LM3424 Datasheet, PDF (13/50 Pages) National Semiconductor (TI) – Constant Current N-Channel Controller with Thermal Foldback for Driving LEDs

If an analog temperature sensor such as the LM94022 is
used, then R_BIAS and the NTC are not necessary and
V_TENSE 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 ca-
pacitor 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. Alter-
natively, a smaller C_REF can be used to create a fade-up
function at start-up (see Application Information section).
Thermal foldback is simply analog dimming according to a
specific profile, therefore any method of controlling the differ-
ential 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:
The CSH pin can also be used to analog dim the LED current
by adjusting the current sense voltage (V_SNS), similar to ther-
mal 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.
In general, analog dimming applications require a lower
switching frequency to minimize the effect of the leading edge
blanking circuit. As the LED current is reduced, the output
voltage and the duty cycle decreases. Eventually, the mini-
mum on-time is reached. The lower the switching frequency,
the wider the linear dimming range. Figure 6 shows how both
CSH methods are physically implemented.
Method 1 uses an external potentiometer in the CSH path
which is a simple addition to the existing circuitry. However,
the LEDs cannot dim completely because there is always
some resistance causing signal current to flow. This method
is also susceptible to noise coupling at the CSH pin since the
potentiometer increases the size of the signal current loop.
Method 2 provides a complete dimming range and better
noise performance, though it is more complex. Like thermal
foldback, it simply sources current into the CSH pin, decreas-
ing the amount of signal current that is necessary. This
method consists of a PNP current mirror and a bias network
consisting of an NPN, 2 resistors and a potentiometer
(R_ADJ), where R_ADJ controls the amount of current sourced
into the CSH pin. A higher resistance value will source more
current into the CSH pin causing less regulated signal current
through R_HSP, effectively dimming the LEDs. Q7 and Q8
should be a dual pair PNP for best matching and perfor-
mance. The additional current (I_ADD) sourced into the CSH pin
can be calculated:
The corresponding I_LED for a specific I_ADD is:
FIGURE 6. Analog Dimming Circuitry
300857k3
THERMAL SHUTDOWN
The LM3424 includes thermal shutdown. If the die tempera-
ture reaches approximately 165°C the device will shut down
(GATE pin low), until it reaches approximately 140°C where
it turns on again.
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