The HM3845 is HeroMicro a fixed-frequency, current-mode PWM controller designed for off-line and DC-to-DC converter applications. Requiring minimal external components, it enables robust and cost-effective switch-mode power supply (SMPS) designs. With features such as low start-up current, precise duty cycle control, built-in under-voltage lockout (UVLO), and cycle-by-cycle current limiting, the HM3845 is ideal for AC/DC adapters, LED drivers, and consumer electronics power supplies.
1. Key Features Overview
Based on the HM3845 datasheet:
Control Architecture: Peak current-mode control
Max Duty Cycle: 48% (typical), preventing transformer saturation
Start-up Current: 0.3 mA typ. @ Vi = 6.5V, suitable for low standby power
Operating Frequency: Up to 500 kHz (set by external RT/CT)
Supply Voltage Range:
Start Threshold (VTH): 8.8 V (typ.)
UVLO Turn-off (VOPR): 7.6 V (typ.)
UVLO Hysteresis: 1.2 V for noise immunity
Output Drive: ±1 A peak current, capable of directly driving power MOSFETs
Reference Voltage (VREF): 5.0 V ±2% (@ Io = 1 mA), powers RT and feedback network
Operating Temperature: 0°C to +70°C (commercial grade only; note: narrower than HM2845’s –25°C~+85°C)
Packages: DIP8 / SOP8
2. Pin Functions (8-Pin)
| Pin | Name | Description |
|---|---|---|
| 1 | COMP | Error amplifier output; connect compensation network (RC) |
| 2 | VFB | Inverting input of error amp; connect output voltage divider |
| 3 | ISENSE | Current-sense input; connect to sense resistor across switch |
| 4 | RT/CT | Connect timing resistor RT (to VREF) and capacitor CT (to GND) |
| 5 | GROUND | Common ground for control and power stages |
| 6 | OUTPUT | Totem-pole output; drives MOSFET gate directly (±1A peak) |
| 7 | Vi | IC supply pin (decouple with capacitor) |
| 8 | VREF | Internal 5V reference output (up to 20mA) |
3. Critical Design Guidelines
(1) Start-up & UVLO Circuit
The IC turns on at Vi > 8.8V and shuts down below 7.6V, with 1.2V hysteresis to prevent chatter.
Use a high-voltage start-up resistor from bulk capacitor to Vi, and place a ≥1 μF low-ESR capacitor close between Vi and GND.
(2) Frequency & Duty Cycle Setting (RT/CT)
Oscillator frequency: f ≈ 1.72 / (RT × CT) (RT in kΩ, CT in nF).
Example: RT = 10 kΩ, CT = 3.3 nF → f ≈ 52 kHz
Max duty cycle is clamped near 50%, making it suitable for flyback topologies.
(3) Current-Mode Control Benefits
Cycle-by-cycle current limiting via ISENSE pin provides fast overcurrent protection.
PWM pulse terminates when ISENSE voltage reaches ~1 V (typ.).
(4) Loop Compensation (COMP Pin)
An external RC compensation network (Type II/III) is required at COMP to stabilize the voltage feedback loop.
Ensure sufficient phase margin (>45°) to avoid oscillation.
(5) MOSFET Gate Drive Optimization
Although OUTPUT can source/sink ±1A, long traces cause ringing.
Add a 5–10 Ω gate resistor between OUTPUT and MOSFET gate to damp oscillations and control dv/dt.
(6) PCB Layout Recommendations
Keep ISENSE trace short and away from noisy nodes (e.g., switch node, OUTPUT).
Place VFB divider resistors close to the IC with a dedicated local ground return.
Decoupling capacitor on Vi must be placed as close as possible to pins 7 and 5.
(7) Temperature Range Limitation
The HM3845 is rated for 0°C to +70°C only (commercial grade). For industrial or outdoor applications requiring wider temperature range, consider the HM2845 (–25°C to +85°C).
4. Typical Applications
Flyback AC/DC converters (5–60 W, indoor use)
Buck or Boost DC/DC converters
Consumer LED drivers (e.g., desk lamps, display backlights)
Auxiliary supplies in home appliances and office equipment
5. Summary
The HM3845 offers an excellent balance of simplicity, integration, and protection features for cost-sensitive, medium-power SMPS designs in commercial environments. By carefully designing the start-up circuit, RT/CT timing, compensation network, and PCB layout, engineers can achieve high efficiency and stability. However, always respect its 0°C to +70°C operating limit to ensure reliable performance.
