A specially designed thermally enhanced leadframe has
been used to house the LED Driver. Below is a graph of the
Normalized Tracking at 25°C
RTCO
Short
1 KΩ
2 KΩ
Tracking %/°C
average Θ
plotted against air flow.
JA
+0.20
+0.52
+0.89
110
100
90
To match the LED chosen, a 1KΩ resistor can be used.
Now that this is known, the value of the voltage at the V
can be substituted into Equation 1 to determine the value of
SET
RSET resistor which, for this example is 10Ω.
80
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 +5V example, the maximum amount of
pre-distortion is desired, so the STRETCH pin is connected
to ground.
70
0
100
200
300
400
500
AIRFLOW (LFPM)
Figure 2. Typical Θ
versus Airflow
JA
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.
In addition a resistor must be placed between I
OUT
and
VCC. 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 2V. For this
example:
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 +5V application, the following
equations can be used:
Pstatic = V
* I
CC CC
(Equation 5)
(Equation 6)
R @ I
= VF/I
MOD
OUT
Pswitching = (V -V -V
CC
)* I
SET MOD
F
°
V
@85 C
Now to calculate the dissipated power on the chip for a
nominal application.
SET
RSET
855mV
10
I
86mA
MOD(max)
V
V
V
I
I
= 5V
CC
F
SET
MOD
CC
= 1.5V
= 0.7V
= 60mA
= 18mA
R @ I
OUT
= 1.5V/86mA = 17Ω
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.
so:
Pd = 5 * 18 + (5 - 1.5 - 0.7) * 60
Pd = 258mW
Thermal Management
This number can be entered into Equation 3 along with the
environmental information to calculate the nominal operating
junction temperature.
LED devices tend to require large amounts of current for
most efficent 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:
Because of the open loop feedback control in the bias
control circuitry, the revised I
value must be determined
MOD
given the tracking rate chosen so that the power dissipation
can be re-calculated. For assessing product reliability, worst
case values should be entered to calculate the maximum
junction temperature.
T = T + P * Θ
(Equation 3)
J
A
D
JA
Reliability of Plastic Packages
Although today’s plastic packages are as reliable as
ceramic packages under most environmental conditions, as
the junction temperature increases a failure mode unique to
plastic packages becomes a significant factor in the long
term reliability of the device.
T
Junction Temperature
Ambient Temperature
Power Dissipation
Average Thermal Resistance
(Junction-Ambient)
J
T
A
P
Θ
D
JA
MOTOROLA
6
High Performance Frequency
Control Products — BR1334
MOTOROLA [ MOTOROLA ]
