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Texas Instruments LM5115SDX
- Part No.:
- LM5115SDX
- Manufacturer:
- Texas Instruments
- Category:
- Power Supply Controllers, Monitors
- Package:
- 16-WFDFN Exposed Pad
- Datasheet:
-
LM5115SDX.pdf
- Description:
- IC SECONDARY SIDE CTRLR 16WSON
- Quantity:
- Payment:
- Shipping:
Inventory:3,113
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Product details
Overview
LM5115SDX 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, delivers 2.5A peak gate drive, and operates with VBIAS input from 4.5V to 30V - used in telecom power supplies to regulate auxiliary rails from transformer secondary waveforms.
For engineers reviewing the LM5115SDX datasheet, LM5115SDX pinout, LM5115SDX application, or LM5115SDX equivalent, key selection considerations include its adaptive dead-time control, voltage-mode operation with current injection, 4MHz error amplifier bandwidth, and compatibility with both SSPR and standalone synchronous buck topologies.
Technical Context
The LM5115SDX uses leading-edge pulse width modulation synchronized to an external phase signal (e.g., transformer secondary waveform), enabling stable auxiliary regulation without interfering with main-channel peak-current-mode control. Its RAMP generator charges with 3× SYNC current and discharges via internal 100Ω MOSFET, supporting line feed-forward through amplitude-proportional ramp slope.
Voltage-mode control is enhanced by current injection: the CS–VOUT sense amplifier (gain = 16, offset = 1.27V) produces CV reference that modulates the PWM comparator threshold in real time. The error amplifier features 4MHz GBW, 300µA COMP pull-up, and 0.75V internal reference referenced to SS pin.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| VBIAS Range | 4.5V to 30V - powers internal LDO and current sense amplifier; enables direct bias from wide-input industrial or telecom supplies. |
| Output Voltage Range | 0.75V to 13.5V - set via FB resistor divider; supports common logic rails (3.3V, 2.5V, 1.8V) and analog subsystems. |
| Gate Drive Current | 2.5A sink / 2A source - drives high- and low-side NMOS FETs directly; reduces need for external drivers in compact designs. |
| Error Amplifier GBW | 4MHz - enables fast transient response and stable loop compensation for high-frequency auxiliary outputs. |
| Current Sense Threshold | 45mV (typical) - sets overcurrent limit at CS–VOUT differential; foldback to 36mV at VOUT = 0V protects during short-circuit. |
| Soft-Start Control | SS pin biased at 0.75V with 60kΩ equivalent impedance - allows precise timing of output ramp via external capacitor. |
| Thermal Shutdown | 150°C activation with 25°C hysteresis - prevents latch-up during overload or poor heatsinking in enclosed systems. |
Pinout & Package
LM5115SDX is packaged in a thermally enhanced 16-pin WSON (NHQ0016A) with exposed pad bonded to system ground for low thermal impedance (θJA = 32°C/W).
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| CS | Current sense amplifier positive input | Connects to high side of sense resistor; triggers current limiting when CS–VOUT ≥ 45mV. |
| VOUT | Current sense amplifier negative input | References sense resistor low side; supports 0V–13.5V common-mode range for full-output-range monitoring. |
| AGND | Analog ground reference | Must tie directly to PGND to minimize noise coupling into error amplifier and current sense paths. |
| CO | Current limit amplifier output | Open-drain sink tied to COMP to override PWM duty cycle during overcurrent events. |
| COMP | Error amplifier output | 300µA internal pull-up; interfaces with R-C compensation network to stabilize voltage loop. |
| FB | Error amplifier inverting input | Connected to output divider; regulates output to 0.75V reference with ±13mV tolerance over temperature. |
| SS | Soft-start control node | Charged to 0.75V via internal 60kΩ divider; controls startup slew rate and inrush current. |
| RAMP | PWM ramp generator node | External capacitor sets ramp slope; charged with 3× SYNC current for line feed-forward capability. |
| SYNC | Synchronization input | Low-impedance (2.5kΩ) current input; accepts square-wave phase signal to lock auxiliary PWM to main converter. |
| PGND | Power ground return | High-current return path for gate drivers and current sense; must be star-connected to AGND. |
| LO | Low-side gate driver output | Drives NMOS source-to-ground switch; 12ns fall time minimizes shoot-through risk. |
| VCC | LDO bias regulator output | 7V ±350mV regulated supply for internal logic; requires local 0.1–10µF ceramic decoupling. |
| HS | High-side MOSFET source connection | Reference point for bootstrap circuit; ties to source of high-side NMOS in buck configuration. |
| HO | High-side gate driver output | Drives NMOS gate with 2.5A sink; 15ns rise time ensures fast turn-on of high-side switch. |
| HB | Bootstrap rail supply | Connects to bootstrap capacitor cathode; supplies gate drive voltage above HS potential. |
| VBIAS | Bias regulator input | Primary supply input; powers internal LDO and current sense amp; UVLO at 4.5V enables brownout protection. |
Key Features
| Feature | Design Value |
|---|---|
| Leading-edge PWM modulation | Enables stable auxiliary regulation in peak-current-mode main converters by placing load on transformer secondary during latter portion of phase pulse. |
| Voltage-mode control with current injection | Eliminates need for lead RC network in feedback loop; improves loop stability and noise immunity while retaining voltage-mode simplicity. |
| Adaptive dead-time control | Prevents shoot-through between HO and LO by dynamically adjusting delay based on gate driver transitions and load conditions. |
| Self-synchronization to main channel | Uses transformer secondary phase signal as timing reference - no external clock required and eliminates frequency drift between main and auxiliary outputs. |
| Wide VBIAS operating range | Supports direct connection to unregulated intermediate bus (e.g., 12V, 24V, 48V) without external pre-regulator, reducing BOM count and board space. |
Applications
| Telecom Power Supply | Server VRM Auxiliary Rail |
|---|---|
Use Scenario: Regulating +2.5V or +3.3V auxiliary rails from the secondary winding of an isolated forward or flyback converter in 48V telecom rectifiers. IC Role / Device Role / Timing Role: Secondary-side post-regulator implementing SSPR; synchronizes to main converter phase signal to maintain tight regulation without optical feedback. Use Value: Achieves <±1% output accuracy across line/load/temperature while eliminating optocoupler and TL431, reducing cost and improving reliability. | Use Scenario: Generating tightly regulated low-voltage rails (e.g., 1.8V, 1.2V) for memory or I/O subsystems in high-density server PSUs using shared transformer windings. IC Role / Device Role / Timing Role: Synchronous buck controller with current-injected voltage-mode control; provides fast transient response to dynamic load steps on auxiliary domains. Use Value: Delivers 4MHz error amplifier bandwidth and 2.5A gate drive to support >10A auxiliary loads with <50µs recovery from 50% load step. |
| Industrial Isolated DC/DC Module | Automotive Infotainment Power |
Use Scenario: Adding isolated +5V or ±12V auxiliary outputs to DIN-rail mounted 24V/48V input DC/DC modules for sensor interface or communication ICs. IC Role / Device Role / Timing Role: Standalone synchronous buck controller powered from auxiliary winding; operates in free-run mode using internal oscillator when phase signal unavailable. Use Value: Supports 4.5V–30V VBIAS input and 0.75V–13.5V output range, enabling flexible rail generation without redesigning transformer turns ratio. | Use Scenario: Regulating +3.3V and +5V supplies for infotainment head units from a 12V automotive battery-fed isolated flyback stage. IC Role / Device Role / Timing Role: Secondary-side regulator with thermal shutdown (150°C) and wide-input bias range, ensuring robust operation under cold-crank (6.5V) and load-dump (40V) conditions. Use Value: Maintains regulation during battery transients and provides overcurrent foldback to protect MOSFETs during short-circuit faults in harsh EMI environments. |
Equivalent & Alternatives
The following parts are listed as comparable options for similar synchronous buck controller applications.
| Alternative Part | Technical Difference | Application Difference | Selection Advice |
|---|---|---|---|
| LM5117MHX/NOPB | Higher VBIAS max (75V vs. 30V); integrated high-side driver; no SSPR mode; requires external current sense amp. | Targeted at standalone high-input-voltage buck converters, not secondary-side post-regulation. | Select when designing non-isolated high-voltage buck regulators where transformer synchronization is unnecessary. |
| UCC28911DR | Integrated MOSFET; flyback controller only; no synchronous buck capability; no RAMP/SYNC pins. | Designed for primary-side regulated flyback with quasi-resonant operation - lacks auxiliary rail flexibility. | Choose only for cost-sensitive, low-power (<10W) isolated flyback designs requiring minimal external components. |
Compared with LM5117MHX/NOPB and UCC28911DR, the LM5115SDX uniquely supports secondary-side post-regulation with self-synchronization, making it irreplaceable for multi-output isolated supplies requiring optocoupler-free auxiliary regulation and leading-edge PWM compatibility with peak-current-mode main controllers.
Availability
LM5115SDX is available at Aetrix Electronics and suitable for telecom power supplies, server VRM auxiliary rails, and industrial isolated DC/DC modules requiring stable component supply, long-term lifecycle support, and guaranteed traceability.
Supply support for LM5115SDX 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 leader delivering analog, embedded processing, and power management solutions with emphasis on reliability, precision, and energy efficiency.
The LM5115 product line was designed specifically for secondary-side post-regulation in isolated multi-output power converters - enabling high-efficiency, optocoupler-free auxiliary rail generation with synchronized leading-edge PWM control.
FAQ
What is the primary function of the LM5115SDX in a power supply design?
The LM5115SDX serves as a secondary-side post-regulator and synchronous buck controller. It regulates auxiliary output voltages (e.g., 2.5V, 3.3V) from the transformer secondary waveform of an isolated main converter using leading-edge PWM. In standalone mode, it functions as a voltage-mode synchronous buck controller with current injection. Its core purpose is to eliminate optocouplers while maintaining tight regulation across line, load, and temperature - a key capability implemented in the LM5115SDX.
How does the LM5115SDX achieve synchronization without an external clock?
The LM5115SDX uses its SYNC pin to accept a square-wave phase signal directly from the transformer secondary winding. A current proportional to the phase signal amplitude flows into SYNC, setting the charge current for the RAMP capacitor. This self-synchronizing architecture locks the auxiliary PWM timing to the main converter's switching cycle - no external oscillator or clock signal is needed. This behavior is intrinsic to the LM5115SDX design and documented in TI SNVS343E.
Can the LM5115SDX operate without connection to a main converter's phase signal?
Yes. When no phase signal is applied to SYNC, the LM5115SDX enters free-run mode using an internal oscillator. In this mode, the RAMP capacitor is charged with a fixed current derived from VCC, enabling standalone synchronous buck operation. The LM5115SDX maintains full functionality including current limiting, soft-start, and thermal shutdown regardless of synchronization source - a feature confirmed in the "Standalone DC/DC Synchronous Buck Mode" section of the LM5115SDX datasheet.
What is the role of the CS and VOUT pins in overcurrent protection?
The CS and VOUT pins form the differential input to the integrated current sense amplifier. A sense resistor placed between CS and VOUT develops a voltage proportional to inductor current. When the CS–VOUT differential exceeds 45mV (typical), the current limit amplifier pulls down the COMP pin via CO, forcing PWM duty cycle reduction. This overcurrent protection operates across the full 0V–13.5V VOUT common-mode range and is a defined specification of the LM5115SDX per Electrical Characteristics table.
Does the LM5115SDX require external components for soft-start, and how is it configured?
Yes. Soft-start is implemented by connecting an external capacitor (CSS) from the SS pin to AGND. The internal 60kΩ equivalent impedance charges CSS toward 0.75V, creating a controlled ramp of the error amplifier's non-inverting input. Output voltage follows VOUT(t) = VOUT(final) × (1 − e^(−t/(60kΩ × CSS))). This RC-based soft-start is mandatory for inrush current limiting and is a standard configuration method for the LM5115SDX across all application circuits.
LM5115SDX Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Texas Instruments
- Series:
- -
- Package/Case:
- 16-WFDFN Exposed Pad
- Packaging:
- Tape & Reel (TR)
- Product Status:
- Obsolete
- Programmable:
- Not Verified
- Applications:
- Secondary-Side Controller
- Voltage - Input:
- 5V ~ 30V
- Voltage - Supply:
- 5V ~ 7.5V
- Current - Supply:
- 4 mA
- Operating Temperature:
- -40°C ~ 125°C
- Grade:
- -
- Qualification:
- -
- Mounting Type:
- Surface Mount
- Supplier Device Package:
- 16-WSON (5x5)
LM5115SDX FAQ
1.How can I place an order for LM5115SDX through Aetrix?
Please submit a Request for Quotation (RFQ) for LM5115SDX on Aetrix. Our sales agent will provide a competitive quotation and guide you through the order confirmation once you accept the terms.
2.Are the price and stock information for LM5115SDX reliable?
The price and inventory of LM5115SDX are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for LM5115SDX is usually 5 days.
3.What payment methods are accepted for LM5115SDX?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for LM5115SDX transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for LM5115SDX?
LM5115SDX orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your LM5115SDX order is processed, you will receive an email with the shipment details and tracking number.
Note: Tracking information may take up to 24 hours to appear. Express delivery typically takes 3–5 business days.
5.How can I obtain technical support or documentation for LM5115SDX?
For technical support, including LM5115SDX datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your LM5115SDX requirements.
6.How does Aetrix verify that LM5115SDX is sourced from the original manufacturer or authorized distributors?
All LM5115SDX products on Aetrix are procured from qualified distributors and authorized channels. Our dedicated quality assurance team conducts strict verification, including traceability checks and, if necessary, third-party testing. This ensures that LM5115SDX meets industry standards.
7.What is the process for return or replacement of LM5115SDX?
All LM5115SDX units undergo pre-shipment inspection (PSI). If there is an issue with LM5115SDX, returns or replacements are accepted under the following conditions:
1.Quantity discrepancies, incorrect items, or visible external defects (such as breakage or corrosion), acknowledged by Aetrix.
2.The issue is reported within 90 days of delivery.
3.The LM5115SDX part is unused and in its original packaging.
Return procedure for LM5115SDX:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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