The LT3796-1 is a DC/DC controller designed to regulate a constant current or constant voltage at the output, which is an essential requirement for driving LEDs. The device uniquely has two independent current sense amplifiers, and its high-side PMOS disconnect switch driver can operate in combination with a switching regulator (using a PWM pin) or independently (using the TGEN pin).
These features make the LT3796-1 suitable for certain special LED applications. For example, in high reliability lighting, the controller can be configured to drive two parallel LED strings to detect a single fault LED in any string (based on another string). Alternatively, for applications that require accurate analog dimming, the two current sense amplifiers can be extended to adjust the LED current in two ranges: high and low, extending the analog dimming capability of a high power LED driver to 100. :1, a 10x improvement over the dimming range typically available with a single current control loop.
Fault detection capability utilizing LED driving two identical parallel LED stringIt is tricky to detect a single LED with a performance degradation or short circuit in the string because the forward voltage can vary greatly over the entire load, temperature, and manufacturing tolerances. One way to eliminate such variations is to use two parallel matched strings so that any forward voltage relative deviation between the two strings can indicate the occurrence of a fault. In this type of solution, the string was originally made from picking parts and was chosen to achieve a total forward voltage drop match.
One potential problem with driving two parallel strings from a single output is that if one of the strings changes to an open or non-conducting state, then twice the current may flow through the other string. The dual current regulation loop in the LT3796-1 can be used to avoid this current disturbance.
Figure 1 shows how to configure ISP/ISN and CSP/CSN current sense amplifiers in a boost LED driver. In normal operation, the ISP/ISN current loop dominates and sets the LED current. The current flowing through LED string 1 is reported on ISMON. The CSP/CSN current loop is typically responsible for providing monitoring on the CSOUT pin because it is set at 25% lower than the regulation point of FB2. The CSOUT pin reports the current of 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, an external controller can be used to trigger the fault signal.
Figure 1: Boost LED driver for paired LED strings with detection and protection for faulty LEDs in any string
Figure 2 shows the oscilloscope waveform when one of the LEDs in the string 1 is shorted. The ISP/ISN current sense amplifier instantly detects an overcurrent event and disconnects both PMOS switches. After a soft start cycle, the ISP/ISN current loop begins to adjust the LED string to a new output voltage that corresponds to the forward voltage drop of the eight LEDs. Since the new output voltage cannot drive 9 LEDs, LED string 2 stops conducting current and CSOUT reports 0V. Similarly, if one of the LEDs 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)
In addition, the converter provides high-performance PWM dimming and robust short-circuit protection for the two strings via the high-side PMOS disconnect switch driver pin TG. The built-in overcurrent comparator located inside the ISP/ISN current sense amplifier protects the string 1 from short-circuit damage, while the circuit formed by D4, Q1 and R9~R11 is used to detect the short circuit of the string 2 and to FB1 The pin is driven high and the PMOS switches (M2 and M3) are turned off.
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