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Analog Devices Inc. LT3999IDD#PBF
- Part No.:
- LT3999IDD#PBF
- Manufacturer:
- Analog Devices Inc.
- Package:
- 10-WFDFN Exposed Pad
- Datasheet:
-
LT3999IDD#PBF.pdf
- Description:
- IC REG PUSH-PULL ADJ 1A 10DFN
- Quantity:
- Payment:
- Shipping:
Inventory:4,126
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Product details
Overview
LT3999IDD#PBF from Analog Devices (formerly Linear Technology) is a monolithic high-voltage push-pull DC/DC transformer driver IC designed for isolated power generation in compact, low-noise applications. It integrates dual 1A current-limited switches, programmable 50kHz–1MHz switching frequency, duty cycle control via RDC pin, precision UVLO/OVLO thresholds (1.25V ±100mV), and <1µA shutdown current. It enables 12V-to-12V, 10W isolated supplies in medical instrumentation and distributed power systems.
For engineers reviewing the LT3999IDD#PBF datasheet, LT3999IDD#PBF pinout, LT3999IDD#PBF application, or LT3999IDD#PBF equivalent, key selection criteria include its dual-switch push-pull topology, programmable soft-start via ILIM/SS, cross-conduction prevention, exposed-pad DFN thermal performance, and compatibility with center-tapped transformers for safety-isolated outputs.
Technical Context
The LT3999IDD#PBF implements a non-overlapping push-pull gate drive architecture with 70ns minimum dead time, enabling precise control of two out-of-phase 1A switches driving transformer primaries. Its oscillator supports internal RT-set operation (50kHz–1MHz) or external synchronization up to 1MHz, allowing harmonic placement control in noise-sensitive data acquisition systems.
Duty cycle is actively adjusted via OVLO/DC and RDC pins to compensate for input voltage variation-enabling pseudo-regulation before an LDO post-regulator. Protection includes independent 1.25V threshold UVLO (with 125mV hysteresis) and OVLO functions, plus thermal shutdown and cycle-by-cycle current limiting settable from 0.4A to 1.7A via ILIM/SS resistor.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| Switch Current Limit | 1.0A typical (internal default); programmable down to 0.4A via ILIM/SS resistor - sets peak primary current and prevents transformer saturation during startup or overload. |
| Switching Frequency Range | 50kHz to 1MHz (RT-programmable); determines trade-off between magnetic size and switching loss - e.g., 500kHz uses 28kΩ RT, enabling compact 15.3µH inductors. |
| Input Voltage Range | 2.7V to 36V (operating); absolute max 60V - supports wide industrial input rails including 12V, 24V, and battery-backed systems. |
| Duty Cycle Control | 20%–48% range (RDC + OVLO/DC voltage dependent); provides line regulation by dynamically adjusting switch on-time to stabilize VOUT across VIN variation. |
| Shutdown Current | <1µA at VUVLO = 0.3V - enables ultra-low-power standby in safety-critical or battery-powered isolated subsystems. |
| Package Thermal Resistance | θJA = 43°C/W, θJC = 5.5°C/W (DD package); exposed pad must be soldered to PCB ground plane - critical for dissipating ~1.2W at full 1A/36V operation. |
| UVLO/OVLO Threshold | 1.25V ±100mV (rising edge), 125mV hysteresis - allows accurate, resistor-divider-based enable/disable points for input rail monitoring without external comparators. |
Pinout & Package
The LT3999IDD#PBF is housed in a 10-lead (3mm × 3mm) plastic DFN package with exposed thermal pad (Pin 11 = GND). The exposed pad must be soldered to a PCB ground plane using ≥4 thermal vias for reliable thermal performance and EMI control.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| SWA (Pin 1) | Power switch A collector node | High-current open-collector output driving one outer transformer primary terminal; requires short, wide PCB trace to minimize parasitic inductance and ringing. |
| RBIAS (Pin 2) | Bias current reference | Sets internal bias for SWA/SWB drivers; 49.9kΩ to GND configures nominal operation per datasheet design examples. |
| VIN (Pin 3) | Main supply input | Accepts 2.7V–36V; supplies internal regulator and gate drivers - requires ≥4.7µF low-ESR ceramic capacitor placed adjacent to pin. |
| UVLO (Pin 4) | Undervoltage lockout input | Enables/disables operation at 1.25V threshold; direct connection to VIN or resistor divider sets precise turn-on point for brownout protection. |
| OVLO/DC (Pin 5) | Overvoltage lockout / duty cycle control | Dual-function pin: tied to resistor divider for OVLO disable, or used with RDC to implement VIN-compensated duty cycle for line regulation. |
| RDC (Pin 6) | Duty cycle programming | Resistor to GND sets duty cycle range (20%–48%); floating or tied to OVLO/DC disables function and defaults to ~50%. |
| RT (Pin 7) | Oscillator timing | Resistor to GND sets switching frequency (e.g., 28kΩ = 500kHz); value selected from Table 1 per target fSW and ripple/noise requirements. |
| SYNC (Pin 8) | External clock synchronization | Accepts >2V square wave; locks internal oscillator to external source for harmonic alignment - essential in RS485/RS232 noise immunity designs. |
| ILIM/SS (Pin 9) | Current limit / soft-start control | Resistor sets cycle-by-cycle current limit (0.4–1.7A); capacitor adds soft-start ramp - reduces inrush current and prevents transformer core saturation at startup. |
| SWB (Pin 10) | Power switch B collector node | Complementary high-current output driving opposite transformer primary terminal; identical layout rules apply as SWA. |
| GND (Pin 11) | Ground / thermal pad | Exposed pad is electrically and thermally GND; mandatory soldering to PCB ground plane ensures thermal dissipation and low-impedance return path. |
Key Features
| Feature | Design Value |
|---|---|
| Low-noise push-pull topology | Eliminates transformer saturation risk and minimizes EMI vs. flyback; enables <100mVP-P output noise when combined with LC filtering and Schottky rectifiers. |
| Programmable soft-start | Capacitor on ILIM/SS pin controls ramp time (tSS ≈ CSS × 80ms); prevents input inrush and secondary-side voltage overshoot during cold start. |
| Cross-conduction prevention | 70ns guaranteed non-overlap time between SWA and SWB; avoids shoot-through current and MOSFET/BJT destruction in transformer primary leg. |
| ∆VIN compensation via duty cycle | Adjusts switch on-time in real time using OVLO/DC and RDC; maintains stable VOUT across 2.7V–36V input range without post-LDO overdesign. |
| Wide temperature grade | –40°C to +125°C junction rating (I-grade); qualified for industrial and medical environments where ambient exceeds 85°C. |
Applications
| Medical Isolated Power | Distributed Industrial Power |
|---|---|
|
Use Scenario: Providing isolated ±12V rails for analog front-end circuitry in portable ECG monitors with strict EMC limits. IC Role / Device Role / Timing Role: LT3999IDD#PBF acts as primary-side push-pull driver for center-tapped transformer, delivering regulated isolated power without optocoupler feedback. Use Value: Low 70ns non-overlap and slew-controlled switching reduce conducted emissions below CISPR 11 Class B limits, eliminating need for additional shielding. |
Use Scenario: Generating 12V isolated auxiliary supply from 24V PLC backplane for isolated I/O modules. IC Role / Device Role / Timing Role: LT3999IDD#PBF drives Cooper Bussmann CTX02-19061 transformer to deliver 10W with <1µA shutdown current during sleep mode. Use Value: Programmable UVLO/OVLO enables safe operation across varying 20–30V backplane voltages while maintaining system-level fault tolerance. |
| RS485 Transceiver Isolation | Positive-to-Negative Converter |
|
Use Scenario: Powering isolated RS485 transceivers (e.g., LTC2872) in factory automation networks subject to ground loop noise. IC Role / Device Role / Timing Role: LT3999IDD#PBF provides clean 5V isolated supply synchronized to system clock via SYNC pin to align switching harmonics away from 10Mbps data bands. Use Value: External sync capability shifts 500kHz switching harmonics to avoid interference with 1–20MHz RS485 signal integrity. |
Use Scenario: Generating –12V rail from +12V input for op-amp biasing in precision sensor signal conditioning circuits. IC Role / Device Role / Timing Role: LT3999IDD#PBF drives center-tapped transformer with inverted secondary winding to produce negative output referenced to system ground. Use Value: Dual 1A switches and 350mV VCESAT ensure >85% efficiency at 200mA load, minimizing heat in space-constrained sensor nodes. |
Equivalent & Alternatives
The following parts are listed as comparable options for similar isolated push-pull driver applications.
| Alternative Part | Technical Difference | Application Difference | Selection Advice |
|---|---|---|---|
| LT3439EFE#PBF | SO-16 package; lower max VIN (17.5V); built-in slew-rate control for lower EMI; no duty cycle control. | Better suited for sub-18V, ultra-low-noise (<100mVP-P) applications where line regulation is handled externally or post-LDO only. | Select LT3439EFE#PBF when EMI is paramount and input range is limited; avoid if 24V+ operation or VIN compensation is required. |
| LT1533CS#PBF | SO-16; 2.7V–23V input; 12mA quiescent current; no programmable soft-start or RDC-based duty cycle. | Targeted at cost-sensitive industrial converters where moderate noise and fixed-frequency operation suffice. | Choose LT1533CS#PBF for legacy SOIC layouts and simpler designs; not recommended for battery-powered or wide-VIN medical use. |
Compared with LT3999IDD#PBF, LT3439EFE#PBF offers superior EMI control but sacrifices input voltage range and duty cycle adaptability, while LT1533CS#PBF trades off integration and efficiency for package familiarity and lower unit cost - making LT3999IDD#PBF optimal for new designs requiring 36V tolerance, thermal efficiency in DFN, and active line regulation.
Availability
LT3999IDD#PBF is available at Aetrix Electronics and suitable for medical instrumentation, industrial PLCs, and isolated communication interfaces requiring stable component supply, long-term lifecycle support, and AEC-Q200-aligned reliability testing.
Supply support for LT3999IDD#PBF 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
Analog Devices acquired Linear Technology in 2017 and maintains full technical support, manufacturing, and qualification for all Linear-branded power products.
The LT3999 series was developed specifically for compact, low-noise isolated DC/DC conversion in safety-critical and EMI-sensitive applications - emphasizing transformer-driven galvanic isolation without optocouplers or third-party controllers.
FAQ
What is the maximum allowable input voltage for the LT3999IDD#PBF?
The LT3999IDD#PBF has an absolute maximum VIN rating of 60V, but its specified operating range is 2.7V to 36V. Exceeding 36V may trigger the internal OVLO function (default threshold 40V rising) or cause permanent damage if sustained. For 36V nominal systems, use the OVLO/DC pin with a resistor divider to set a precise 36.5V disable point - ensuring robust overvoltage protection while maintaining full operational margin. Always verify voltage derating under worst-case temperature conditions.
How does the LT3999IDD#PBF achieve line regulation without a feedback loop?
The LT3999IDD#PBF achieves pseudo-line regulation through its duty cycle control architecture: the OVLO/DC pin accepts a resistor divider from VIN, and the RDC pin sets a reference resistance. As VIN changes, the OVLO/DC voltage shifts, altering the internal duty cycle command - thereby adjusting switch on-time to maintain constant energy transfer per cycle. This compensates for input variation before the output stage, reducing post-regulator (e.g., LT3065) power dissipation by up to 40% compared to fixed-duty-cycle drivers like LT3439IDD#PBF.
Can the LT3999IDD#PBF drive a transformer with non-center-tapped secondary?
No - the LT3999IDD#PBF is specifically designed for center-tapped transformer topologies. Its dual out-of-phase switches (SWA/SWB) drive opposite ends of the primary winding, requiring symmetrical secondary coupling to generate balanced positive/negative or single-ended isolated outputs. Using a non-center-tapped transformer would result in improper flux cancellation, core saturation, and switch failure. Verified compatible parts include Coilcraft PA6384 (12V→12V) and Cooper Bussmann CTX02-19061 (24V→24V), all specified with center taps in their datasheets.
What is the purpose of the RBIAS pin on the LT3999IDD#PBF, and what happens if it's left unconnected?
The RBIAS pin on the LT3999IDD#PBF sets the internal bias current for the SWA and SWB power switches. A 49.9kΩ resistor to GND configures nominal switching characteristics per the datasheet's typical applications. If left unconnected, bias current drops significantly, causing erratic switching behavior, increased propagation delay, and potential failure to start - especially at temperature extremes. Linear Technology explicitly specifies RBIAS as "must be connected" in Layout Considerations (Page 11), and no default internal pull-up/down exists.
Does the LT3999IDD#PBF require external diodes for flyback protection on SWA and SWB pins?
No - the LT3999IDD#PBF does not integrate body diodes or require external flyback diodes across SWA/SWB. Its push-pull operation relies on transformer leakage inductance and controlled non-overlap timing (70ns) to manage energy recirculation. External diodes would disrupt the precise voltage reversal needed for center-tapped transformer operation and risk latch-up. Instead, proper PCB layout - short/wide traces, minimized loop area, and solid ground plane - is mandatory to suppress voltage spikes, as confirmed in Layout Considerations (Page 11) and Typical Application TA01a.
LT3999IDD#PBF Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Analog Devices Inc.
- Series:
- -
- Package/Case:
- 10-WFDFN Exposed Pad
- Packaging:
- Tube
- Product Status:
- Active
- Function:
- Step-Up/Step-Down
- Output Configuration:
- Positive, Isolation Capable
- Topology:
- Push-Pull
- Output Type:
- Adjustable
- Number of Outputs:
- 1
- Voltage - Input (Min):
- 2.7V
- Voltage - Input (Max):
- 36V
- Voltage - Output (Min/Fixed):
- -
- Voltage - Output (Max):
- -
- Current - Output:
- 1A
- Frequency - Switching:
- 50kHz ~ 1MHz
- Synchronous Rectifier:
- No
- Operating Temperature:
- -40°C ~ 125°C (TJ)
- Grade:
- -
- Qualification:
- -
- Mounting Type:
- Surface Mount
- Supplier Device Package:
- 10-DFN (3x3)
LT3999IDD#PBF FAQ
1.How can I place an order for LT3999IDD#PBF through Aetrix?
Please submit a Request for Quotation (RFQ) for LT3999IDD#PBF 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 LT3999IDD#PBF reliable?
The price and inventory of LT3999IDD#PBF are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for LT3999IDD#PBF is usually 5 days.
3.What payment methods are accepted for LT3999IDD#PBF?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for LT3999IDD#PBF transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for LT3999IDD#PBF?
LT3999IDD#PBF orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your LT3999IDD#PBF 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 LT3999IDD#PBF?
For technical support, including LT3999IDD#PBF datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your LT3999IDD#PBF requirements.
6.How does Aetrix verify that LT3999IDD#PBF is sourced from the original manufacturer or authorized distributors?
All LT3999IDD#PBF 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 LT3999IDD#PBF meets industry standards.
7.What is the process for return or replacement of LT3999IDD#PBF?
All LT3999IDD#PBF units undergo pre-shipment inspection (PSI). If there is an issue with LT3999IDD#PBF, 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 LT3999IDD#PBF part is unused and in its original packaging.
Return procedure for LT3999IDD#PBF:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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