Datasheet

16
MSL2021 [DATASHEET]
42062A–LED–02/2013
where MREF is the decimal equivalent of the value in register 0x20. The default value for MREF is 0x64, for a feedback
voltage of 0.2V. Change the feedback voltage by changing the value in register 0x20 using the serial interface. LED
average current is within ±3% of the targeted value when a 1% resistor is used for R
S
.
11.3 Setting AC/DC Output Voltage
The efficiency optimizer output, FBO, connects to the AC/DC or DC/DC converter’s output voltage feedback node, and
pulls current from the node to force the converter’s output voltage up. The MSL2021 works with any input power
converter topology that uses a resistor divider to set its output voltage. Operation with a AC/DC PFC converter is
described below.
Select the two resistors that set the nominal AC/DC LED power supply’s output voltage by first determining the minimum
output voltage using:
where V
fMIN
is the minimum LED forward voltage for the Main string LEDs at the expected LED current, N is the number
of LEDs in the string, and 0.2V is the minimum overhead required for the current sense resistor and the FET. Then
determine the maximum output voltage using:
where V
fMAX
is the maximum LED forward voltage for the Main string LEDs at the operating LED current, N is the number
of LEDs in the string, and 1.2V is the maximum overhead required for the current sense resistor and the FET. Determine
the value for the upper voltage setting resistor using:
where 170A is the minimum FBO full scale current. Determine the lower resistor using:
where V
FB
is the feedback regulation voltage of the switch mode converter.
11.4 Selecting the Main String MOSFET
The Main string MOSFET sinks the string current to ground through current sense resistor R
S
. Output of pin G drives the
gate of the MOSFET at up to VIN - 2V. Select a MOSFET with a maximum drain-source voltage of at least 20% above:
where 340µA is the maximum FBO full scale current.
11.5 Selecting the Drain Resistor – R
D
The drain resistor, R
D
, connects the MSL2021 to the drain of the main string external MOSFET. Use a 100k for R
D
.
V
SFB
0.002 MREFV=
V
OUT MIN
V
fMIN
N 0.2V+
V
OUT MAX
V
fMAX
N 1.2V+=
R
TOP
V
OUT MAX
V
OUT MIN 
–
170 10
6–

-----------------------------------------------------------------

R
BOTTOM
R
TOP
V
FB
V
OUT MIN
V
FB
–
------------------------------------------- -
=
V
fb
R
TOP
R
BOTTOM
------------------------ 1+


340AR
TOP
+