Datasheet

15Atmel LED Drivers-MSL3163/MSL3164
Connecting the Efficiency Optimizer to an LED
String Power Supply and Selecting Resistors
The MSL3163/4 are designed to control LED string
power supplies that use a voltage divider (R
TOP
and
R
BOTTOM
in Figure 6) to set output voltage, and whose
regulation feedback voltage is not more than 3.5V.
The Efficiency Optimizer improves power efficiency
by injecting a current of between 0µA and 255µA
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’s minimum and
maximum forward voltage drops at the peak current
set by R
ILED
(page 10). 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 the LEDs are 35V and 38V. Adding
an allowance of 0.5V of for the string drive MOSFET
headroom brings V
OUT(MIN)
to 35.5V and V
OUT(MAX)
to
38.5V. Do not to exceed the 40V maximum specification
of the string drivers STR1 thru STR15. Then determine
R
TOP
using:
)(
)()(
MAXFBOn
MINOUTMAXOUT
TOP
I
VV
R
−
= ,
where I
FBOn(MAX)
is the 255µA maximum output current
of the Efficiency Optimizer outputs FBOn (if cascading
multiple MSL3163/4s determine I
FBOn(MAX)
as shown in
the next section). 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 diode (1N4148 or similar) between
FBOn and the supply’s feedback node to protect the
MSL3163/4 against current flow into FBOn.
Using Multiple Atmel LED Drivers-
MSL3163/4s to Control a Common Power
Supply
Cascade multiple MSL3163/4 devices into a chain
configuration, with the FBIn of one device connected to
the FBOn of the next (Figure 7). Connect the first FBOn
to the power supply feedback resistor node through a
diode, and the unused FBIn inputs (and any unused
FBOn outputs) to GND as close to the MSL3163/4
as possible. Assign all strings powered by a common
supply to the proper FBOn output using String Set
registers (STRnSET) 0x20 thru 0x3F. The chained
devices work together to ensure that the system
operates at optimum efficiency. Note that the accuracy
of the feedback chain may degrade through each link of
the FBIn/FBOn chain by as much as 2%. Determine the
potential worst case maximum FBOn current I
FBOn(MAX/MIN)
using:
I
FBOn(MAX / MIN)
= 255µA* (0.98)
N-1
,
where N is the number of MSL3163/4s connected in
series. Use this result in the above R
TOP
resistor equation
for the term I
FBOn(MAX)
instead of using 255µA.
Take care in laying out the traces for the Efficiency
Optimizer connections. Minimize the FBIn/FBOn 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 FBIn. If these traces must
be routed near noisy signals, shield them from noise by
using ground planes or guard traces.
Atmel LED Drivers-MSL3163 and MSL3164
16-string White & RGB LED Drivers with Adaptive Power
Control, E²PROM, and SPI/I²C/SMBus Serial Interface