Datasheet

20
MSL2023/2024 [DATASHEET]
42063A–LED–02/2013
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 G drives the gate
of the MOSFET at up to VIN - 2.0V. Select a MOSFET with a low RDS(ON) and a maximum drain-source voltage of at
least 20% greater than:
where 340µA is the maximum FBO full scale current.
11.5 Selecting the Drain Resistor – R
D
The drain resistor, R
D
in the “Typical Application Circuit ” on page 16, connects the MSL2023/24 to the Drain of the Main
string external MOSFET. Use a 100k for R
D
.
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
+