The **HT7939** is a high-efficiency, step-up (boost) DC/DC converter specifically designed to drive white LEDs. It is commonly used in backlighting for portable devices, such as smartphones, tablets, and GPS units. ### 1. Key Specifications and Features The HT7939 operates primarily as a constant-current source to ensure uniform brightness across all LEDs in a string. | Feature | Specification | | :--- | :--- | | **Input Voltage Range** | 2.6V to 5.5V | | **Output Voltage** | Up to 28V (Supports up to 8 LEDs in series) | | **Switching Frequency** | 1.2MHz (Fixed) | | **Feedback Voltage** | 200mV (Low voltage for high efficiency) | | **Protection** | Over Voltage (OVP), Over Temperature (OTP), Under Voltage (UVLO) | | **Package** | SOT23-6 | --- ### 2. Main Internal Components The IC integrates several critical electronic functions into its small 6-pin footprint: 1. **Power MOSFET:** An internal N-Channel MOSFET acts as the high-speed switch that builds energy in the external inductor. 2. **Error Amplifier:** Compares the feedback voltage (FB) against an internal 0.2V reference to regulate the output current. 3. **PWM Control Logic:** Operates at 1.2MHz. The high frequency allows for the use of very small external inductors and capacitors. 4. **Dimming Control:** The **EN (Enable)** pin can be used for PWM dimming. By applying a PWM signal to this pin, the average LED current can be adjusted. --- ### 3. Required External Components To function, the HT7939 requires a few passive components. Their selection is critical for stability and efficiency: | Component | Function | Recommended Value | | :--- | :--- | :--- | | **Inductor (L1)** | Stores energy during the "ON" cycle. | 10µH to 22µH | | **Schottky Diode (D1)** | Rectifies the current when the internal switch is "OFF". | Low forward voltage (e.g., SS14) | | **Input/Output Caps** | Filters noise and stabilizes voltage. | 1µH to 4.7µH Ceramic | | **Sense Resistor ($R_{SET}$)** | Sets the LED current ($I_{LED} = 0.2V / R_{SET}$). | Calculated based on desired mA | --- ### 4. Typical Application Circuit Diagram The basic flow of the circuit is as follows: 1. **VIN** is connected to the battery/power source. 2. **SW** pin is connected to the junction of the Inductor and the Schottky Diode. 3. **FB (Feedback)** pin is connected between the last LED in the string and a resistor to ground. 4. **EN** pin receives the turn-on signal or PWM dimming signal. ---