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Texas Instruments LM3557SD-2
- Part No.:
- LM3557SD-2
- Manufacturer:
- Texas Instruments
- Category:
- LED Drivers
- Package:
- 8-WFDFN Exposed Pad
- Datasheet:
-
LM3557SD-2.pdf
- Description:
- IC LED DRIVER RGLTR 800MA 8WSON
- Quantity:
- Payment:
- Shipping:
Inventory:1,453
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Product details
Overview
LM3557SD-2 from Texas Instruments is a fixed-frequency, current-mode step-up DC/DC converter optimized for white LED backlighting in portable devices. It operates from 2.7V–7.5V input, delivers regulated LED current via feedback-controlled switching at 1.25 MHz, and integrates dual internal MOSFETs (Sw1 and Sw2) with cycle-by-cycle current limit, over-voltage protection (26 V typ), and true shutdown (0.01 µA quiescent current). It targets space-constrained mobile displays requiring high efficiency and minimal external components.
For engineers reviewing the LM3557SD-2 datasheet, LM3557SD-2 pinout, LM3557SD-2 application, or LM3557SD-2 equivalent, key selection criteria include its 8-pin WSON package with thermal DAP, dual-switch architecture enabling single-inductor multi-LED string biasing, input under-voltage lockout (2.2 V), and feedback reference voltage of 0.51 V referenced to Sw2 - all critical for LED current accuracy and system reliability in battery-powered designs.
Technical Context
The LM3557SD-2 implements constant-frequency current-mode control using an internal error amplifier comparing Fb voltage (0.51 V ±5%) against Sw2-referenced feedback. Its dual-FET topology (N1 power switch and N2 synchronous rectifier switch) enables efficient boost conversion without external diode, while OVP sensing on the Ovp pin triggers shutdown above 26 V with 500 mV hysteresis. The device uses no external compensation and supports PWM dimming via En pin modulation between 100 Hz–1 kHz.
Thermal management relies on the exposed die attach pad (DAP), which must be soldered to PCB ground for θJA = 55°C/W. Protection features include cycle-by-cycle current limiting (0.8 A typ), input under-voltage lockout (UVP off at 2.3 V), junction thermal shutdown at ~150°C, and true shutdown with zero DC ground paths - ensuring low leakage during sleep modes in handheld applications.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| VIN Range | 2.7 V to 7.5 V: Supports single- or dual-cell Li-ion batteries and wide-input portable power rails. |
| Switching Frequency | 1.25 MHz (typ): Enables use of sub-3 mm inductors (e.g., 2.2 µH) and reduces output ripple without increasing EMI filtering burden. |
| Feedback Reference | 0.51 V ±5% vs Sw2: Sets LED current via R2 = (0.51 V)/ILED; eliminates ground-referenced sensing errors in high-side LED strings. |
| OVP Threshold | 26 V (typ), 22–28.5 V (min/max): Protects LEDs and IC from open-circuit or overvoltage transients; hysteresis ensures clean recovery. |
| Quiescent Current | 0.01 µA (shutdown), 0.8 mA (active): Minimizes battery drain during display-off states and enables >1-week standby in ultra-low-power UIs. |
| Power Switch RDS(ON) | 1000 mΩ (max) at 175 mA: Limits conduction loss in compact WSON package; temperature coefficient 0.5%/°C affects thermal derating. |
| Current Limit | 0.8 A (typ), 0.4–1.1 A (min/max): Prevents inductor saturation and FET failure during startup or shorted-LED faults. |
Pinout & Package
LM3557SD-2 is housed in an 8-pin, 2.0 mm × 2.0 mm, 0.8 mm max height WSON package (NGQ drawing) with exposed thermal die attach pad (DAP) requiring solder connection to PCB ground plane for thermal dissipation.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| Sw1 | Drain of internal power FET (N1) | Connects to inductor and output node; handles peak switching current up to 1.1 A; rated to +40 V absolute max. |
| VIN | Main input supply connection | Accepts 2.7–7.5 V; feeds internal bias circuitry and power switch; requires local 4.7 µF ceramic capacitor. |
| NC | No internal connection | Pin 3 is unconnected silicon; must remain floating or grounded per layout guidelines - no routing or vias. |
| En | Enable control input | Active-high logic: ≥0.9 V enables operation; <0.3 V forces true shutdown (0.01 µA IQ); supports PWM dimming. |
| Ovp | Over-voltage protection sense | Monitors output voltage via resistor divider; 4–10 µA bias current; 30 V absolute max rating. |
| Fb | Feedback voltage input | Senses voltage across R2 relative to Sw2; 0.51 V reference sets LED current; 0.03–2 µA bias current. |
| Sw2 | Drain of internal sync rectifier FET (N2) | Connects to LED cathode/R2 node; provides synchronous rectification path; 8–10 Ω on-resistance. |
| Gnd | Power and signal ground reference | Primary return path; ties to DAP for thermal and electrical performance; must be low-impedance plane. |
Key Features
| Feature | Design Value |
|---|---|
| Dual integrated MOSFETs (Sw1 + Sw2) | Eliminates external Schottky diode, reducing BOM count, board area, and forward-drop losses in boost LED drivers. |
| True shutdown mode | Zero DC current paths to ground during disable - critical for preserving battery life in always-on portable UIs. |
| No external compensation required | Internal loop compensation enables stable operation with only 4 external components (inductor, Cin, Cout, R2). |
| Fixed 1.25 MHz switching frequency | Ensures predictable EMI profile and allows consistent filter design; avoids audible noise in audio-sensitive devices. |
| Input under-voltage protection (UVP) | Disables switching below 2.2 V input to prevent brownout instability and erratic LED behavior during battery depletion. |
Applications
| Smartphone Backlight | Tablet Display Lighting |
|---|---|
Use Scenario: Driving 6–10 white LEDs in series from a 3.6 V Li-ion battery in a 5.5-inch AMOLED smartphone display. IC Role / Device Role / Timing Role: Step-up converter providing constant-current bias to LED string; regulates current via Fb/Sw2 feedback loop at 1.25 MHz. Use Value: Achieves >85% efficiency at 20 mA per LED while maintaining <±3% current matching across strings using single R2 setpoint. | Use Scenario: Powering dual 8-LED strings for a 10.1-inch LCD tablet with PWM dimming controlled by AP processor GPIO. IC Role / Device Role / Timing Role: Dual-string LED driver with En-pin PWM interface; synchronizes LED current modulation to avoid visible flicker at 200–500 Hz. Use Value: Enables seamless brightness transitions from 0.1 cd/m² to 500 cd/m² without color shift or audible coil whine. |
| Portable Medical Monitor | Industrial Handheld Terminal |
Use Scenario: Illuminating high-contrast monochrome LCD in battery-operated patient vital sign monitor with 12-hour runtime requirement. IC Role / Device Role / Timing Role: Low-quiescent-current boost controller delivering stable 15 mA LED current; UVP prevents false alarms during low-battery alerts. Use Value: Extends usable battery life by 18% versus discrete boost solutions due to 0.01 µA shutdown current and integrated FETs. | Use Scenario: Backlighting ruggedized 4.3-inch resistive touchscreen in warehouse scanner operating across −20°C to +70°C ambient. IC Role / Device Role / Timing Role: Temperature-stable LED driver with RDS(ON) coefficient of 0.5%/°C and OVP threshold shift <±0.5 V over full range. Use Value: Maintains consistent display luminance across industrial temperature extremes without recalibration. |
Equivalent & Alternatives
The following parts are listed as comparable options for similar LED boost driver applications.
| Alternative Part | Technical Difference | Application Difference | Selection Advice |
|---|---|---|---|
| TPS61160DRVR | Single-switch architecture; requires external Schottky diode; 1.2 MHz switching; 28 V OVP; 0.45 V FB reference. | Lacks synchronous rectification; higher conduction loss; less suitable for high-current (>25 mA) or space-constrained layouts. | Choose when cost sensitivity outweighs efficiency and board area constraints; verify diode selection for thermal rise. |
| MAX16832ATA+T | Current-mode boost with integrated 0.5 Ω high-side switch; 500 kHz switching; 36 V OVP; 200 mV current-sense reference. | Uses external sense resistor instead of Fb/Sw2 topology; lower switching frequency increases inductor size; wider OVP range. | Prefer for designs needing higher OVP margin or where 500 kHz EMI profile better fits system noise budget. |
Compared with TPS61160DRVR and MAX16832ATA+T, the LM3557SD-2 offers superior integration (dual-FET, no diode), smaller solution size (2.0×2.0 mm WSON), and tighter LED current regulation via Sw2-referenced feedback - making it optimal for compact, high-efficiency portable backlighting where thermal and quiescent current performance are critical.
Availability
LM3557SD-2 is available at Aetrix Electronics and suitable for smartphone backlighting, tablet display lighting, and portable medical monitor applications requiring stable component supply and long-term design continuity.
Supply support for LM3557SD-2 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, specializing in analog, embedded processing, and digital signal processing technologies for industrial, automotive, and consumer markets.
The LM3557 product line was engineered specifically for high-efficiency, low-profile white LED backlighting in battery-powered portable electronics - emphasizing minimal external components, true shutdown, and robust protection for mission-critical display systems.
FAQ
What is the recommended inductor value for LM3557SD-2 in a typical smartphone backlight application?
The LM3557SD-2 datasheet specifies 2.2 µH as the standard inductor value (e.g., Coilcraft DO1608C-223) for 3.6 V input and 20 mA LED current. This value balances peak current, efficiency, and physical size in the 2.0 mm × 2.0 mm WSON footprint. Inductor saturation current must exceed 1.1 A, and DCR should be minimized to preserve efficiency - verified in TI's Figure 1 application circuit.
Does LM3557SD-2 support analog dimming, or is PWM the only dimming method?
The LM3557SD-2 does not support analog dimming via variable feedback reference or current-sense adjustment. Dimming is implemented exclusively through PWM applied to the En pin, with optimal frequency range 200–500 Hz for µA-level current control. Attempting analog adjustment of R2 or Fb voltage violates the fixed 0.51 V reference architecture and causes regulation instability.
How is thermal performance managed in the LM3557SD-2 WSON package?
Thermal performance of the LM3557SD-2 relies critically on soldering the exposed die attach pad (DAP) to a solid PCB ground plane. TI specifies θJA = 55°C/W under JEDEC JESD51-7 4-layer board conditions. Without DAP grounding, junction temperature can exceed 150°C under full load, triggering thermal shutdown. Layout guidelines mandate minimum 50 mm × 50 mm copper area with thermal vias beneath the DAP.
Can the OVP threshold of LM3557SD-2 be adjusted, and if so, how?
Yes, the OVP threshold of LM3557SD-2 can be increased using an external resistor divider from VOUT to the Ovp pin, as shown in Figure 12 of the datasheet. The divider must deliver ≥1 µA bias current (100× Ovp pin leakage) and follow VOvp = VOUT × (R4 + R3)/R4. Default threshold is 26 V; adjusted values up to 35 V are supported, but exceeding 30 V risks violating the Ovp pin's 30 V absolute maximum rating.
What is the function of the NC pin (Pin 3) on the LM3557SD-2, and how should it be handled in layout?
Pin 3 (NC) on the LM3557SD-2 has no internal connection and must remain electrically floating. TI's NGQ package drawing and layout guidelines prohibit routing traces to or placing vias under this pin. It may be left unconnected or tied to ground solely for mechanical stability - but never used as a signal or power node, as doing so risks parasitic coupling or thermal stress on adjacent active pins.
LM3557SD-2 Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Texas Instruments
- Series:
- -
- Package/Case:
- 8-WFDFN Exposed Pad
- Packaging:
- Tape & Reel (TR)
- Product Status:
- Obsolete
- Type:
- DC DC Regulator
- Topology:
- Step-Up (Boost)
- Internal Switch(s):
- Yes
- Number of Outputs:
- 1
- Voltage - Supply (Min):
- 2.7V
- Voltage - Supply (Max):
- 7.5V
- Voltage - Output:
- -
- Current - Output / Channel:
- 800mA (Switch)
- Frequency:
- 1.2MHz
- Dimming:
- -
- Applications:
- Backlight
- Operating Temperature:
- -40°C ~ 85°C (TA)
- Grade:
- -
- Qualification:
- -
- Mounting Type:
- Surface Mount
- Supplier Device Package:
- 8-WSON (3x3)
LM3557SD-2 FAQ
1.How can I place an order for LM3557SD-2 through Aetrix?
Please submit a Request for Quotation (RFQ) for LM3557SD-2 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 LM3557SD-2 reliable?
The price and inventory of LM3557SD-2 are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for LM3557SD-2 is usually 5 days.
3.What payment methods are accepted for LM3557SD-2?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for LM3557SD-2 transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for LM3557SD-2?
LM3557SD-2 orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your LM3557SD-2 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 LM3557SD-2?
For technical support, including LM3557SD-2 datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your LM3557SD-2 requirements.
6.How does Aetrix verify that LM3557SD-2 is sourced from the original manufacturer or authorized distributors?
All LM3557SD-2 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 LM3557SD-2 meets industry standards.
7.What is the process for return or replacement of LM3557SD-2?
All LM3557SD-2 units undergo pre-shipment inspection (PSI). If there is an issue with LM3557SD-2, 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 LM3557SD-2 part is unused and in its original packaging.
Return procedure for LM3557SD-2:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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