Some multi-string LED modules use a common anode configuration; however, this common anode connection reduces the number of wires between the LED module and its driver from 2N to N + 1, where N is the number of LED strings in the module. This article will clarify how to drive such a common anode LED module and simultaneously limit the LED string voltage when an LED string changes to an open state.
Figure 1 shows the LT3496 triple output LED driver in buck mode configuration where the LED string is placed between the PVIN and 200mΩ sense resistors to achieve a common anode connection on PVIN. This is completely different from the common buck mode configuration for 3 free floating LED strings. In standard steady-state operation, the circuit delivers 500mA to each LED string.
In the buck mode LED driver circuit, programming overvoltage protection (OVP) is not always required. Unlike boost, buck-boost, and SEPIC drivers, the switching voltage of the buck mode LED driver will drop when an LED string is open. In this case, the OVP function is not required. However, the CAP1 pin can be used as an open indicator. In addition, an open collector buffer may be required in some applications. For the sake of simplicity, we only used the reference indicator for channel 1 in the description below.
If an LED string changes to an open state and is subsequently reconnected, a problem may occur. This can happen, for example, if the cable connection between the LED driver and the LED module is not a constant connection (which needs to be disconnected and reconnected from time to time). In this case, when the LED string is reconnected, it experiences a large inrush current (for a few microseconds). This large inrush current is caused by the discharge of capacitor C4. The magnitude of this inrush current is related to the difference between the PVIN and the LED string voltage - the greater the voltage difference, the greater the inrush current. For example, in Figure 1, for a 24V input and 3-LED configuration, the measured peak inrush current is 1.2A.
If you are concerned about inrush current, when the LED string is open, it is necessary to clamp the voltage across the LED string to a voltage that is only slightly higher than the LED string voltage. Figure 2 shows a circuit that limits the voltage across the LED string to an OVP level set by resistors R1 and R3. In this example, the OVP level will be 15V. However, in order for the OVP circuit to be active, the CAP1 pin voltage must be pulled up after the OVP logic has disconnected the main switch. Resistor R4 provides hundreds of microamps (μA) of pull-up current to the CAP1 pin. When R4 is not used, the CAP1 pin is held low, making the OVP circuit inoperative.
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