LM3424 Datasheet, PDF (15/69 Pages) National Semiconductor (TI) – Constant Current N-Channel Controller with Thermal Foldback for Driving LEDs

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LM3424, LM3424-Q1
SNVS603C – AUGUST 2009 – REVISED AUGUST 2015
Feature Description (continued)
During operation, if V_DIF < 0V, then the sensed temperature is less than T_BK and the differential sense amplifier
will regulate 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:
R_REF1
R_REF1 + R_REF2
=
R_NTC-BK
R_NTC-BK + R_BIAS
(7)
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 ultimately sets the slope of the LED current
decrease with respect to increasing temperature by changing I_TF:
V_TREF - V_TSENSE
I_TF =
R_GAIN
(8)
If an analog temperature sensor such as the LM94022 is used, then R_BIAS and the NTC are not necessary and
V_TSENSE will be the direct voltage output of the sensor.
Since the NTC is not usually local to the controller, a bypass capacitor (C_NTC) is suggested from TSENSE to
GND. If a capacitor is used at TSENSE, then a capacitor (C_REF) of equal or greater value should be placed from
TREF to GND in order to ensure the controller does not start-up in foldback. Alternatively, a smaller C_REF can be
used to create a fade-up function at start-up (see Application Information).
Thermal foldback is simply analog dimming according to a specific profile, therefore any method of controlling the
differential voltage between TREF and TSENSE can be use to analog dim the LED current. The corresponding
LED current for any V_DIF > 0V is defined:
I_LED
=
(I_CSH
-
I_TF)
x
¨§
©
R_HSP¸·
R_SNS¹
(9)
The CSH pin can also be used to analog dim the LED current by adjusting the current sense voltage (V_SNS),
similar to thermal foldback. There are several different methods to adjust V_SNS using the CSH pin:
1. External variable resistance: Adjust a potentiometer placed in series with R_CSH to vary V_SNS.
2. External variable current source: Source current (0 µA to I_CSH) into the CSH pin to adjust V_SNS.
Variable Current Source
LM3424
V_S
V_CC
Q7
Q8
R_MAX
Q6
R_ADJ
R_BIAS
CSH
R_CSH
R_ADJ
Variable
Resistance
Figure 21. Analog Dimming Circuitry
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