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STMicroelectronics UC3845BNST
- Part No.:
- UC3845BNST
- Manufacturer:
- STMicroelectronics
- Category:
- DC DC Switching Controllers
- Package:
- 8-DIP (0.300", 7.62mm)
- Datasheet:
-
UC3845BNST.pdf
- Description:
- IC REG CTRLR BST/FLYBK 8-MINIDIP
- Quantity:
- Payment:
- Shipping:
Inventory:3,692
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Product details
Overview
UC3845BNST from STMicroelectronics is a high-performance current-mode PWM controller IC designed for fixed-frequency DC-DC and off-line power supplies. It features an internally trimmed 5 V reference, under-voltage lockout with 8.4 V turn-on/7.6 V turn-off thresholds, 50% maximum duty cycle (due to internal toggle flip-flop), oscillator frequency guaranteed at 250 kHz (up to 500 kHz), and ±1 A totem-pole output capable of directly driving N-channel MOSFETs in flyback or forward converters.
For engineers reviewing the UC3845BNST datasheet, UC3845BNST pinout, UC3845BNST application, or UC3845BNST equivalent, this device is selected for cost-sensitive, low-to-mid power AC-DC adapters, industrial SMPS, and isolated DC-DC modules where precise current-mode control, latch protection, and low start-up current (<0.5 mA) are critical design requirements.
Technical Context
The UC3845BNST implements current-mode control via a high-gain (3 V/V) current-sense comparator that terminates switch conduction cycle-by-cycle, enabling inherent slope compensation and improved transient response. Its oscillator uses RT/CT programming with discharge current trimmed to 7.5–8.8 mA, ensuring stable 250 kHz operation across temperature and supply voltage variations.
Internal circuitry includes a precision error amplifier (90 dB open-loop gain, 1 MHz bandwidth), latched PWM logic for fault containment, and UVLO hysteresis (0.8 V) to prevent erratic startup. The 5 V reference provides regulated charging current for CT and supports external compensation networks without additional regulators.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| Oscillator Frequency | Guaranteed 250 kHz at 25°C; programmable up to 500 kHz via RT/CT - enables compact magnetics and EMI optimization. |
| Max Duty Cycle | 47–50% - enforced by internal toggle flip-flop, limiting peak primary current in flyback designs for safety compliance. |
| UVLO Thresholds | 8.4 V (on), 7.6 V (off) - ensures reliable startup from bulk capacitors in universal-input AC-DC supplies. |
| Output Drive | ±1 A peak sink/source - directly drives gate capacitance of ≤1 nF MOSFETs without external buffers. |
| Reference Voltage | 4.90–5.10 V (25°C, 1 mA load) - trimmed accuracy enables stable feedback loop with <1% output regulation tolerance. |
| Start-up Current | ≤0.5 mA at 6.5 V - reduces auxiliary winding losses and enables high-impedance VCC startup circuits. |
| Current Sense Gain | 2.85–3.15 V/V - converts sense resistor voltage to precise 1 V trip point, supporting accurate overcurrent protection. |
Pinout & Package
UC3845BNST is housed in an 8-pin SO8 (Small Outline Integrated Circuit) package with standard 1.27 mm pitch, JEDEC MS-012AC compliant, rated for 800 mW power dissipation at 25°C ambient.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| 1 - COMP | Error amplifier output | Provides access to EA output for Type II/III compensation; high-impedance node requiring direct RC network connection to VFB. |
| 2 - VFB | Inverting input of error amplifier | Connected to output voltage divider; sets regulation point when COMP = 2.5 V (internal EA reference). |
| 3 - ISENSE | Current sense input | Receives voltage from primary-side sense resistor; triggers PWM latch at 1 V threshold for cycle-by-cycle current limiting. |
| 4 - RT/CT | Oscillator timing node | RT connects to VREF (pin 8), CT to ground; sets fOSC and max duty cycle; CT also receives 5 V charging current from VREF. |
| 5 - GROUND | Signal and power return | Single-point ground for control circuitry and power stage; requires low-inductance layout to minimize noise coupling into ISENSE/VFB. |
| 6 - OUTPUT | PWM output driver | Totem-pole stage sinks or sources up to 1 A; low-state voltage ≤0.4 V at 20 mA - ensures clean MOSFET turn-off. |
| 7 - VCC | Supply input | Operates from 7.6–30 V; powers internal bias, reference, and logic; draws ≤17 mA during operation. |
| 8 - VREF | 5 V reference output | Supplies regulated 5 V at ≤20 mA; used to charge CT and power external circuitry (e.g., optocoupler LED bias). |
Key Features
| Feature | Design Value |
|---|---|
| Latching PWM with cycle-by-cycle current limit | Prevents multiple fault pulses during overload; forces shutdown until reset via UVLO cycling or external latch release. |
| Internally trimmed 5 V reference with ±20 mV line/load regulation | Eliminates need for external reference IC; maintains tight output regulation across input and load transients. |
| Under-voltage lockout with 0.8 V hysteresis | Ensures clean startup and shutdown sequencing; prevents brown-out oscillation during AC line dips. |
| High-current totem-pole output (±1 A) | Drives MOSFET gates directly without buffer stages, reducing BOM count and PCB area in sub-50 W designs. |
| Automatic feed-forward compensation | Compensates for input voltage variation in real time via oscillator frequency modulation, improving line regulation. |
Applications
| AC-DC Adapter | Flyback SMPS |
|---|---|
|
Use Scenario: 5–24 W universal-input (85–265 VAC) wall adapters powering consumer electronics. IC Role / Device Role / Timing Role: Primary-side PWM controller implementing current-mode flyback topology with fixed 250 kHz switching. Use Value: Low start-up current and 8.4 V UVLO enable reliable startup from small auxiliary windings; 50% duty cycle limit simplifies transformer design for safety isolation. |
Use Scenario: Isolated 12–48 V output industrial DC-DC converters with wide input range (18–75 VDC). IC Role / Device Role / Timing Role: Fixed-frequency current-mode controller regulating output via optocoupler feedback to VFB pin. Use Value: 3 V/V current sense gain and 1 V trip threshold provide accurate overcurrent protection; ±1 A output drives medium-size MOSFETs without gate drivers. |
| LED Driver | Industrial Power Module |
|
Use Scenario: Constant-current LED drivers for signage and architectural lighting (20–40 V, 350–700 mA). IC Role / Device Role / Timing Role: Secondary-side controlled flyback controller using current-sense resistor on LED string for regulation. Use Value: Latched shutdown prevents thermal runaway during LED open-circuit faults; 5 V VREF powers optocoupler and sensing circuitry. |
Use Scenario: DIN-rail mounted 15–30 W power modules for PLC I/O and sensor interfaces. IC Role / Device Role / Timing Role: Core PWM engine in ruggedized, conformal-coated SMPS with extended temperature operation (–40°C to +105°C). Use Value: Guaranteed 250 kHz operation and 150°C junction rating support high-density layouts; SO8 package enables automated assembly and reflow compatibility. |
Equivalent & Alternatives
The following parts are listed as comparable options for similar current-mode PWM controller applications.
| Alternative Part | Technical Difference | Application Difference | Selection Advice |
|---|---|---|---|
| UC3845D8TR | Same electrical specs; SO8 package with tape-and-reel packaging and RoHS-compliant finish. | No functional difference; optimized for high-volume SMT production with automated pick-and-place. | Select when requiring full RoHS compliance and reel-based manufacturing logistics. |
| NCP1207ADR2G | Fixed-frequency current-mode controller with integrated soft-start, higher VCC range (10–28 V), no internal VREF. | Requires external reference; better suited for high-efficiency, low-noise applications with tighter EMI constraints. | Select when soft-start control and lower standby current (<1 mA) are prioritized over integrated 5 V reference. |
Compared with UC3845BNST, UC3845D8TR offers identical performance in a production-optimized package, while NCP1207ADR2G trades integrated reference for enhanced soft-start and lower quiescent draw - making it preferable for energy-efficient designs where external reference is acceptable.
Availability
UC3845BNST is available at Aetrix Electronics and suitable for AC-DC adapters, flyback SMPS, and industrial power modules requiring stable component supply, long-term lifecycle support, and consistent parametric performance across batches.
Supply support for UC3845BNST 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
STMicroelectronics is a global semiconductor leader headquartered in Geneva, Switzerland, delivering silicon solutions for automotive, industrial, and power management applications.
The UC384xB family was developed specifically for cost-effective, high-reliability current-mode PWM control in off-line and DC-DC power conversion, targeting applications where minimal external components and robust fault handling are essential.
FAQ
What is the maximum operating frequency of UC3845BNST?
The UC3845BNST oscillator is guaranteed at 250 kHz under standard conditions but supports operation up to 500 kHz using appropriate RT/CT values. At higher frequencies, propagation delays and output rise/fall times (50–150 ns) must be verified against MOSFET gate drive requirements and layout parasitics to maintain stable regulation.
How does the UC3845BNST implement current limiting?
It uses a dedicated current-sense comparator with a fixed 1 V threshold referenced to ground. A voltage proportional to primary current - developed across a sense resistor on the MOSFET source - is fed to pin 3 (ISENSE). When this voltage reaches 1 V, the PWM latch resets, terminating the switch conduction immediately - enabling true cycle-by-cycle current limiting.
Can UC3845BNST replace UC3842BN in an existing design?
No - UC3845BNST has 8.4 V/7.6 V UVLO thresholds and 50% max duty cycle due to its internal toggle flip-flop, whereas UC3842BN uses 16 V/10 V UVLO and supports ~100% duty cycle. Substituting without circuit revision risks failure to start or insufficient output voltage under light load.
What is the purpose of the COMP pin, and how should it be configured?
The COMP pin is the error amplifier output and serves as the control node for loop compensation. It must be connected to VFB (pin 2) via a Type II or Type III network (typically RC + C or RC + R+C) to stabilize the feedback loop. Incorrect compensation causes oscillation or slow transient response; STMicroelectronics Application Note AN1057 provides validated networks for common topologies.
UC3845BNST Specifications
- Product attributes
- Attribute value
- Manufacturer:
- STMicroelectronics
- Series:
- -
- Package/Case:
- 8-DIP (0.300", 7.62mm)
- Packaging:
- Tube
- Product Status:
- Obsolete
- Output Type:
- Transistor Driver
- Function:
- Step-Up, Step-Up/Step-Down
- Output Configuration:
- Positive, Isolation Capable
- Topology:
- Boost, Flyback
- Number of Outputs:
- 1
- Output Phases:
- 1
- Voltage - Supply (Vcc/Vdd):
- 7.6V ~ 30V
- Frequency - Switching:
- Up to 500kHz
- Duty Cycle (Max):
- 48%
- Synchronous Rectifier:
- No
- Clock Sync:
- No
- Serial Interfaces:
- -
- Control Features:
- Frequency Control
- Operating Temperature:
- -40°C ~ 150°C (TJ)
- Grade:
- -
- Qualification:
- -
- Mounting Type:
- Through Hole
- Supplier Device Package:
- 8-Mini DIP
UC3845BNST FAQ
1.How can I place an order for UC3845BNST through Aetrix?
Please submit a Request for Quotation (RFQ) for UC3845BNST 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 UC3845BNST reliable?
The price and inventory of UC3845BNST are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for UC3845BNST is usually 5 days.
3.What payment methods are accepted for UC3845BNST?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for UC3845BNST transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for UC3845BNST?
UC3845BNST orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your UC3845BNST 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 UC3845BNST?
For technical support, including UC3845BNST datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your UC3845BNST requirements.
6.How does Aetrix verify that UC3845BNST is sourced from the original manufacturer or authorized distributors?
All UC3845BNST 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 UC3845BNST meets industry standards.
7.What is the process for return or replacement of UC3845BNST?
All UC3845BNST units undergo pre-shipment inspection (PSI). If there is an issue with UC3845BNST, 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 UC3845BNST part is unused and in its original packaging.
Return procedure for UC3845BNST:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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