The multi-function LED driver under different LED string level input voltage

LTM8042 is a module LED driver that can support LEDs with up to 1 A current and 3000:1 dimming ratio. Its working input voltage range is 3 V to 30 V, and the frequency range can reach 250 kHz to 2 MHz, so it can be widely used in scanners, automobiles and aviation lighting and other fields.

The LTM8042 can be easily configured into three topologies (boost, buck, and buck-boost) to meet a variety of specific application requirements. The boost topology is suitable for low input voltage and high string voltage, and the buck topology is more suitable for high input voltage and low string voltage. The buck-boost topology is used for a wide input range where the voltage may be lower or higher than the LED string. This article describes how to choose the appropriate topology and its corresponding connections.

Boost LED Driver

The most commonly used topology for LED drivers is boost applications, which may be used for LED arrays powered by 12 V input rails with VIN <VF. Figure 1 shows the LTM8042 boost solution, and Figure 2 shows the block diagram. The input voltage is connected to the BSTIN/BKLED- terminal, and the cathode of the LED string is connected to GND. When the transistor Q is turned on, current accumulates in the inductor L. Once Q is turned off, the voltage on L changes polarity, and the inductor current starts to flow to the output filter capacitor C2. LED dimming is implemented in the PWM section, which adjusts the duty cycle and even the average LED current (set by the resistor RCLR). Capacitor C1 is the input voltage filter.

Step-down LED driver

The buck topology is used for relatively high input voltages such as automotive and industrial 24 V power rails. Figure 3 shows the block diagram of the VIN> VF configuration. The input voltage is connected to the BSTOUT/BKIN terminal, and the LED cathode is connected to the BSTIN/BKLED- terminal. When the transistor Q is turned on, current flows from the input terminal through the LED string and the inductor L to GND. Once Q is turned off, the voltage on L will change polarity, and the diode D will be forward biased, thereby pulling the LED cathode below the input voltage level and providing the set current value in the LED string. C5 creates an output filter for this topology.

Buck-boost LED driver

In many commercial, battery and solar power applications, the input voltage varies widely. In these situations, it is most appropriate to use the buck-boost topology shown in Figure 4. The input voltage and LED cathode are connected to the BSTIN/BKLED- terminal. When the transistor Q is turned on, current accumulates in the inductor L. Once Q is turned off, the voltage on L changes polarity, causing diode D to be forward biased, while the voltage climbs above the input level. PWM keeps the current flowing through the LED and C5 at the set value, and C2 acts as an output filter. The LED string voltage VF can be lower or higher than VIN.

Test results of three topologies

All three topologies are tested using the DC1511 demonstration circuit of the LTM8042-using the same LED string, output current and switching frequency in all cases. To ensure that the bias power consumption of the three settings is the same, the same VCC is also provided (as shown in Figure 1). In most cases, the VCC pin can be connected to VIN. Figure 5 shows the resulting efficiency curve. These three topologies can also be used in LTspice? Modeling in the environment, simulation files related to LTM8042 can be found in the data sheet.