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STMicroelectronics UC3845BN
- Part No.:
- UC3845BN
- Manufacturer:
- STMicroelectronics
- Category:
- DC DC Switching Controllers
- Package:
- 8-DIP (0.300", 7.62mm)
- Datasheet:
-
UC3845BN.pdf
- Description:
- IC REG CTRLR BOOST/FLYBACK 8DIP
- Quantity:
- Payment:
- Shipping:
Inventory:1,811
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Product details
Overview
UC3845BN from STMicroelectronics is a high-performance current-mode PWM controller IC designed for DC-DC and off-line power supplies. It features 8.4 V turn-on / 7.6 V turn-off UVLO, ≤50% maximum duty cycle (due to internal toggle flip-flop), 250 kHz guaranteed oscillator frequency, ±1 A totem-pole output, and 5.0 V ±100 mV reference output. It is used in low-duty-cycle isolated flyback converters requiring precise current-sense-based cycle-by-cycle limiting.
For engineers reviewing the UC3845BN datasheet, UC3845BN pinout, UC3845BN application, or UC3845BN equivalent, key selection criteria include UVLO threshold compatibility with 12 V input rails, <50% duty cycle constraint for transformer reset margin, current-mode stability with slope compensation, and SO8/Minidip package thermal limits (150 °C/W junction-to-ambient).
Technical Context
The UC3845BN implements fixed-frequency current-mode control using an internally trimmed oscillator (RT/CT programmable), a high-gain error amplifier (90 dB open-loop gain, 1 MHz bandwidth), and a latched PWM comparator that terminates switch conduction when sensed current exceeds the error amp output voltage. Its internal toggle flip-flop forces every-other-cycle blanking, limiting max duty cycle to <50%.
It integrates a 5.0 V reference (±100 mV over temperature and load), under-voltage lockout with hysteresis (8.4 V on / 7.6 V off), and a 1 A capable totem-pole output stage optimized for N-channel MOSFET gate drive. Startup current is <0.5 mA, enabling direct bias from auxiliary windings without startup resistors.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| Oscillator Frequency | Guaranteed 250 kHz at 25°C; supports up to 500 kHz with external RT/CT tuning. |
| UVLO Threshold | 8.4 V (turn-on) / 7.6 V (turn-off); enables reliable start-up from 12 V input rails with hysteresis. |
| Max Duty Cycle | <50%; enforced by internal toggle flip-flop to ensure safe transformer reset in flyback topologies. |
| Reference Voltage | 5.00 V ±100 mV (0–70°C, 1–20 mA load); powers RT/CT timing network and provides stable feedback reference. |
| Output Drive | ±1 A peak sink/source; directly drives N-MOSFET gates without external buffers in sub-100 W designs. |
| Current Sense Gain | 3.0 V/V ±5%; converts sense resistor voltage to comparator trip point with minimal offset. |
| Start-up Current | <0.5 mA at 6.5 V; allows direct auxiliary winding biasing without power-wasting startup resistors. |
Pinout & Package
UC3845BN is available in 8-pin Minidip (PDIP-8) and SO8 packages. Both share identical pinout and electrical functionality.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| 1 - COMP | Error amplifier output | Provides access to EA output for loop compensation (e.g., Type II/III networks); high-impedance node requiring low-leakage capacitor. |
| 2 - VFB | Inverting input of error amplifier | Connected to output voltage divider; sets regulation point via 2.5 V internal reference at EA non-inverting input. |
| 3 - ISENSE | Current sense comparator input | Receives voltage from primary-side sense resistor; terminates switch on-cycle when ≥ (COMP voltage)/3.0 V/V. |
| 4 - RT/CT | Oscillator timing interface | RT connects to VREF (pin 8), CT to GND (pin 5); sets fOSC and influences max duty cycle via discharge current. |
| 5 - GROUND | Signal and power ground reference | Common return for control circuitry and power stage; requires star grounding near pin 5 to minimize noise coupling. |
| 6 - OUTPUT | PWM output driver | Totem-pole stage sourcing/sinking up to 1 A; directly drives N-MOSFET gate; low-state impedance ensures fast turn-off. |
| 7 - VCC | Supply input | Accepts 7.6–30 V; powers internal bias circuits; UVLO monitors this pin to enable/disable operation. |
| 8 - VREF | 5 V reference output | Supplies regulated 5 V (100 mA max); used to bias RT and charge CT; load regulation ±25 mV (1–20 mA). |
Key Features
| Feature | Design Value |
|---|---|
| Latched PWM with cycle-by-cycle current limiting | Prevents multiple switching events during overload; ensures single-pulse shutdown per fault event for predictable stress control. |
| Internally trimmed 5 V reference with ±100 mV accuracy | Eliminates need for external reference IC; maintains tight output regulation across line/load/temperature in cost-sensitive SMPS. |
| Low-startup current (<0.5 mA) | Enables self-biasing from auxiliary winding in flyback converters, removing startup resistor losses and improving light-load efficiency. |
| Automatic feed-forward compensation | Uses VCC variations to modulate oscillator timing, inherently compensating for input voltage changes without external circuitry. |
| High-current totem-pole output (±1 A) | Drives standard N-channel MOSFETs (Qg ≤ 20 nC) directly; reduces BOM count and layout complexity in sub-100 W applications. |
Applications
| AC-DC Adapter | Flyback LED Driver |
|---|---|
|
Use Scenario: 12 V/1 A isolated AC-DC adapter for industrial sensors. IC Role / Device Role / Timing Role: Primary-side PWM controller enforcing current-mode regulation and cycle-by-cycle overcurrent protection. Use Value: <50% max duty cycle ensures safe transformer reset at 250 kHz; UVLO thresholds match 12 V auxiliary winding voltage profile. |
Use Scenario: Constant-current 36 V/350 mA LED driver for signage lighting. IC Role / Device Role / Timing Role: Flyback controller regulating LED current via primary-side sensing and optocoupler feedback. Use Value: 5.0 V reference powers optocoupler bias and timing network; ±1 A output drives 650 V MOSFET with 15 nC gate charge. |
| Industrial DC-DC Converter | Isolated Telecom Supply |
|
Use Scenario: 24 V input to ±15 V dual-output converter for PLC I/O modules. IC Role / Device Role / Timing Role: Dual-rail flyback controller managing cross-regulation via VFB divider and current-mode stability. Use Value: 90 dB EA gain and 1 MHz bandwidth support tight transient response; low Ib (−1 µA) minimizes divider error. |
Use Scenario: 48 V telecom input to 5 V/2 A isolated supply for baseband processing. IC Role / Device Role / Timing Role: High-reliability current-mode controller with latch protection against short-circuit faults. Use Value: Latching PWM prevents repetitive fault stress; 150 °C/W SO8 thermal resistance supports derated operation at 70°C ambient. |
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 |
|---|---|---|---|
| UC3845BD1 | SO8 package (vs. Minidip); 800 mW max power dissipation (vs. 1.25 W); identical electrical specs. | Better thermal performance in surface-mount layouts; lower profile; requires reflow-compatible PCB design. | Select for automated assembly and space-constrained boards where thermal management favors SO8 mounting. |
| NCP1200DR2G | Fixed 65 kHz frequency; no RT/CT pins; built-in soft-start; 0.65 A output; 16 V UVLO. | Simpler implementation for low-cost adapters; lacks programmable frequency and high-current drive. | Choose when fixed frequency, lower gate drive, and integrated soft-start outweigh flexibility and drive strength needs. |
Compared with UC3845BN, UC3845BD1 offers identical functionality in a surface-mount package with tighter thermal resistance, while NCP1200DR2G trades programmability and drive strength for integration and ease-of-use in cost-driven, fixed-frequency applications.
Availability
UC3845BN is available at Aetrix Electronics and suitable for AC-DC adapters, flyback LED drivers, industrial DC-DC converters, and isolated telecom supplies requiring stable component supply, long-lifecycle support, and legacy design continuity.
Supply support for UC3845BN 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 applications with emphasis on reliability and longevity.
The UC384xB series was developed specifically for cost-sensitive, high-volume current-mode switch-mode power supplies-targeting off-line and DC-DC flyback, forward, and push-pull topologies requiring minimal external components and robust fault handling.
FAQ
What is the maximum operating junction temperature for UC3845BN?
The UC3845BN has a specified junction operating temperature range of −40°C to +150°C. In Minidip packaging, its thermal resistance is 100°C/W (junction-to-ambient), meaning at 25°C ambient and 1.25 W dissipation, junction temperature reaches 150°C-its absolute maximum. Derating is required above 25°C ambient to maintain reliability.
Can UC3845BN be used in a boost converter topology?
Yes, UC3845BN supports boost configurations, but requires careful attention to slope compensation due to right-half-plane zero instability. Its <50% max duty cycle is compatible with typical boost duty ranges, and the 5.0 V reference can bias the feedback network. However, continuous conduction mode boost demands external ramp injection at ISENSE to maintain stability.
How does the UC3845BN's UVLO behavior differ from UC3842BN?
UC3845BN uses 8.4 V turn-on / 7.6 V turn-off UVLO thresholds, optimized for 12 V input rails and auxiliary winding biasing. UC3842BN uses 16 V / 10 V thresholds, targeting higher-voltage off-line applications. This difference prevents direct substitution without verifying startup voltage compatibility and may require redesigning the VCC bias network.
Is external slope compensation required for UC3845BN in flyback operation?
External slope compensation is strongly recommended for UC3845BN in discontinuous conduction mode (DCM) flyback and mandatory in continuous conduction mode (CCM) to prevent subharmonic oscillation. The internal current-sense gain (3.0 V/V) and fixed oscillator do not provide inherent compensation; a sawtooth ramp injected at ISENSE (typically 25–50% of sensed signal slope) is required for stable operation above 50% duty cycle.
UC3845BN Specifications
- Product attributes
- Attribute value
- Manufacturer:
- STMicroelectronics
- Series:
- -
- Package/Case:
- 8-DIP (0.300", 7.62mm)
- Packaging:
- Tube
- Product Status:
- Active
- 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
UC3845BN FAQ
1.How can I place an order for UC3845BN through Aetrix?
Please submit a Request for Quotation (RFQ) for UC3845BN 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 UC3845BN reliable?
The price and inventory of UC3845BN are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for UC3845BN is usually 5 days.
3.What payment methods are accepted for UC3845BN?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for UC3845BN transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for UC3845BN?
UC3845BN orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your UC3845BN 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 UC3845BN?
For technical support, including UC3845BN datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your UC3845BN requirements.
6.How does Aetrix verify that UC3845BN is sourced from the original manufacturer or authorized distributors?
All UC3845BN 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 UC3845BN meets industry standards.
7.What is the process for return or replacement of UC3845BN?
All UC3845BN units undergo pre-shipment inspection (PSI). If there is an issue with UC3845BN, 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 UC3845BN part is unused and in its original packaging.
Return procedure for UC3845BN:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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