The PJ3846, introduced by HeroMicro, is a high-performance high-frequency current-mode PWM controller. It integrates the core functions needed to build reliable and efficient switch-mode power supplies, including a precise reference voltage source, a high-speed oscillator, an error amplifier, a differential current-sense amplifier, and multiple protection circuits. This IC is an ideal choice for designing AC-DC or DC-DC converters based on forward, flyback, push-pull, and half/full-bridge topologies.
1. Key Features Overview
Based on the datasheet provided by HeroMicro (PJ3846.pdf), key features include:
Operating Mode: Peak current-mode control, offering excellent line regulation, load transient response, and inherent cycle-by-cycle current limiting.
High Operating Frequency: Supports switching frequencies up to 500kHz, enabling smaller magnetics and capacitors for high power density designs.
Integrated Protection:
Under-Voltage Lockout (UVLO): Ensures the chip remains off during insufficient input voltage, preventing erratic startup.
External Shutdown: Allows the outputs to be forcibly disabled via an external signal.
Programmable Cycle-by-Cycle Current Limiting: The over-current protection point can be set via an external resistor network.
Advanced Features:
Automatic Feed-Forward Compensation: Simplifies loop compensation design.
Automatic Symmetry Correction for Push-Pull: Improves efficiency and reliability of push-pull converters.
Parallel Operation Capability: Supports paralleling multiple modules for higher power output.
Double Pulse Suppression: Prevents abnormal drive signals.
Drive Capability: Totem-pole outputs with a peak source/sink current of up to 500mA, capable of directly driving small-to-medium MOSFETs or IGBTs.
Package: Available in DIP16, SOIC16, and SOP16 packages for various application needs.
2. Key Pin Functions
Pin 1 (C/S SS): Current Limit / Soft Start. All outputs are disabled if this pin's voltage falls below 0.5V. An external capacitor to GND enables soft-start; an external resistor divider sets the cycle-by-cycle current limit threshold.
Pin 2 (VREF): 5.1V Precision Reference Output (±1%). Provides a stable bias for external circuits (e.g., error amplifier, current sense network).
Pin 3 & 4 (C/S-, C/S+): Differential Current Sense Inputs. Used to connect the current sense signal (e.g., voltage drop across a sense resistor).
Pin 5 & 6 (E/A+, E/A-): Error Amplifier Non-inverting/Inverting Inputs. E/A- is typically connected to the feedback divider from the output voltage, while E/A+ can be tied to a reference or used for other control loops.
Pin 7 (COMP): Error Amplifier Output. An RC compensation network is connected between this pin and E/A- to stabilize the control loop.
Pin 8 & 9 (CT, RT): Oscillator Timing Capacitor/Resistor. The switching frequency is set by external CT and RT (f ≈ 1 / (2.2 * RT * CT)). CT is recommended to be >1000pF to reduce noise sensitivity.
Pin 10 (Sync): Synchronization Input/Output. Can be used to synchronize multiple PJ3846s or to be synchronized by an external clock source.
Pin 11 & 14 (AOUT, BOUT): Complementary Totem-Pole Outputs. These two outputs are out of phase, with dead time automatically generated internally to prevent shoot-through.
Pin 13 (VC) & Pin 15 (VIN): Output Stage Bias Voltage and Chip Supply Voltage. VC is usually tied to VIN, but can be independently powered to optimize drive capability.
Pin 16 (Shutdown): External Shutdown Input. The chip operates normally when this pin's voltage is >350mV; it is forcibly shut down when <250mV.
3. Critical Design Guidelines
(1) Startup & Power Supply
The VIN pin requires a stable operating voltage between 9V and 25V. The Under-Voltage Lockout (UVLO) threshold is approximately 7.7V with 0.75V of hysteresis. Ensure the startup supply can deliver sufficient current.
The VC pin provides bias for the output stage. Connecting it to VIN is the simplest approach. For stronger drive capability, VC can be connected to a lower-impedance supply.
(2) Frequency Setting & Synchronization
Select RT (1kΩ - 500kΩ) and CT (>1000pF) according to the formula fOSC ≈ 1 / (2.2 * RT * CT). For example, with RT=10kΩ and CT=4.7nF, the typical frequency is about 43kHz.
For synchronization, connect the master controller's Sync signal to the slave controller's Sync pin, and ground the slave's CT pin.
(3) Current Sensing & Limiting
The current sense signal should be connected to Pin 4 (C/S+), while Pin 3 (C/S-) is typically grounded (for low-side sensing).
The cycle-by-cycle current limit threshold is determined by the voltage at Pin 1 (C/S SS). A resistor divider (R1, R2) from VREF can set the threshold: Vpin1 = VREF * R2 / (R1 + R2). Outputs are disabled when Vpin1 < 0.5V.
Always add an RC filter network (as shown in Fig 4-2) on the current sense path to filter out high-frequency spikes caused by parasitic inductance/capacitance, preventing false triggering.
(4) Loop Compensation
Design a Type II or Type III compensation network between Pin 6 (E/A-) and Pin 7 (COMP). VREF can provide a reference for E/A+. The design of this network is critical for system stability and must be tailored to the specific topology and parameters.
(5) Soft Start
Connect a capacitor Css between Pin 1 (C/S SS) and GND. Upon power-up, an internal current source charges Css, causing the voltage at Pin 1 to ramp up slowly from 0V, thereby gradually increasing the PWM duty cycle to achieve soft start. A larger Css results in a longer soft-start time.
(6) Layout Considerations
Current sense traces should be as short as possible and kept away from noise sources.
CT/RT traces should be short and close to the IC pins to minimize noise coupling.
Output pins (AOUT/BOUT) should use sufficiently wide traces to handle the peak 500mA drive current.
4. Typical Applications
Desktop/Laptop Adapters
Industrial and Telecom Power Supplies
LED Drivers
Battery Chargers
5. Summary
HeroMicro's PJ3846 is a versatile and robust current-mode PWM controller. By properly configuring the frequency setting, current sensing, loop compensation, and protection circuits, engineers can quickly develop efficient and stable switch-mode power supplies. Its built-in advanced features (such as automatic symmetry correction and parallel capability) further simplify the design of complex power systems.
