12

Atmel LED Driver-MSL3082

Connecting an LED String Power Supply

to an Efficiency Optimizer

The MSL3082 is designed to control an external LED

string power supply that uses a voltage divider (R

R

regulation feedback voltage is not more than 1.5V. The

efficiency optimizer improves power efficiency by injecting

a current of between 0 and 45µA into the voltage divider

of the external power supply, dynamically adjusting the

power supply's output to the minimum voltage required

by the LED strings. To select the resistors first determine

V

supply voltage limits, using:

V

and

V

where V

maximum forward voltage drops at the peak current

set by R

are V

used in a string, then the total minimum and maximum

voltage drops across a string are 35V and 38V,

respectively. Adding an allowance of 0.5V for the string

drive MOSFET headroom brings V

V

R

V

I

FBO(MAX)

where I

the efficiency optimizer output, FBO. Finally, determine

R

R

*

V

FB

,

V

where V

power supply. Place a Schottky diode (CMPSH-3 or

similar) between FBO and the supply’s feedback node

to protect the MSL3082 against current flow into FBO.

Figure 4. FBO Connection to Power Supply Voltage Divider

Using Multiple Atmel LED Driver-MSL3082s

to Control a Single Power Supply

Cascade multiple MSL3082 devices into a chain

configuration (Figure 5) with the FBI of one device

connected to the FBO of the next. Connect the first FBO

to the power supply feedback resistor node through a

CMPSH-3 or similar Schottky diode, and the unused FBI

input to ground as close to the MSL3082 as possible.

The chained devices work together to ensure that the

system operates at optimum efficiency. Note that the

accuracy of the feedback chain has the potential to

degrade through each link of the FBI/FBO chain by as

much as 2%. Determine the worst case maximum FBO

current, I

I

N-1

,

where N is the number of MSL3082s connected in

series. Use this result in the above R

for the term I

shown here.

Take care in laying out the traces for the efficiency

optimizer connections. Minimize the FBI/FBO 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 FBI. If these traces must

be routed near noisy signals, shield them from noise by

using ground planes and/or guard traces.