LM3424 Datasheet, PDF (14/69 Pages) National Semiconductor (TI) – Constant Current N-Channel Controller with Thermal Foldback for Driving LEDs

LM3424, LM3424-Q1
SNVS603C – AUGUST 2009 – REVISED AUGUST 2015
Feature Description (continued)
I_LED
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L
1
R_GAIN
0
T
T_BK
T_END
Figure 19. Ideal Thermal Foldback Profile
I_LED
R_HSP
V_SNS R_SNS
R_HSN
R_CSH
C_CMP
HSP
High-Side
Sense Amplifier
LM3424
V_S
2.45V
100k 100k _TREF
R_REF2
NTC
HSN
I_CSH
CSH
COMP
I_TF
100k
1.24V
Error Amplifier
To PWM
Comparator
V_DIF
TSENSE
100k
TGAIN
R_REF1
R_BIAS
R_NTC
R_GAIN
LM94022
Figure 20. Thermal Foldback Circuitry
Precision Temp Sensor
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 temperature (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 20 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 temperature increases, voltage should decrease).
An NTC thermistor is the most cost effective device used to sense temperature. As the temperature of the
thermistor increases, its resistance decreases (albeit non-linearly). Usually, 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 temperature. 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 (that is, R₈₅ multiplier is 0.161 therefore R₈₅ = 0.161 x R₂₅).
Given a desired 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 implemented with a resistor connected between V_S and
TSENSE and the NTC thermistor placed at the desired location 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 20. 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.
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