Datasheet

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
TOP
and
R
BOTTOM
in Figure 4) to set the output voltage, and whose
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
OUT(MIN)
and V
OUT(MAX)
, the minimum and maximum string
supply voltage limits, using:
V
OUT(MIN)
= (V
f (MIN)
*
[#ofLEDs])+ 0.5 ,
and
V
OUT(MAX)
= (V
f (MAX)
*
[#ofLEDs])+ 0.5 ,
where V
f(MIN)
and V
f(MAX)
are the LED minimum and
maximum forward voltage drops at the peak current
set by R
Sn
(page 11). For example, if the LED data
are V
f(MIN)
= 3.5V and V
f(MAX)
= 3.8V, and ten LEDs are
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
OUT(MIN)
to 35.5V and
V
OUT(MAX)
to 38.5V. Then determine R
TOP
using:
R
TOP
=
V
OUT (MAX)
_ V
OUT(MIN)
,
I
FBO(MAX)
where I
FBO(MAX)
is the 45µA maximum output current of
the efficiency optimizer output, FBO. Finally, determine
R
BOTTOM
using:
R
BOTTOM
= R
TOP *
V
FB
,
V
OUT(MAX)
_
V
FB
where V
FB
is the regulation feedback voltage of the
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
FBO(MAX/MIN)
, using:
I
FBO(MAX / MIN)
= 45µA* (0.98)
N-1
,
where N is the number of MSL3082s connected in
series. Use this result in the above R
TOP
resistor equation
for the term I
FBO(MAX)
instead of using the 45µA figure
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.