The HM3843 is HeroMicro a current-mode pulse-width modulation (PWM) controller IC specifically designed for switch-mode power supplies. Compared to voltage-mode control, it offers superior load transient response, line regulation, and system stability. With features such as ultra-low start-up current, built-in under-voltage lockout (UVLO), high-current totem-pole output, and operation up to 500 kHz, the HM3843 is widely used in flyback, forward, boost, and other medium-power AC/DC and DC/DC converters.
1. Key Features Overview
Based on the HM3843 datasheet:
Control Architecture: Peak current-mode control
Maximum Duty Cycle: 97% (typical) — ideal for topologies requiring high duty cycle
Start-up Current: 0.12 mA (typ.) — among the lowest in class, enabling ultra-low standby power
Operating Frequency: Up to 500 kHz (set by external RT/CT)
Supply Voltage Range:
Start Threshold (VTH): 8.8 V (typ.)
Min Operating Voltage: 7.6 V (typ.)
Built-in UVLO with hysteresis
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
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 | IC ground (use single-point grounding for power and control) |
| 6 | OUTPUT | Totem-pole output; drives MOSFET gate directly (±1A peak) |
| 7 | Vi | Supply pin (decouple with ≥1 μF capacitor) |
| 8 | VREF | Internal 5V reference output (up to 20mA) |
3. Critical Design Guidelines
(1) High Duty Cycle Advantage
The 97% max duty cycle distinguishes HM3843 from HM3845/HM2845 (~48%).
Ideal for Boost, SEPIC, or CCM flyback topologies where high duty cycle is essential.
⚠️ Not recommended for DCM flyback due to risk of transformer saturation.
(2) Ultra-Low Start-up Current
At just 0.12 mA, start-up power loss through the high-voltage resistor is minimized—helpful for meeting Energy Star and other low-standby regulations.
(3) RT/CT Frequency Setting
Oscillator frequency: f ≈ 1.72 / (RT × CT) (RT in kΩ, CT in nF).
Example: RT = 10 kΩ, CT = 3.3 nF → f ≈ 52 kHz
Ensure sufficient magnetic reset time when operating at high duty cycles.
(4) Current-Mode Control & Protection
PWM pulse terminates when ISENSE voltage ≥ 1 V (typ.), providing cycle-by-cycle current limiting.
Keep the sense resistor and ISENSE trace short and away from noise sources.
(5) Loop Stability & Slope Compensation
Use a Type II/III compensation network at COMP for stable voltage regulation.
⚠️ Slope compensation is mandatory when duty cycle > 50% to prevent subharmonic oscillation. This can be implemented via an external ramp injected into ISENSE or derived from the CT waveform.
(6) PCB Layout Best Practices
Route ISENSE trace short, wide, and isolated from switching nodes.
Place VFB divider resistors close to the IC with a dedicated local ground.
Use a low-ESR ceramic capacitor (≥1 μF) between Vi and GND, placed as close as possible to the IC.
4. Typical Applications
PFC Boost pre-regulator stages
SEPIC or Cuk isolated DC/DC converters
CCM flyback converters with high input-to-output voltage ratio
Battery charger boost modules
5. Summary
The HM3843 stands out with its exceptionally high duty cycle capability, ultra-low start-up current, and robust current-mode control. It is the controller of choice for topologies that demand >50% duty cycle. Successful implementation requires careful attention to slope compensation, magnetic reset, and PCB layout to ensure stability and reliability. Ideal for cost-sensitive, high-performance medium-power applications where dynamic response and efficiency are critical.
