Get the most out of LED-based lighting systems with inductive converters
Achieving the required brightness in mid-sized LCD panel applications requires a driver to provide stable current to the LED under all operating conditions. Generally, two types of LED driver topologies can be used: capacitive charge pumps and inductor-based switching regulators. This article focuses on inductive converter LED drive circuits that provide 1W to 6W power for LEDs.
Charge-pump LED drivers are widely used in mobile phones and other small-size LCD backlight applications because of their high efficiency, low cost, and ease of implementation. The external components required for the charge pump consist only of three or four capacitors and no inductors. However, there are limitations in terms of output power.
Although some high-power flash LED charge pumps can provide up to 2W of power, the output voltage of the charge pump is limited to about 6V and therefore cannot drive more than two LEDs in series. The number of LEDs is determined by the number of channels in the charge pump (usually 6 more). Therefore, mid-sized panel applications may be limited by charge pump packaging, since more channels mean more pins and larger package size.
The combination of LED forward voltage (VF), LED current and supply voltage range determines the type of inductor converter LED drive circuit required. LED VF varies with current, temperature and LED model.
When choosing an LED driver architecture (linear, buck or boost), the VF occurring over temperature is a critical parameter, as is the overvoltage protection (OVP) level. In this article, it is assumed that VF is 3.8V.
When selecting an LED driver IC, the key parameters are the switch current limit; the output voltage; and the overvoltage detection threshold required to prevent an LED open circuit condition. External components such as inductors and capacitors should also be selected carefully.
Figure 1: Shown is an example of an 8-inch LCD module backlight with 9 strings of 3 white LEDs each. When the LED forward voltage (VF) is typically 3.3V, the total string voltage is approximately 10V.
As an example, consider an 8-inch LCD module that includes a backlight with a total of nine strings of three white LEDs each, as shown in Figure 1 above. When the LED forward voltage (VF) is typically 3.3V, the total string voltage is approximately 10V (3 x3.3V). With 20mA per LED, the total current is 180mA (9 x 20) and the total power consumption is 1.8W. 5V power is provided by the AC power adapter. Inductor-based LED drivers are ideal for this application.
First, let's understand what device switching current is required to handle a 2W load. Assuming the efficiency (?) is 80%, the input current is equal to Vout x Iout / Vin x ? = 10 x 0.18 / 5 x 0.8 = 450mA. The CAT4139 inductive boost LED driver has a switch current limit of 750mA (value), making it ideal for this application.
The inductor current rating should be able to handle the peak LED driver switching current without going into saturation. Once saturation occurs, there will be some current spikes because the inductor behaves like a resistor and the circuit no longer works as expected. An inductor with a current rating of 800mA or higher will do.
The output voltage of the LED should be kept below the output voltage when it is working. For three LEDs in series, the total forward voltage can be as high as 11.4V (3 x3.8V) at low temperatures.
The 24V open LED detection threshold is well above this limit. If the LED is disconnected, the output voltage increases and remains around 30V, and the device is in a low-power mode, drawing only a few milliamps from the supply. A 30V rated output capacitor is sufficient.
Now, we consider a 6W LED lamp powered by a 12V power supply. This can be achieved by using six high-brightness white LEDs connected in series and driven at a fixed current of 300mA, with a typical forward voltage of 3.3V.
LED string voltage is typically 20V, which can increase to 23V (6 x 3.8V) at low temperatures. This voltage is too high for a device like the CAT4139. A higher voltage boost LED driver (such as the CAT4240) is required to drive this load. The CAT4240 boost LED driver features a higher overvoltage detection threshold of 40V and is compatible with strings of up to 10 LEDs in series.
Figure 2: Using the CAT4201 BuckLED driver to drive five 1W LEDs from a 24V supply to provide accurate average current.
When the supply voltage is higher than the total LED forward voltage, a linear current source or a switching buck regulator can be used to provide a constant current to the LED.
Linear current sources have one disadvantage - the power dissipated in the regulator IC is proportional to the voltage difference from the source to the load. The switching solution has higher efficiency and prevents a lot of heat from dissipating in the IC, keeping it close to or slightly above ambient temperature.
The example in Figure 2 above shows how to drive five 1W LEDs from a 24V supply using the CAT4201 buck driver. LED current is set by external resistor R1. The CAT4201 step-down LED driver provides average current achieved in two-phase switching operation. During the phase, the internal CAT4201 FET switch connects the SW pin to ground so that the current rises and charges the inductor.
The voltage across the inductor is essentially 24V minus the LED voltage drop. Once the current reaches a defined peak, the internal switch closes and current continues to flow through the Schottky diode until the inductor discharges.
When the inductor current drops to zero, the above process is repeated to make the inductor current waveform appear triangular. In this case, the switching frequency is approximately 260kHz. Capacitor C2 across the LED minimizes LED current ripple.
The larger the capacitor size, the smaller the ripple. The overall converter efficiency (LED power divided by VBAT power) in this example is as high as approximately 94%.
As long as VBAT is higher than total VF plus 3V, the LED current remains well regulated. Below this level, the LED current decreases linearly. The switching regulator and its external components must be configured correctly for each specific application.
The high efficiency of switching regulators makes heat dissipation in power management circuits no longer an issue, and users can benefit from energy savings.
Linear current regulator ICs have the advantage of inherently low-noise operation (no switching) but are primarily suitable for lower current applications due to package temperature limitations.
When it comes to driving mid-sized panels and general lighting applications, inductive converter LED drivers are the solution, enabling well-regulated LEDs and optimal overall lighting efficiency. Choosing the correct inductive converter (boost or buck) depends on the application power supply and LED configuration.
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