Ambient Temperature Compensation
Drive circuitry can be designed which
compensates for increasing ambient temperature
by decreasing the forward current to the LED
array. This allows the lamp designer to drive the
LED array at a higher forward current by reducing
the amount of current derating.
Temperature compensation is achieved by
incorporating temperature sensitive components
into the drive circuitry, such as positive
temperature coefficient (PTC) resistors. An
example of the resistance vs. temperature
characteristics of a PTC resistor is shown in
Figure 4.10.
Figure 4.9 LED driver module for automotive
lighting applications.
“Switching” Power Supplies
Current sources, which operate efficiently over a
wide range on input voltages, can be designed
using pulseꢀwidth modulation (PWM) circuitry.
Such circuits have the advantage of low heat
dissipation, and large input voltage compliance.
This type of power supply is traditionally used in
applications where electrical efficiency and heat
dissipation are of critical importance, such as a
laptop computer. Due to their widespread
adoption in other applications, the cost of
components has decreased, and their
availability has increased, making this an
interesting alternative for driving LED arrays.
A block diagram of a simple switching current
source is shown in Figure 4.9.
Figure 4.10 Resistance-Temperature curve for
PTC resistor.
The PWM module varies the pulse width based
on the input and feedback voltages. The
feedback voltage is proportional to the current
through the LED array, where voltage is
measured directly above a small fixed
resistance connected to ground. The filter
circuitry is used to smooth out the output
voltage of the PWM / transistor switch. With
minor modifications, this type of circuit can be
used to drive multiple LED arrays and a variety
of drive circuits.
It can be seen that the resistance of such a device
radically increases when the body temperature of
the PTC resistor reaches the switching
temperature. By designing a drive circuit such that
the switching temperature occurs at a
temperature less than Ta(max), full current derating
is not necessary.
Consider the case in which the switching
temperature of the PTC resistor is achieved at an
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