply support for LM2585S-5.0 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.

The LM2585 series was designed specifically for cost-effective, low-component-count flyback and boost DC-DC conversion in industrial, automotive, and telecom systems - emphasizing ruggedness, thermal efficiency, and ease of implementation.

FAQ

What is the maximum continuous output current supported by the LM2585S-5.0 in a flyback configuration?

The LM2585S-5.0 supports up to 1.1 A continuous output current in flyback mode under specified conditions (VIN = 4–12 V, TJ ≤ +125°C, proper PCB copper area). This is limited by internal current sensing and thermal dissipation - exceeding 1.1 A requires derating or active cooling. The LM2585S-5.0 datasheet guarantees performance up to this level with standard transformers like Coilcraft S6057-A.

Does the LM2585S-5.0 require external feedback resistors to set the output voltage?

No. The LM2585S-5.0 integrates a precision 5.0 V reference and internal resistor divider, so no external feedback resistors are needed - unlike the adjustable LM2585-ADJ version. This simplifies layout, improves reliability, and ensures factory-trimmed output accuracy of ±4% across temperature and load. The LM2585S-5.0 is optimized for fixed 5 V applications.

Can the LM2585S-5.0 be used in boost topology, and what are the key layout considerations?

Yes, the LM2585S-5.0 supports boost operation per Figure 30 in SNVS120E. Critical layout practices include placing the 0.1 μF ceramic bypass capacitor within 5 mm of Pin 1 (Input) and Pin 2 (Emitter), routing the switch node (Pin 4) with minimal loop area, and using a low-ESR output capacitor (≥680 μF) close to the diode cathode. The LM2585S-5.0 requires careful grounding to avoid noise coupling into the feedback path.

What is the purpose of the Compensation (COMP) pin on the LM2585S-5.0, and how is it configured?

The COMP pin is the error amplifier output and serves as the loop compensation node. For LM2585S-5.0 flyback designs, it is typically connected to a series RC network (e.g., 2 kΩ + 0.47 μF) from COMP to ground, as shown in Figure 2. This stabilizes the control loop and prevents oscillation. The LM2585S-5.0's transconductance (0.750 mmho typical) and voltage gain (165 V/V) determine the required compensation values.

How does the LM2585S-5.0 handle short-circuit conditions in flyback versus boost configurations?

In flyback mode, the LM2585S-5.0 enters frequency foldback (to 25 kHz) during output short, allowing safe energy release and limiting peak switch current via internal cycle-by-cycle limiting. In boost mode, however, the LM2585S-5.0 cannot limit output current directly - external current limiting (e.g., at input or output) is mandatory to protect both load and LM2585S-5.0 switch.