Datasheet

21Atmel LED Drivers-MSL2160/MSL2161
The MSL2160/1 are designed to control external LED string power supplies that use voltage dividers (R
TOP
and R
BOTTOM
in Figure 5) to set the 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 0 and 255µA 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
S
(page
12). 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 255µA maximum output current of the efficiency optimizer outputs FBOn. 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 diode (1N4148 or similar) between FBOn and
the supply’s feedback node to protect the MSL2160/1 against current flow into FBOn. Assign all strings powered by a
common supply to the proper FBO output using string set registers 0x40 - 0x5F.
Once configured, determine the change in power supply output voltage in response to a change in FBO output current using:
TOPFBOOUT
RIV ∗∆=∆
.
Direct PWM Control of the LED Strings
An external PWM signal applied to the PWM input allows direct PWM control over the strings when bits PWMEN and
PWMDIRECT (bits D0 and D1 in PWM control register 0x2D) are set to 1. This configuration bypasses PHI and GSC, but
allows LED string phase delay via the phase delay enable bit, PHADLYEN (bit D0 of register 0x2E). With phase delay
enabled, a progressive delay of 1/16 the PWM frame is calculated and applied successively to each string drive signal.
The PWM input can also be configured as a gate for the output of the PWM engine using PWM global enable
(bit D2 of the PWM control register 0x2D).
Connecting the Efficiency Optimizer to an LED String Power Supply and Selecting Resistors
Atmel LED Drivers-MSL2160/MSL2161
16-string, White and RGB LED Drivers with Adaptive Configuration,
EEPROM, and SPI/I
2
C/SMBus Serial Interface