LED Controller with Dual Current Regulation Loops Detects Faulty LEDs and Provides 100:1 Analog Dimming

The LT3796-1 is a DC/DC controller designed to regulate a constant current or constant voltage at the output, an essential requirement for driving LEDs. The device uniquely features two independent current sense amplifiers, and its high side PMOS disconnect switch driver can operate either in combination with the switching regulator (using the PWM pin) or independently (using the TGEN pin).

These features enable the LT3796-1 to meet the needs of certain special LED applications. For example, in high-reliability lighting, the controller can be configured to drive two parallel LED strings so that a single faulty LED in either string can be detected (using the other string as a reference). Or, for applications that require accurate analog dimming, the two current sense amplifiers can be expanded to regulate the LED current in two ranges: high and low, thereby extending the analog dimming capability of a high power LED driver to 100:1, a 10x improvement over the dimming range typically available when using a single current control loop.

Driving two identical LED strings in parallel using faulty LED detection capability

Detecting a single degraded or shorted LED in a string is tricky because the forward voltage varies greatly across load, temperature, and manufacturing tolerances. One way to eliminate this variation is to use two matched strings in parallel so that any relative deviation in forward voltage between the two strings indicates a fault. In this type of solution, the strings are initially made from selected parts that are chosen to match the total forward voltage drop.

A potential problem with driving two parallel strings from a single output is that if one string becomes open or non-conducting, twice the current may flow in the other string. The dual current regulation loops in the LT3796-1 can be used to avoid this current disturbance.

Figure 1 shows how the ISP/ISN and CSP/CSN current sense amplifiers are configured in a boost LED driver. In normal operation, the ISP/ISN current loop dominates and sets the LED current. The current through LED string 1 is reported on ISMON. The CSP/CSN current loop is typically responsible for providing monitoring at the CSOUT pin, as it is set 25% below the regulation point of FB2. The CSOUT pin reports the current in LED string 2 as follows:

If the two strings are unbalanced, the reporting pins ISMON and CSOUT will show the relative LED current deviation as a voltage. By comparing the two analog signals externally, a fault signal can be triggered by an external controller.

Figure 1: Boost LED driver for paired LED strings with detection and protection for a faulty LED in either string

Figure 2 shows the oscilloscope waveform when one LED in string 1 is shorted. The ISP/ISN current sense amplifier detects the overcurrent event instantly and disconnects both PMOS switches. After a soft-start period, the ISP/ISN current loop begins to regulate the LED string at a new output voltage that corresponds to the forward voltage drop of 8 LEDs. Since the new output voltage cannot drive 9 LEDs, LED string 2 stops conducting current and CSOUT reports 0V. Similarly, if one LED in string 2 is shorted, the CSP/CSN current loop takes over control and regulates the output current to 625mA through the FB2 pin, as shown in Figure 3. LED string 1 stops conducting current and ISMON reports 0V.

Figure 2: Shorting one LED in string 1 (for the circuit shown in Figure 1)

Figure 3: Shorting one LED in string 2 (for the circuit shown in Figure 1)

The converter also provides high-performance PWM dimming and robust short-circuit protection for both strings through the high-side PMOS disconnect switch driver pin TG. The built-in overcurrent comparator inside the ISP/ISN current sense amplifier protects string 1 from short-circuit damage, while the circuit formed by D4, Q1 and R9~R11 is used to detect the short circuit of string 2, drive the FB1 pin to a high level and turn off the PMOS switches (M2 and M3).

LED Driver with 100:1 Analog Dimming Ratio

Many high power LED applications require a high analog dimming ratio, which is difficult to achieve with a single current sense path. The problem is dynamic range: at high currents, a low differential voltage (typically 250mV or less) is needed to limit power dissipation in the sense resistor, but because the available signal is so small, the current sense amplifier accuracy of a few mV can have a significant impact on the error in the sense voltage even with 10% analog dimming. The LT3796-1's two current sense loops enable it to produce a high analog dimming ratio by splitting the current regulation task between the two loops. One loop has a low value sense resistor to limit power dissipation in the high current path, while the other loop uses a higher sense signal in the low current path to improve accuracy where power dissipation is less of a concern. Figure 4 shows the LT3796-1 configured to produce a 100:1 analog dimming ratio in SEPIC mode using the LTC1541, a precision reference, op amp and comparator in a single package.

Figure 4: A SEPIC mode LED driver with 100:1 analog dimming ratio

Assuming that the RDS(ON) of M2 is negligible, in the high current range between 200mA and 1A, the ISP/ISN current loop regulates the output current at:

When VCTRL drops below 1.2V, the LTC1541’s comparator forces the TGEN pin low and disconnects M2. The LED string is then sensed and regulated only by the CSP/CSN loop. When the CSP/CSN loop is in control, the FB2 pin voltage is regulated at 1.25V, and the CTRL_IN input sets the CSP/CSN threshold by pulling current from the CSOUT pin (via R6). The CSP/CSN dimming range is from 668mV to 33.4mV, allowing detection of low LED currents between 200mA and 10mA while maintaining accuracy. In summary, the combined dual current loops provide a 100:1 analog dimming range, as shown in Figure 5.

Figure 5: Relationship between ILED and CTRL (for the solution in Figure 4)

in conclusion

The LT3796-1 is an LED controller that features two independent current sense amplifiers with reporting, two FB pins and a high side disconnect switch driver. In addition, the device has robust fault protection and a versatile toolset to address challenging LED applications such as high reliability and high performance analog dimming.