LM3424 Datasheet, PDF (12/50 Pages) National Semiconductor (TI) – Constant Current N-Channel Controller with Thermal Foldback for Driving LEDs

FIGURE 4. Thermal Foldback Circuitry
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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 Fig-
ure 5. The LM3424 allows the user to program both the
breakpoint and slope of the thermal foldback profile.
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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.
Figure 4 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 (R₂₅) 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. R₈₅
multiplier is 0.161 therefore R₈₅ = 0.161 x R₂₅). 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 Figure 4. Either method can be used
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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