300857a1
FIGURE 4. Thermal Foldback Circuitry
THERMAL FOLDBACK / ANALOG DIMMING
Thermal foldback is necessary in many applications due to
the extreme temperatures created in LED environments. In
general, two functions are necessary: a temperature break-
point (T
BK
) after which the nominal operating current needs to
be reduced, and a slope corresponding to the amount of LED
current decrease per temperature increase as shown in
. The LM3424 allows the user to program both the
breakpoint and slope of the thermal foldback profile.
300857c2
FIGURE 5. Ideal Thermal Foldback Profile
Foldback is accomplished by adding current (I
TF
) to the CSH
summing node. As more current is added, less current is
needed from the high side amplifier and correspondingly, the
LED current is regulated to a lower value. The final tempera-
ture (T
END
) is reached when I
TF
= I
CSH
causing no current to
be needed from the high-side amplifier, yielding I
LED
= 0A.
shows how the thermal foldback circuitry is physically
implemented in the system. I
TF
is set by placing a differential
voltage (V
DIF
= V
TREF
– V
TSENSE
) across TSENSE and TREF.
V
TREF
can be set with a simple resistor divider (R
REF1
and
R
REF2
) supplied from the V
S
voltage reference (typical 2.45V).
V
TSENSE
is set with a temperature dependant voltage (as tem-
perature increases, voltage should decrease).
An NTC thermistor is the most cost effective device used to
sense temperature. As the temperature of the thermistor in-
creases, its resistance decreases (albeit non-linearly). Usu-
ally, the NTC manufacturer's datasheet will detail the
resistance-temperature characteristic of the thermistor. The
thermistor will have a different resistance (R
NTC
) at each tem-
perature. The nominal resistance of an NTC is the resistance
when the temperature is 25°C (R
25
) and in many datasheets
this will be given a multiplier of 1. Then the resistance at a
higher temperature will have a multiplier less than 1 (i.e. R
85
multiplier is 0.161 therefore R
85
= 0.161 x R
25
). Given a de-
sired T
BK
and T
END
, the corresponding resistances at those
temperatures (R
NTC-BK
and R
NTC-END
) can be found.
Using the NTC method, a resistor divider from V
S
can be im-
plemented with a resistor connected between V
S
and
TSENSE and the NTC thermistor placed at the desired loca-
tion and connected from TSENSE to GND. This will ensure
that the desired temperature-voltage characteristic occurs at
TSENSE.
If a linear decrease over the foldback range is necessary, a
precision temperature sensor such as the LM94022 can be
used instead as shown in
to set V
TSENSE
according to the temperature. However, for the
rest of this datasheet, the NTC method will be used for thermal
foldback calculations.
During operation, if V
DIF
< 0V, then the sensed temperature
is less than T
BK
and the differential sense amplifier will regu-
late its output to zero forcing I
TF
= 0. This maintains the
nominal LED current and no foldback is observed.
At T
BK
, V
DIF
= 0V exactly and I
TF
is still zero. Looking at the
manufacturer's datasheet for the NTC thermistor, R
NTC-BK
can
be obtained for the desired T
BK
and the voltage relationship
at the breakpoint (V
TSENSE-BK
= V
TREF
) can be defined:
A general rule of thumb is to set R
REF1
= R
REF2
simplifying the
breakpoint relationship to R
BIAS
= R
NTC-BK
.
If V
DIF
> 0V (temperature is above T
BK
), then the amplifier will
regulate its output equal to the input forcing V
DIF
across the
resistor (R
GAIN
) connected from TGAIN to GND. R
GAIN
ulti-
mately sets the slope of the LED current decrease with re-
spect to increasing temperature by changing I
TF
:
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LM3424