Texas Instruments LM2585SX-3.3

Overview

LM2585SX-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, NPN internal switch rated for 65V standoff and 3.0A current, and ±4% system output voltage tolerance over line and load. It operates from 4V to 40V input and is used in isolated DC-DC power supplies for industrial control interfaces requiring stable low-noise 3.3V rails.

For engineers reviewing the LM2585SX-3.3 datasheet, LM2585SX-3.3 pinout, LM2585SX-3.3 application, or LM2585SX-3.3 equivalent, key selection criteria include its TO-263-5 (S-package) thermal performance, flyback topology support with transformer-coupled isolation, soft-start function for in-rush control, and current-mode control for transient response in multi-output systems.

Technical Context

The LM2585SX-3.3 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 3.3V reference (measured at FB pin), generating a compensation signal that modulates switch duty cycle via a transconductance amplifier (GM = 1.193 mmho typical) and comparator.

Protection includes undervoltage lockout (3.30V threshold), thermal shutdown, and internal current limit (4.0A typical). The NPN switch exhibits 0.45V saturation voltage at 3.0A and sustains up to 65V, enabling operation in flyback topologies where reflected output voltage plus input defines peak switch stress.

Key Specifications

Pinout & Package

LM2585SX-3.3 is housed in a 5-lead TO-263 (S) surface-mount package (NSC TS5B), with exposed thermal pad for enhanced heat dissipation. The package is RoHS-compliant and designed for high-power density PCB layouts.

Key Features

Applications

Equivalent & Alternatives

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

Compared with LM2585SX-3.3, LM2585T-3.3 trades thermal performance for mechanical robustness and ease of hand-soldering, while LM2585S-3.3 offers identical functionality in non-reel packaging - all three share identical regulation accuracy, protection features, and flyback/boost topology support.

Availability

LM2585SX-3.3 is available at Aetrix Electronics and suitable for industrial power conversion, isolated sensor interface design, and telecom remote terminal units requiring stable component supply with long-term lifecycle visibility.

Supply support for LM2585SX-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 supply chain responsibility for the SIMPLE SWITCHER® portfolio, including legacy regulators like the LM2585 series.

The LM2585 product line was engineered for cost-sensitive, high-reliability flyback and boost DC-DC conversion in industrial, telecom, and medical equipment - emphasizing minimal external components and proven transformer-based isolation.

FAQ

What topology does the LM2585SX-3.3 support?

The LM2585SX-3.3 supports flyback, step-up (boost), and forward converter topologies. Its internal 3A/65V NPN switch and current-mode control architecture make it especially suited for flyback designs requiring galvanic isolation. In boost mode, external current limiting is required to protect the switch since output current isn't internally limited. The LM2585SX-3.3 datasheet provides reference schematics for both configurations, including transformer and inductor selection guides.

Does the LM2585SX-3.3 require external feedback resistors?

No, the LM2585SX-3.3 does not require external feedback resistors because it integrates a trimmed 3.3V reference directly tied to the FB pin. Unlike the adjustable LM2585-ADJ variant, this fixed-output version configures the feedback network internally - simplifying layout, reducing BOM count, and improving long-term voltage stability. The LM2585SX-3.3 achieves ±4% output tolerance across line, load, and temperature without any external resistor divider.

What is the thermal performance of the LM2585SX-3.3 in its TO-263 package?

The LM2585SX-3.3 in TO-263 (TS5B) package achieves a junction-to-ambient thermal resistance (θ_JA) of 26°C/W when mounted on 7.4× the package copper area (1.0064 in² of 1 oz. copper). This enables continuous 3A operation at +70°C ambient without a heatsink. With less copper, θ_JA rises to 56°C/W (minimum copper) or 35°C/W (3.6× copper), requiring derating per the thermal model in TI's Switchers Made Simple software. The exposed thermal pad must be soldered to the PCB for optimal performance.

How does the LM2585SX-3.3 handle short-circuit conditions?

In flyback configurations using standard transformers (e.g., T7), the LM2585SX-3.3 survives output shorts by dropping switching frequency to 25 kHz, extending off-time to fully discharge transformer energy before next cycle - enabling zero-current turn-on and activation of internal current limit. In boost mode, however, the LM2585SX-3.3 cannot limit output current during a short; external current limiting (e.g., input fuse or active current limiter) is mandatory to prevent switch failure. This behavior is documented in the LM2585SX-3.3 Application Hints section.

Can the LM2585SX-3.3 be used in a non-isolated boost configuration?

Yes, the LM2585SX-3.3 can be used in non-isolated boost configurations, delivering regulated 3.3V from inputs as low as 4V. Its 100 kHz fixed-frequency oscillator and current-mode control provide stable regulation under dynamic loads. However, output current must be externally limited to ≤3A due to lack of internal output current limiting in boost topology. The LM2585SX-3.3 reference design in Figure 30 of the datasheet shows a complete +5V to +12V boost example - scaling to 3.3V requires adjusting inductor and diode ratings accordingly.