Texas Instruments | Automotive LED Lighting with Adjustable Shunt References | Application notes | Texas Instruments Automotive LED Lighting with Adjustable Shunt References Application notes

Texas Instruments Automotive LED Lighting with Adjustable Shunt References Application notes
Automotive LED Lighting With Adjustable Shunt
References
Automotive suppliers are developing innovative ways
to further enhance the user experience within vehicles.
For instance, the external rear lights have various
functions such as sequential illuminating turn signal
indicators, dynamic braking lights, and multiple design
elements for aesthetics - all possible with the adoption
of LED lighting. There are various methods to
implement how these LEDs are powered, one discrete
method is by using a simple voltage reference. This
method could yield design advantages such as ease in
design efforts and reduction in solution cost.
VCC
R1
IKA
1
TL431-Q1
2
3
RS
IOUT
GND
Figure 1. Constant Current Source Circuit
The TL431-Q1 is an automotive grade adjustable
shunt voltage reference. The three-terminal device has
a 2.5 V internal bandgap voltage reference with a
feedback pin to adjust the voltage reference from 2.5 V
to 36 V. In addition, the feedback pin can be utilized to
create a constant-current topology which can be used
for automotive LEDs applications.
LEDs require proper current drive for illumination. A
constant current source is one way to achieve correct
current regulation, as shown in Figure 1. The current
source circuit uses a bipolar junction transistor (BJT)
and sense resistor. The base current of the transistor
is being controlled by the TL431-Q1 such that the
transistor is conducting to produce a 2.5 V current
sense voltage at Rs. The current sense voltage is fed
back to the feedback pin of the TL431-Q1 for a
negative feedback control loop. In case of any
transients, the TL431-Q1 can regulate properly to
provide the constant current required.
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VCC
R1 = I
OUT
IKA
hFE
IOUT =
VREF
RS
IKA
where
•
•
•
•
•
•
•
R1 = Input bias resistor (Ω)
RS = Current sense resistor(Ω)
IOUT = Output current source (A)
IKA = Cathode current (A)
VREF = Internal feedback reference (V)
VCC = Input supply voltage (V)
hFE = Transistor DC current gain
(1)
Equation 1 shows, the cathode current of the TL431Q1, IKA, contributes to the overall current source. The
TL431-Q1 will require at most 1 mA of cathode current
to function. If the required constant current is low, the
varying IKA across devices can affect the accuracy of
the current source.
One solution to omit the cathode current is to use a
constant current sink circuit instead of a sourcing
circuit, as shown in Figure 2. The circuit is designed to
sink the proper current for proper lighting of the LEDs.
The relationship between the transistor, sense resistor,
and TL431-Q1 are the same as the circuit shown in .
Equation 2 shows, the current sink circuit does not
need to take the cathode current into consideration
when selecting the current sink.
Automotive LED Lighting With Adjustable Shunt References
Copyright © 2018, Texas Instruments Incorporated
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resistors accuracy varies and can be chosen
depending on the application. For the transistor, the
DC current gain, hFE, will be the main characteristic for
inaccuracy. The manufacture’s datasheet will provide
minimum and maximum values.
VCC
VCC
VCC
IOUT
VCC
R1
R1
1
2
1
TL431-Q1
3
2
TL431-Q1
RS
3
GND
R1 = I
OUT
hFE
IOUT =
IKA
IOUT = Output current sink
Others = Same as before
Alternate Device Recommendations
VREF
RS
(2)
In some applications, multiple LED strings are
required. In both circuit topologies, parallel LED strings
can be implemented using one TL431-Q1 as long as
the strings of LEDs require the same current. Figure 3
shows a constant-current sink with three parallel LED
strings. The cathode is connecting to the base of each
transistor. One of the sense resistors is fed back to the
feedback pin of the TL431-Q1 as a sense voltage for
regulation. One thing to note is the current of other
strings may vary due varying transistor characteristics
and resistor accuracy.
The accuracy of the current depends on the TL431-Q1
grade, current sense resistor, and transistor. R1 must
be a properly sized to provide minimum cathode
current to the TL431-Q1 for regulation and enough
base current to conduct the transistor. Current sense
1
RS3
Figure 3. Current Sink With Parallel LED Strings
where
•
•
RS2
GND
Figure 2. Constant Current Sink Circuit
VCC
RS1
If the TL431-Q1 cathode current becomes an issue for
high side current sourcing, consider using an alternate
device featured in Table 1. The devices are all
adjustable shunt references with IKA(MIN) less than 100
µA. The devices also have an internal bandgap
reference of 1.24 V compared to the 2.5 V of the
TL431-Q1. Ensure that the sense resistors are
designed properly.
Table 1. Alternative Device Recommendations
Device
Optimized
Parameters
Performance Trade-Off
LM4041-NQ1
Minimum IKA;
Temperature
coefficient
Cathode current rating
TLV431-Q1
Minimum IKA
Initial accuracy; Cathode
current rating;
Temperature coefficient
TLVH431-Q1
Minimum IKA
Initial accuracy; Cathode
current rating;
Temperature coefficient
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Automotive LED Lighting With Adjustable Shunt References
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