Overview

Boost converters (see Figure 6.1) are ideal for LED driver applications where the LED string voltage is greater than the input voltage. Normally, a boost converter would only be used when the output voltage minimum is about 1.5 times the input voltage.

• The converter can easily be designed to operate at efficiencies greater than 90%.

• Both the MOSFET and LED string are connected to a common ground. This simplifies sensing of the LED current, unlike the buck converter where we have to choose either a high side MOSFET driver or a high side current sensor.

• The input current can be continuous, which makes it easy to filter the input ripple current and thus easier to meet any required conducted EMI standards.

Figure 6.1: Simplified Boost Converter Circuit

Boost converters have some disadvantages, especially when used as LED drivers, due to the low dynamic impedance of the LED string.

• The output current of the boost converter is a pulsed waveform. Thus, a large output capacitor is required to reduce the ripple in the LED current.

• The large output capacitor makes PWM dimming more challenging. Turning the boost converter on and off to achieve PWM dimming means the capacitor will have to be charged and discharged every PWM dimming cycle. This increases the rise and fall times of the LED current.

• Open loop control of the boost converter to control the LED current (as in the case of an HV9910-based buck control) is not possible. Closed loop is required to stabilize...