Datasheet

I
TF
=
V
TREF
- V
TSENSE
R
GAIN
=
R
REF1
R
REF1
+ R
REF2
R
NTC-BK
R
NTC-BK
+ R
BIAS
T
I
LED
0
T
BK
T
END
v
1
R
GAIN
LM3424
SNVS603B –AUGUST 2009–REVISED OCTOBER 2009 www.ti.com
Figure 23. 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 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 22 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 (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 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 22. 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 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:
(8)
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
:
(9)
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.
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