SOME MULTI-STRING LED MODULES come with a common-anode configuration. The commonanode 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. In this idea, we’ll drive a commonanode LED module while simultaneously limiting the LED string voltage when an LED string becomes open.
Figure 1 shows the LT3496 triple-output LED driver in a buck-mode configuration. Its LED strings reside between PVIN and the 200-m sense resistors, enabling the common-anode connection at PVIN. (A typical buck-mode configuration has three free-floating LED strings.) In a normal steady-state operation, this circuit delivers 500 mA to each LED string.
Programmed overvoltage protection (OVP) isn’t always needed in a buck-mode LED driver circuit. Unlike boost, buck-boost, and SEPIC drivers, the switch voltage of a buck-mode LED driver droops when an LED string is opened. In this case, OVP isn’t needed. However, the CAP1 pin can be used as an open circuit indicator. Furthermore, an open collector buffer may be needed in some applications. For simplicity, the reference designators of Channel 1 are used exclusively.
A potential issue arises if an LED string goes open and is subsequently reconnected. This could occur if, say, a cable connection between the LED driver and the LED module isn’t a constant connection and intermittently disconnects and reconnects.
Under these circumstances, the LED string can experience a large inrush current for a number of microseconds after it’s reconnected. This large current is due to the discharge of capacitor C4. The amplitude of this inrush current is related to the difference between PVIN and the LED string voltage—the larger the difference, the higher the inrush current. For example, in Figure 1’s setup, should the LED be disconnected and then be reconnected, one could expect 1.2-A inrush current in the LEDs.
If inrush current is a concern, then the voltage across the LED string terminals needs to be clamped to a voltage that’s just slightly higher than the LED string voltage when the string is open. Figure 2 shows a circuit that limits the voltage across the LED string to an OVP level set by resistors R1 and R3. This would be 15 V in this example.
For an OVP circuit to be effective, though, CAP1 must be brought up after the OVP logic turns off the main switch. Resistor R4 provides a few hundred microamperes of pull-up current for CAP1. Without R4, CAP1 is held low, making the OVP circuit moot.