Current Mode PWM Control Circuit with 50% Max Duty Cycle # Technical Documentation: CS3845B Current Mode PWM Controller
## 1. Application Scenarios
### 1.1 Typical Use Cases
The CS3845B is a fixed-frequency current-mode PWM controller IC primarily employed in switch-mode power supply (SMPS) designs. Its core functionality centers on regulating output voltage by controlling power switch conduction times through pulse-width modulation.
Primary applications include:
- DC-DC Converters : Both isolated (flyback, forward) and non-isolated (buck, boost) topologies
- AC-DC Power Supplies : Offline converters with universal input voltage ranges (85-265VAC)
- Battery Chargers : For lead-acid, Li-ion, and other rechargeable battery systems
- LED Drivers : Constant-current supplies for lighting applications
- Auxiliary Power Supplies : Standby/auxiliary rails in larger power systems
### 1.2 Industry Applications
Consumer Electronics :
- Power adapters for laptops, monitors, and televisions
- Set-top boxes and gaming consoles
- Printer and scanner power modules
Industrial Systems :
- Motor control auxiliary power
- PLC (Programmable Logic Controller) power supplies
- Test and measurement equipment
Telecommunications :
- Network switch/router power modules
- Base station power systems
- Telecom rectifiers
Automotive (secondary systems):
- Infotainment system power supplies
- LED lighting drivers
- DC-DC converters for 12V/24V systems
### 1.3 Practical Advantages and Limitations
Advantages:
- Current-Mode Control : Provides inherent cycle-by-cycle current limiting, simplified feedback loop compensation, and automatic line voltage compensation
- Fixed Frequency Operation : Typically 52kHz, reducing EMI filter design complexity
- Low Startup Current : <1mA typical, enabling efficient startup circuits
- Integrated Features : Includes undervoltage lockout (UVLO), precision reference, and error amplifier
- Wide Operating Range : Suitable for universal input voltage applications
- Cost-Effective : Mature technology with competitive pricing
Limitations:
- Fixed Frequency : Less flexible than variable-frequency controllers for optimizing efficiency across load ranges
- Maximum Duty Cycle : Limited to approximately 50% (typical), restricting topology choices
- Current Sensing : Requires external current sense resistor, adding power dissipation
- No Integrated MOSFET : Requires external power switch, increasing component count
- Frequency Sensitivity : PCB layout critical due to high-frequency switching operation
## 2. Design Considerations
### 2.1 Common Design Pitfalls and Solutions
Pitfall 1: Current Sense Signal Integrity
- Problem : Noise pickup on current sense line causing erratic switching or premature shutdown
- Solution :
- Use Kelvin connection to current sense resistor
- Place RC filter (100Ω + 1nF) close to IC current sense pins
- Route sense traces as differential pair away from switching nodes
Pitfall 2: Insufficient UVLO Hysteresis
- Problem : Power supply "chattering" during startup or brownout conditions
- Solution :
- Implement proper hysteresis using external resistors on VCC pin
- Typical hysteresis: 6V turn-on, 10V turn-off (adjustable via external components)
Pitfall 3: Feedback Loop Instability
- Problem : Output oscillations or poor transient response
- Solution :
- Proper compensation network at COMP pin
- Type II compensation typical: Series RC from COMP to GND with capacitor to VREF
- Ensure adequate phase margin (>45°) through careful component selection
Pitfall 4: Thermal Management
- Problem : Overheating during high ambient temperature operation
- Solution :
- Adequate
