Datasheet
R
SNS
I
LED
R
HSP
R
HSN
HSN
HSP
High-Side
Sense Amplifier
CSH
1.24V
C
CMP
R
CSH
COMP
Error Amplifier
V
SNS
To PWM
Comparator
LM3424
I
CSH
I
TF
100k100k
100k 100k
TREF
TSENSE
TGAIN
2.45V
V
S
R
REF2
R
REF1
R
BIAS
R
NTC
R
GAIN
V
DIF
LM94022
Precision Temp Sensor
NTC
R
SNS
I
LED
R
HSP
R
HSN
HSN
HSP
High-Side
Sense Amplifier
CSH
1.24V
C
CMP
R
CSH
COMP
Error Amplifier
V
SNS
To PWM
Comparator
LM3424
I
TF
Thermal Foldback Current
I
CSH
I
LED
=
R
SNS
1.24V
R
SNS
V
SNS
R
CSH
R
HSP
=
x
LM3424
www.ti.com
SNVS603B –AUGUST 2009–REVISED OCTOBER 2009
I
LED
can then be calculated:
(7)
The selection of the three resistors (R
SNS
, R
CSH
, and R
HSP
) is not arbitrary. For matching and noise performance,
the suggested signal current I
CSH
is approximately 100 µA. This current does not flow in the LEDs and will not
affect either the off-state LED current or the regulated LED current. I
CSH
can be above or below this value, but
the high-side amplifier offset characteristics may be affected slightly. In addition, to minimize the effect of the
high-side amplifier voltage offset on LED current accuracy, the minimum V
SNS
is suggested to be 50 mV. Finally,
a resistor (R
HSN
= R
HSP
) should be placed in series with the HSN pin to cancel out the effects of the input bias
current (~10 µA) of both inputs of the high-side sense amplifier.
Note that he CSH pin can also be used as a low-side current sense input regulated to 1.24V. The high-side
sense amplifier is disabled if HSP and HSN are tied to GND.
Figure 21. LED Current Sense Circuitry
Figure 22. Thermal Foldback Circuitry
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 breakpoint (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 Figure 23. The LM3424 allows the user to program both the breakpoint and
slope of the thermal foldback profile.
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