Texas Instruments | Different Methods to Drive LEDs Using TPS63XXX Buck-Boost Converters (Rev. C) | Application notes | Texas Instruments Different Methods to Drive LEDs Using TPS63XXX Buck-Boost Converters (Rev. C) Application notes

Texas Instruments Different Methods to Drive LEDs Using TPS63XXX Buck-Boost Converters (Rev. C) Application notes
Application Report
SLVA419C – June 2010 – Revised July 2019
Different Methods to Drive LEDs Using TPS63xxx BuckBoost Converters
Juergen Neuhaeusler, Minqiu Xie
ABSTRACT
This application note describes how to drive LEDs using standard DC/DC converters. The circuit examples
used here are based on devices from the TPS63xxx buck-boost converter family. Buck-boost converters
offer high flexibility regarding the supported input voltage range or supported battery configuration. The
devices can also support a wide variety of LEDs by using the same circuit optimized for a certain LED
forward current.
1
Introduction
Nowadays, there are many applications that use a battery as the power supply to drive an LED, such as
wireless security camera or electronic tags. For most of the battery types, the voltage of the battery
changes during discharge. If the forward voltage drop of the LED ends up in the middle of the battery
voltage range, a buck-boost converter can work as a highly efficient LED driver regardless of the battery
voltage being higher or lower than the LED forward voltage. This report illustrates several solutions to
drive LEDs using standard DC/DC converters from the TPS63xxx buck-boost converter family.
2
Simple Configuration with Sense Resistor Used for Voltage Feedback
L1
VIN
L1
L2
VIN
VOUT
C2
C1
PGND/GND
LED
FB
Rsense
TPS63XXX
Figure 1. Constant Current with Single Sense Resistance
The basic schematic in Figure 1 shows the simplest configuration. To configure the DC/DC converter from
operating as a voltage source to operating as a current source, the current is measured through a sense
resistor and fed back into the control loop. For that, the voltage feedback input is used directly. The sense
resistor is placed in series with the LED. Therefore, the LED current is flowing through the sense resistor
as well. For calculating the required resistor value for Rsense for a given LED current ILED, use Equation 1.
VFB is the feedback voltage of the DC/DC converter. In the case of the TPS63xxx devices, VFB is typically
0.5 V or 0.8 V.
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1
Improving the Power Conversion Efficiency
R Sense
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VFB
I LED
(1)
Depending on the LED current, the power dissipation can become critical for the resistor. It may be
necessary to use a larger resistor, or multiple resistors in parallel or in series, to split the dissipated power.
Calculate the power PS, which must be dissipated by Rsense, with Equation 2.
PS
3
2
I LED
u R Sense
(2)
Improving the Power Conversion Efficiency
L1
VIN
L1
L2
VIN
VOUT
C2
C1
PGND/GND
R1
LED
FB
R2
Rsense
TPS63XXX
Figure 2. Constant Current with Resistance Net
The power losses in the sense resistor of the circuit explained in Section 2 lower the efficiency of the
circuit significantly, which is a major drawback. Although the feedback voltage of most TPS63xxx devices
is already low at 0.5 V, it is still causing significant power losses, especially when dealing with high-LED
currents.
How this can be improved is shown in Figure 2. The sense resistor for measuring the LED current, Rsense,
is still in series with the LED, but the way R1 is connected, a bias current into the feedback network is
introduced. This bias current causes a voltage drop across R2, which adds to the voltage drop across the
sense resistor Rsense. Because the feedback voltage is not changed, the required voltage drop across the
sense resistor is lower for a given LED current compared to the solution described in Section 2.
Equation 3 gives the calculation for the LED current (ILED). VFB is the feedback voltage of the DC/DC
converter, and VLED is the typical forward voltage of the LED.
I LED
VFB
R Sence
VLED
R1
R2
VLED u R 2
R Sense u R 1
R2
(3)
The regulated LED current in this circuit depends on the forward voltage of the LED. How much the LED
current varies is defined by the forward voltage variation of the LED and the values of resistors R1 and
R2. With setting the value of R1 as high as possible and the value of R2 as low as possible, the current
variation is at its minimum. The theoretical extreme, when R1 is nonexistent and R2 is shorted, is basically
the circuit explained in Section 2, so doing trade-offs is required. Another benefit of the circuit shown in
Figure 2 is the output voltage regulation in case the LED is disconnected. This is required if the DC/DC
converter used does not have a built-in output over-voltage protection. In this case, the maximum output
voltage can be programmed with resistors R1 and R2+Rsense using the equations of the datasheet for
calculating the feedback divider of the respective device. Rsense has a value that is significantly lower
compared to R1 and R2, so it is negligible.
Programming the LED current is done by selecting the appropriate value for Rsense. Equation 4 shows how
to calculate the value for Rsense and Equation 5 shows how to calculate the losses in Rsense, PS.
2
Different Methods to Drive LEDs Using TPS63xxx Buck-Boost Converters
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Improving the LED Current Control Accuracy
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R Sense
PS
R 1 u VFB
R 2 u VLED
I LED u R 1
R2
VFB
VLED
(4)
2
I LED
u R Sense
(5)
See PMP15037 Test Results for detailed design guidance and calculation.
4
Improving the LED Current Control Accuracy
L1
VIN
L1
L2
VIN
VOUT
C2
LED
C1
FB
PGND/GND
R2
R1
VREF
Rsense
TPS63XXX
Figure 3. Dimming Solution with Resistance Net
To overcome the problem with the LED current changing with the LED forward voltage, resistor R1 can be
connected to any fixed reference voltage; for example, VREF in Figure 3. This reference voltage can be
implemented with a RC filtered PWM signal from a microprocessor, for example, or just from any available
DC source. The only requirement is that it must be higher than the feedback voltage. Together with R1, it
feeds in a constant bias current into the feedback node, which generates a constant voltage drop across
R2. This voltage adds to the voltage drop across the sense resistor Rsense. The sum of both voltages is the
feedback voltage. The equation for the LED current is given with Equation 6.
I LED
VFB u
R1
R2
R Sense
R 1 u R Sense
VREF u
R2
R Sense
R 1 u R Sense
(6)
According to Equation 6, a change in the reference voltage VREF changes the LED current that might be an
advantage in some systems. The output load of this reference voltage is basically defined by the series
connection of resistors R1, R2 and RSense, which usually has relatively high impedance. Therefore, almost
any low-power reference voltage source can be used directly; for example, a PWM-controlled output of a
D/A converter. Because the sensitivity to reference voltage changes can be programmed by selecting
appropriate values for R1 and R2, and of course by selecting the reference voltage level itself, it is also an
ideal circuit implementation if the LED current must be calibrated. This, for example, is very beneficial in
applications like projectors, where it is required to make sure that the wavelength of the emitted light is at
the correct value. For calculating the losses in the sense resistor, use Equation 5.
5
Improving the LED Assembly Options
All the applications described above use a sense resistor connecting the cathode of the LED to the GND
return of the power circuit. This causes difficulties with mechanical assembly since the cathode needs to
be isolated from the ground and most LEDs use the cathode for sinking heat. Designing heat sinks for the
LED is much easier if no isolation is required. Figure 4 illustrates an LED driver circuit that supports that.
In this circuit, the LED current is measured between the output of the DC/DC converter and the anode of
the LED. As in all the other circuits shown in this application report, a resistor in series to the LED is used
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Dimming Solutions with Op-Amp
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for sensing the current. The differential voltage across this resistor is connected to the input of an current
sense amplifier; for example, INA180. This voltage is amplified and directly fed into the feedback pin. Use
Equation 7 to calculate the value of the required sense resistor. G in this equation is the input-to-output
voltage gain of the current sense amplifier. It is recommended to add the 100-nF capacitor C3 to help the
start-up.
L1
L1
L2
Rsense
VIN
VIN
VOUT
C1
C2
IN+
INLED
C3
PGND/GND
FB
OUT
INA180
TPS63XXX
Figure 4. High-Side LED Driver Using INA180
R Sense
VFB
I LED u G
(7)
The solution based on the INA180 (current sense amplifier) is easy to design with only a few external
components. It is flexible to set the sense resistor before or after the LED. This solution achieves high
precision and high noise tolerance owing to the INA part. Besides, for LEDs connected in series, if one of
the LEDs is shorted, the circuit still works, as this solution controls the current.
PLoss
2
I LED
u R Sense
(8)
Users can replace the INA180 with an operational amplifier-based circuit. But this solution needs more
external parts, which harms the precision and increases the design complexity. Due to the additional
external parts, the solution size is slightly larger.
6
Dimming Solutions with Op-Amp
Some applications need adjustable LED brightness. In security cameras, the LED needs to be dimmed by
changing the current to avoid the rolling shutter effect that may occur if PWM dimming is used. Otherwise,
the camera catches the flicker. For this purpose, the solution controls the current through the LED with a
signal from the MCU or processor. A popular way is to use a reference voltage to control the LED current
to achieve the dimming function. Below is a schematic using a TPS63xxx device.
4
Different Methods to Drive LEDs Using TPS63xxx Buck-Boost Converters
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SLVA419C – June 2010 – Revised July 2019
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Dimming Solutions with Op-Amp
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L1
VIN
L1
L2
VIN
VOUT
C1
C2
R1
C3
R2
FB
+
PGND/GND
Rsense
VFB
±
R3
TPS63XXX
R4
R5
VREF
LED
Figure 5. Dimming Solution with Op-Amp
VRsense
PLoss
R4
u VFB
R3
R4
u VREF
R5
(9)
2
I LED
u R Sense
(10)
A larger sense voltage provides better signal-to-noise ratio. However, it brings more power loss. These
two have to come together and compromise.
In some cases, the system needs to control the LED current more precisely for smaller currents. It needs
a larger gain or larger sense resistor to meet this requirement. A larger gain causes a larger bias offset by
the offset voltage and bias current of the amplifier. As a drawback, the larger Rsense increases the losses
and lowers the efficiency.
In that case, this application report recommends the circuit shown in Figure 6. This circuit is derived from
the prior solution with a small change. The difference is that R5 is connected to the positive input of the op
amp. Here, an increase of VREF decreases ILED. For this circuit, the gain could be much smaller. There is
no need to just amplify the Vsense to target VFB when the current is low, as VREF helps to lift the voltage.
L1
VIN
L1
L2
VIN
VOUT
C1
C2
R1
R5
VREF
C3
R2
FB
+
PGND/GND
VFB
Rsense
±
TPS63XXX
R3
R4
LED
Figure 6. Another Dimming Solution with Op-Amp
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Summary
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The calculations are similar to the previous solution. If R3 = R1||R5 and R2 = R4 then:
VRsense
PLoss
R4
u VFB
R3
R4
u VREF
R5
(11)
2
I LED
u R Sense
(12)
Both circuits show high efficiency, but the efficiency of the second solution is slightly higher. Both achieve
analog dimming with simple circuit design and calculations. As both control the current of the LED well,
they also work well even if one of the LED in series is shorted. That is another benefit when compared to
the discontinuous current dimming solution, which only adjusts the duty cycle of LED but keeps Vout fixed.
Moreover, for the last dimming solution, zero VREF leads to the maximum LED current. It might be
necessary to consider this in a system, as it might add risk.
7
Summary
This application report demonstrates several solutions for driving LEDs using an IC out of the TPS63xxx
device family. Depending on the design requirements, such as cost, efficiency, or size, some solutions
have advantages over the others. This is summarized in Table 1.
Table 1. Comparison of Different Solutions for Driving LEDs
Constant Current
Constant Current
Res. Based
Dimming
INA180 Solution
Dimming Solution with
Op-Amp
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5, Figure 6
Dimming
No
No
Analog dimming
No
Analog dimming
Design
complexity
Easy
Easy
Medium
Easy
Complex
Additional
components
Sense resistor only
Three resistors
Three resistors
Current sense
monitor
op-amp and resistors
Efficiency
Low
Medium
Medium
High
High
Cost
Low
Low
Low
High
Medium
Solution size
Small
Small
Small
Medium
Large
Operation with
shorted LEDs
Remains well
Fails to work
Remains well
Remains well
Remains well
Sense side
Low side
Low side
Low side
High/Low side
High/Low side
References
• Texas Instruments, Dynamically Adjustable Output Using TPS63000 Application Report
• Texas Instruments, PMP15037 Test Results
• Texas Instruments, Analog Engineer’s Circuit Cookbook: Op Amps
• Texas Instruments, TPS63802 2-A, high-efficient, low IQ buck-boost converter with small solution size
Data Sheet
6
Different Methods to Drive LEDs Using TPS63xxx Buck-Boost Converters
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SLVA419C – June 2010 – Revised July 2019
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Revision History
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Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Original (June 2010) to C Revision ......................................................................................................... Page
•
•
•
Updated app report for clarity............................................................................................................ 1
Rewrote sections 2 and 3 ................................................................................................................ 1
Added sections 5, 6, and 7. .............................................................................................................. 3
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Revision History
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