MSL3085BT Datasheet, PDF (14/20 Pages) ATMEL Corporation – 8-string White and RGB LED Driver with Adaptive Power Control, EEPROM, and I2C/SMBus Serial Interface

Using Multiple Atmel LED Driver-MSL3085s to Control a Common Power Supply
Cascade multiple MSL3085 devices into a chain configuration (Figure 5), with the FBI1 of one device connected to the
FBO1 of the next. Connect the first FBO1 to the power supply feedback resistor node through a diode, and the unused
FBI1 input to ground as close to the MSL3085 as possible. Assure that all strings powered by the common supply are
assigned to FBO1 using string set registers (STRnSET) 0x20 - 0x3D (FBO1 is the power-up default setting for all
strings). The chained devices work together to ensure that the system operates at optimum efficiency. Note that the
accuracy of the feedback chain can degrade through each link of the FBI1/FBO1 chain by as much as 2%. Determine the
potential worst case maximum FBO1 current, I , _{FBOn(MAX/MIN)} using:
I_{FBO(MAX / MIN)} = 255µA* (0.98)^N-1 ,
where N is the number of MSL3085s connected in series. Use this result in the above R_TOP resistor equation for the term
I
instead of using 255µA.
FBOn(MAX)
Take care in laying out the traces for the efficiency optimizer connections. Minimize the FBI1/FBO1 trace lengths as much
as possible. Do not route the signals close to traces with large variations in voltage or current because noise may couple into
FBI1. If these traces must be routed near noisy signals, shield them from noise by using ground planes or guard traces.
Figure 5. Example of Cascading Multiple Devices to Optimize a Common String Power Supply.
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Atmel LED Driver-MSL3085