Datasheet

14 Atmel LED Driver-MSL2100
Setting LED String Currents
The maximum string current, I
LED
, for each string is
set by a resistor, R
Sn
, connected to ground from the
source terminal of the external string drive MOSFET.
The feedback threshold is 500mV. Determine the
resistor value using:
LED
Sn
I
R
5.0
=
, where I
LED
is in amperes and R
Sn
is in
ohms.
For example, a full-scale LED current of 500mA
returns R
Sn
= 1.00Ω. The current for the LED strings is
individually reduced from the full-scale resistor set value
with 6-bit resolution using ISTR
n
, the string current
control registers 0x10 through 0x1F.
Connecting an LED String Power Supply
to an Efficiency Optimizer Output
The MSL2100 is designed to control up to three
external LED string power supplies that use voltage
dividers (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 dividers of the external
power supplies, dynamically adjusting their outputs
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:
[ ]
( )
5.0#
)()(
+∗= ofLEDsVV
MINfMINOUT
,
and
[ ]
( )
5.0#
)()(
+∗= ofLEDsVV
MAXfMAXOUT
,
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_). 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:
)(
)()(
MAXFBOn
MINOUTMAXOUT
TOP
I
VV
R
−
= ,
where I
FBOn(MAX)
is the 45µA maximum output current
of the Efficiency Optimizer outputs, FBO1, FBO2, and
FBO3. Finally, determine R
BOTTOM
using:
FBMAXOUT
FB
TOPBOTTOM
VV
V
RR
−
∗=
)(
,
where V
FB
is the regulation feedback voltage of the
power supply. Place a Schottky diode (CMPSH-3 or
similar) between FBOn and the supply’s feedback node
to protect the MSL2100 against current flow into FBOn.
Figure 4. FBOn Connection to Power Supply Voltage Divider