The HM2845 is HeroMicro a fixed-frequency, current-mode PWM controller specifically 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 HM2845 is widely used in AC/DC adapters, LED drivers, industrial power supplies, and consumer electronics.
1. Key Features Overview
Based on the HM2845 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, ideal 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: –25°C to +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.
4. Typical Applications
Flyback AC/DC converters (5–60 W)
Forward or Buck DC/DC converters
Constant-current LED drivers
Auxiliary supplies in home appliances, set-top boxes, printers
5. Summary
The HM2845 offers an excellent balance of simplicity, integration, and protection features for medium-power SMPS designs. By carefully designing the start-up circuit, RT/CT timing, compensation network, and PCB layout, engineers can achieve high efficiency, stability, and reliability in a wide range of cost-sensitive power applications.
