A boost converter operating in buck mode to drive white LEDs

The circuit shown in the figure provides a solution to drive high-power white LEDs, that is, using a standard boost converter working in "buck" mode to drive white LEDs. This solution has an efficiency of up to 96%, which has many practical advantages compared to the standard solution with an efficiency of only 85%.

Figure ZXSC310 Typical Application Circuit

When the MOSFET (VT1) is turned on, the current flows from the input through the white LED, the parallel filter capacitor (C2), the inductor (L1), VT1 and the sense resistor (R1). The current value is determined by the sense resistor value and the sense voltage threshold of the ZXSC310 (usually 19mV).

Once the current reaches the corresponding peak current set, the MOSFET is turned off and maintained for 1.7ms. During this time, the energy stored in the inductor is transferred to the white LED through the Schottky diode, thereby maintaining the brightness of the white LED.

This circuit has no restrictions on the input voltage and the number of white LEDs in series. To apply a higher input voltage, the values ​​of C1, R2, VT1, C2, and VT1 must be appropriately adjusted to adapt to the change in input voltage. For a larger number of white LEDs, the minimum input voltage must be greater than the forward voltage drop of the series white LEDs.

By adopting a buck-mode boost converter solution, a low-side N-channel MOSFET can be used to replace the high-side P-channel MOSFET commonly seen in a typical buck converter. The inherent conduction loss of an N-channel MOSFET device is 3 times lower than that of a P-channel MOSFET device of the same size. Of course, an N-channel MOSFET can also be used in a typical buck converter circuit, but an additional bootstrap circuit is required to drive it. The peak sense current of the low-side switch can also be referenced to ground. Compared with high-side current sensing, it can improve accuracy and reduce noise.

By using the boost method in the discontinuous working mode, the control loop can work in the current mode and provide periodic control for the converter, which makes the converter fundamentally stable. Compared with the voltage mode buck converter, the design can be simplified.

Another feature of the above scheme is that because the current flows through the white light LED when the inductor is in the charging state, the peak value of the white light LED current will be reduced, so that the peak current can be set smaller at the same white light LED brightness, thereby further improving efficiency, reliability and input noise performance.