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.
