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MC10SX1130
http://onsemi.com
7
First, the RSET resistor must be chosen to set the desired
nominal modulation current based on the following
equation:
RSET = V
SET
/I
MOD
(Equation 1)
The voltage at VSET is a function of the RTCO tracking
resistor, so the desired tracking rate (VTR) must also be
chosen. To determine this, the equation must be normalized
to correspond to how the LED has been specified.
Temp Co = V
TR
/V
SET
(Equation 2)
The data sheet has three temperature tracking rates for
different values of the RTCO resistor. By using the V
SET
values at 25
°
C and substituting those numbers into
Equation 2, normalized tracking rates can be calculated.
Table 5. Normalized Tracking at 25
°
C
RTCO
Tracking %/
°
C
Short
+0.20
1 K
+0.52
2 K
+0.89
To match the LED chosen, a 1 k resistor can be used.
Now that this is known, the value of the voltage at the V
SET
can be substituted into Equation 1 to determine the value of
RSET resistor which, for this example is 10 .
The Stretch circuit can be used to compensate for the
turn-on/turn-off delay of the LED. The circuit has been
designed for ease of use so the pin is designed to be strapped
to one of the two power plane levels to select the
pre-distortion value. If no pre-distortion is desired, the pin
can be left open. In this +5 V example, the maximum amount
of pre-distortion is desired, so the STRETCH pin is
connected to ground.
In addition a resistor must be placed between I
OUT
and
V
CC
. In selecting this resistor, just as in the case of the RSET,
the resistor type should be chosen to dissipate the worst case
power and derated for the worst case temperature. As a rule
of thumb, the voltage drop across the resistor should match
the forward voltage across the diode. The voltage can be
larger to minimize the power dissipated on chip when the
LED is not ’ON’. Although, the voltage drop across this
resistor should not be greater than 2 V. For this example:
R @ I
OUT
= VF/I
MOD
IMOD(max)
VSET@85 C
RSET
855mV
10
86mA
R @ I
OUT
= 1.5V/86mA = 17
Because of the positive tracking circuitry in the LED
driver, the modulation current will increase over
temperature. It is important to now go back and re-calculate
the numbers under the worst case environmental conditions
to ensure that operating conditions have not been exceeded.
Thermal Management
LED devices tend to require large amounts of current for
most efficient operation. This requirement is then translated
into the design of the LED Driver. When large modulation
currents are required, power dissipation becomes a critical
issue and the user must be concerned about the junction
temperature of the device. The following equation can be
used to estimate the junction temperature of a device in a
given environment:
T
J
= T
A
+ P
D
*
JA
(Equation 3)
T
J
Junction Temperature
T
A
Ambient Temperature
P
D
Power Dissipation
JA
Average Thermal Resistance
(Junction-Ambient)
A specially designed thermally enhanced leadframe has
been used to house the LED Driver. Below is a graph of the
average
JA
plotted against air flow.
Figure 4. Typical
JA
versus Airflow
70
80
90
100
110
0
100
200
300
400
500
AIRFLOW (LFPM)
J
Θ
°
The power dissipation of the device has two components;
the quiescent power drain related to the pre-drive circuitry,
and the power dissipated in the current switch when driving
the LED.
Pd = Pstatic + Pswitching
(Equation 4)
The power dissipated in the current switch is a function of
the IMOD current, the LED forward voltage, and the value
of RSET. For example in a +5 V application, the following
equations can be used:
Pstatic = V
CC
* I
CC
(Equation 5)
Pswitching = (V
CC
-V
F
-V
SET
)* I
MOD
(Equation 6)