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Choosing the Drain Resistor R D
The drain resistor R D connects the MSL2041/2 to
the Drain of the external MOSFET. Choose R D using:
Table 2. Some Typical I DARK and V F(DARK) Values Determined
Using Figure 7
Ω ,
LED
TYPE
LOW
POWER
LED
PART #
LW Y1SG
I DARK (μA)
1.72
V F(DARK) (V)
2.285
LW G6SP
1.67
2.276
LXLW-PWC1
1.72
2.195
where V OUT(MAX) is the value calculated above in the
section “ Connecting the Efficiency Optimizer to an LED
String Power Supply and Selecting Resistors ” beginning
on page12, N is the number of LEDs in the string, I DARK
is the maximum allowable string off current and V F(DARK)
is the LED forward voltage drop at I DARK . When the value
calculated for R D < 0 use 0 Ω .
LED manufacturers typically do not publish I DARK and
V F(DARK) information. One way to determine these
numbers is to use the following method.
Set up the test circuit of Figure 7. Adjust R1
until the current meter indicates I DARK (choose
I DARK < 1mA). Use a volt meter to measure
the voltage at the anode of the LED (A),
MEDIUM
POWER
HIGH
POWER
Large values of R D may cause false LED short circuit
faults. Discharge of the parasitic capacitance at the Dn
node through a large R D holds the node above the
string fault threshold for longer than the LED short
circuit verification time. The addition of a feed-forward
capacitor, C FF in Figure 8, mitigates this issue. The
value for C FF depends upon the amount of parasitic
capacitance at the Dn node and the size of R D , but C FF
= 15pF is an appropriate first approximation.
and then at the cathode of the LED (B).
Subtract the voltage measured at B from that
measured at A to determine V F(DARK) . Some
typical values determined using this method
are listed in Table 2.
?
Figure 8. Feed Forward Capacitor
Figure 7. Test Circuit for Determining
Figure 7. Test Circuit for Determining V F(DARK)
18
Figure 8. Feed Forward Capacitor
Atmel LED Drivers-MSL2041/2042
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