Texas Instruments LM5115MTCX

Overview

LM5115MTCX from Texas Instruments is a secondary-side post-regulator and synchronous buck controller for isolated multi-output DC/DC converters. It implements leading-edge PWM control, supports 0.75V–13.5V output regulation, operates with VBIAS input up to 30V, integrates 7V LDO bias regulator, and drives external NMOS high-side/low-side switches in buck topology for auxiliary rail generation-e.g., generating 2.0V or 3.3V from transformer secondary waveforms.

For engineers reviewing the LM5115MTCX datasheet, LM5115MTCX pinout, LM5115MTCX application, or LM5115MTCX equivalent, key selection criteria include its SSPR capability, voltage-mode control with current injection, adaptive dead-time gate drivers, 4MHz error amplifier bandwidth, and compatibility with both current-mode and voltage-mode main-channel controllers.

Technical Context

The LM5115MTCX uses leading-edge pulse width modulation synchronized to an external phase signal (e.g., transformer secondary waveform), enabling stable auxiliary regulation without destabilizing peak-current-mode main converters. Its RAMP generator charges with 3×SYNC current, and internal CRMIX combines error amplifier output (COMP) with current-injected CV reference for dynamic threshold adjustment.

Voltage-mode control is enhanced by current injection: the CS–VOUT sense amplifier (gain = 16, offset = 1.27V) feeds a transconductance stage (16 mA/V) that modulates the PWM comparator's CV threshold in real time, eliminating need for lead compensation networks. Adaptive dead-time control prevents shoot-through during HO/LO transitions, while thermal shutdown triggers at 150°C with 25°C hysteresis.

Key Specifications

Pinout & Package

TSSOP-16 package (PW0016A) with exposed pad for thermal dissipation; θ_JA = 125°C/W. Pin 16 (VBIAS) is primary supply input; pins 11 (LO) and 14 (HO) drive low-side and high-side NMOS gates respectively; pin 1 (CS) and pin 2 (VOUT) form differential current sense interface.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar synchronous buck controller applications.

Compared with LM5115MTCX, LM5117MTCX offers higher input voltage tolerance but lacks SSPR and dual gate drivers, while UCC28950PW targets primary-side full-bridge control-not auxiliary rail generation-making neither a functional substitute for LM5115MTCX's unique secondary-side post-regulation architecture.

Availability

LM5115MTCX is available at Aetrix Electronics and suitable for isolated telecom power supplies, industrial PLC I/O modules, medical imaging subsystems, and automotive infotainment DC/DC stages requiring stable component supply, long-term lifecycle support, and consistent parametric performance.

Supply support for LM5115MTCX 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 is a global semiconductor company headquartered in Dallas, Texas, delivering analog, embedded processing, and digital signal processing technologies for industrial, automotive, and communications markets.

The LM5115MTCX belongs to TI's high-performance power management controller product line, designed specifically for efficient, isolated multi-output DC/DC conversion with secondary-side regulation-enabling compact, reliable auxiliary rail generation without optocouplers.

FAQ

What is the primary function of the LM5115MTCX in an isolated power supply?

The LM5115MTCX functions as a secondary-side post-regulator and synchronous buck controller. It generates tightly regulated auxiliary output voltages (e.g., 2.0V or 3.3V) directly from the transformer secondary switching waveform-eliminating the need for optocouplers or third windings. In standalone mode, it also operates as a conventional synchronous buck controller with VBIAS input up to 30V. This dual-role capability makes LM5115MTCX especially valuable in space-constrained, high-efficiency isolated designs.

How does the LM5115MTCX achieve stable regulation without optical feedback?

The LM5115MTCX achieves stable regulation through Secondary Side Post Regulation (SSPR), which synchronizes to the main converter's phase signal (e.g., transformer secondary waveform) via the SYNC pin. Leading-edge PWM modulation adjusts the auxiliary buck duty cycle in real time based on this signal, while the internal current-injected voltage-mode control loop maintains tight output accuracy. This eliminates optocoupler latency and drift, enabling ±1% regulation across line, load, and temperature-confirmed in TI's SNVS343E datasheet for LM5115MTCX.

What are the key design considerations for the RAMP and SYNC pins on the LM5115MTCX?

For the SYNC pin, a resistor (RSYNC) must be selected so that SYNC current stays within 50–150µA across the full phase signal amplitude range-TI recommends sizing RSYNC using VPHASE(max)/150µA minus 2.5kΩ. The RAMP capacitor (CRAMP) then sets the sawtooth amplitude (1–1.75V) using CRAMP = (3 × ISYNC × TON)/VRAMP. These values directly impact line feed-forward performance and transient response. Incorrect RAMP/SYNC values on LM5115MTCX cause poor regulation or instability, especially during input voltage steps.

Can the LM5115MTCX operate without being synchronized to a main converter?

Yes, the LM5115MTCX supports standalone operation in free-run mode when powered from a DC source. In this mode, the SYNC pin is biased with a DC current to set oscillator frequency, and the RAMP capacitor generates a self-timed sawtooth. The controller retains all core features-including voltage-mode control with current injection, soft-start, current limiting, and gate drive-making LM5115MTCX fully functional as a standard synchronous buck controller even without external phase signals.

What thermal management guidance applies to the LM5115MTCX in TSSOP-16 package?

The LM5115MTCX in TSSOP-16 (PW0016A) has θ_JA = 125°C/W. To maintain junction temperature ≤125°C, PCB layout must include a minimum 100mm² copper pour connected to the exposed pad (Pin 16 ground tie), multiple thermal vias (≥6×0.3mm), and low-thermal-resistance connections to system ground. Derating is required above 70°C ambient; TI specifies thermal shutdown at 150°C with 25°C hysteresis. Proper thermal design ensures sustained 2.5A gate drive performance and long-term reliability of LM5115MTCX in continuous operation.