■ MAX25510·MAX25511 Drive up to 4 LED Strings

This article describes several methods for applying dimming signals to backlight high-brightness LED drivers.

The MAX25510 and MAX25511 are standalone, peak-current-mode LED drivers that can drive up to four strings of LEDs. Each of the four integrated current outputs can sink up to 120mA, and the LED current can be continuously adjusted using true pulse-width modulation (PWM) dimming, analog dimming, or a combination of the two, called hybrid dimming.

Figure 1 shows a typical application circuit for the MAX25510 and MAX25511.

True PWM and analog dimming functions are enabled by injecting square wave signals of the appropriate frequency and desired duty cycle into the DIM and ADIM pins of the aforementioned devices, respectively. Hybrid dimming combines the first two methods to reduce electromagnetic interference (EMI), increase dimming ratios, and reduce audio noise in applications.

Below we briefly look at the pros and cons of PWM dimming, analog dimming, and hybrid dimming.
■ True PWM dimming

True PWM dimming turns the LEDs on and off according to the programmed frequency and duty cycle of the dimming signal. The LED peak current remains constant throughout the entire dimming range, so there is no change in color temperature and very low minimum brightness levels can be achieved without significant errors in light output.

However, this method is vulnerable in the high brightness range. In the case of high brightness, the high conduction loss due to the resistive elements of this circuit when the LED current is on affects the efficiency in relation to the load current. In addition, when using ceramic capacitors, significant switching noise occurs at the output, which causes acoustic noise if the frequency of the PWM signal is within the audible range.

The MAX25510 and MAX25511 can track an external PWM dimming input at the DIM pin and provide output pulses with a minimum on-time of 300ns. The PWM dimming frequency can be from 100Hz to any value that does not unduly degrade the dimming ratio.

■ Analog dimming

Analog PWM dimming takes a PWM input and converts it into an analog control signal. The LED peak current value decreases linearly by a factor depending on the duty cycle of the applied signal.

This dimming method is advantageous when noise reduction is required, since the LED current is always continuous and any possibility of acoustic noise from the vibration of the output ceramic capacitor at high brightness is eliminated. Also, the conduction loss is low, which improves the efficiency per load current at low brightness levels.

On the other hand, when controlled purely by current, the minimum brightness level is limited due to the nonlinearity of the LED. Because of this, very small current values cannot be provided without loss of resolution.

The MAX25510 and MAX25511 implement the analog dimming function described above by accepting a square-wave signal (frequency range 10kHz to 100kHz) with a variable duty cycle on the ADIM pin. Figure 2 shows the relationship between ADIM setting and LED current (0% duty cycle corresponds to maximum LED current). The ADIM signal is internally converted to an 8-bit word, which is then converted into 256 brightness levels.
■ Hybrid dimming

Hybrid dimming combines the advantages of analog dimming and PWM dimming by dividing LED current control into two parts. In particular, the advantages of hybrid dimming are as follows:

- Reduced EMI due to low current amplitude during PWM dimming
- Reduced audio noise when using ceramic output capacitors
- Higher dimming ratio

In the high brightness range, the LED current varies linearly between the maximum display brightness level and the preset current threshold. Below the threshold, PWM dimming can control the brightness by modulating the duty cycle of the signal while maintaining the preset peak current.

A typical hybrid dimming case applied to the MAX25510 and MAX25511 is shown in Figure 3. The transition from analog dimming to PWM dimming occurs when the maximum LED current is reduced by a factor of X and the analog dimming duty cycle increases by a factor of 100*(X-1)/X. Before the transition, the duty cycle of PWM dimming is maintained at 100%, but after the transition, it is random between 100% and 0%.
Hybrid dimming can be implemented by using the DIM and ADIM inputs simultaneously in the MAX25510 and MAX25511. This requires two different PWM signals, one to control standard PWM dimming (connected to the DIM pin), and one to set the peak LED current between 0% and 100% (connected to the ADIM pin).

The following example demonstrates hybrid dimming operation with the MAX25512 EV kit featuring the MAX25511 IC.

By applying a 10kHz square wave signal with a duty cycle of 0% to 67% to the ADIM pin and maintaining a 1kHz-100% duty cycle square wave signal on the DIM pin, the LED current of up to 100mA per string is linearly reduced to 33mA. When the LED current reaches the breakpoint level of 33mA, the duty cycle of the signal on DIM is reduced to control the LED brightness via PWM dimming.

Figure 4 shows the MAX25511 operating circuit and actual external component values. This device is configured to have the phase shift feature disabled.
The results of the bench tests performed with this EV kit can be seen in the following oscilloscope screens (Figures 5–8), which show the various steps in the transition process between analog dimming and PWM dimming.
The relative LED brightness changes according to ADIM and DIM settings in the previous examples are summarized in Table 1.
Table 2 shows more examples of relative brightness in terms of total LED current and associated ADIM and DIM settings.
■ MAX25510·MAX25511, Hybrid Dimming Implementation

We've briefly looked at the three main categories of PWM dimming used in backlight LED drivers and shown how the MAX25510 and MAX25511 control LED current via true PWM dimming and analog dimming signals on dedicated pins. This architecture allows for hybrid dimming, which combines two dimming methods.

For more detailed information, see the MAX25510/MAX25511 data sheets and the MAX25512 EVKIT data sheet.

※ writing
Analog Devices, Inc.