Texas Instruments | Step-Down LED Driver With Dimming With the TPS621-Family and TPS821-Family (Rev. A) | Application notes | Texas Instruments Step-Down LED Driver With Dimming With the TPS621-Family and TPS821-Family (Rev. A) Application notes

Texas Instruments Step-Down LED Driver With Dimming With the TPS621-Family and TPS821-Family (Rev. A) Application notes
Application Report
SLVA451A – November 2011 – Revised June 2017
Step-Down LED Driver With Dimming With the
TPS621-Family and TPS821-Family
Anthony Fagnani and Leo Li............................................................................ Battery Power Applications
ABSTRACT
The TPS62150 is used normally as a buck (step-down) voltage regulator. However, it is not limited to the
standard voltage regulating topology. This application report demonstrates the TPS62150 as a small,
simple, and easy way to implement a high-brightness LED driver. Regulating the LED with a constant
current for constant brightness is desired, rather than constant voltage regulation. The desired current
must be maintained with an output voltage that varies with changes in the LED forward voltage due to
analog dimming or varying temperatures. To achieve current regulation, the voltage across a known
resistance (RCS) is regulated. RCS is connected from the feedback pin (FB) to GND. Due to the relatively
high FB voltage, the power dissipated by RCS can lower efficiency and reduce battery life. By putting a
resistor from the SS/TR pin to GND, the FB voltage can be reduced. The LEDs are connected from the
output of the inductor to the FB pin. Dimming can be realized in this application by either analog or PWM
methods. This topology can be used with any of the TPS62130, TPS62140, or TPS62150 step-down
converters.
1
2
3
4
5
Contents
Schematic ....................................................................................................................
Design Procedure ............................................................................................................
2.1
LED Current Set ....................................................................................................
2.2
Output Voltage ......................................................................................................
2.3
Output Inductor ......................................................................................................
2.4
Output Capacitor ....................................................................................................
2.5
Input Capacitor ......................................................................................................
2.6
PWM Dimming ......................................................................................................
2.7
Analog Dimming ....................................................................................................
2.8
Comparison Between Dimming Methods ........................................................................
Extending Battery Life with TPS62150 LED Driver......................................................................
Conclusion ....................................................................................................................
References ...................................................................................................................
2
2
2
3
3
4
4
5
6
6
7
7
7
List of Figures
........................................................................................
1
TPS62150 LED Driver Schematic
2
Dimming Linearity With 100-Hz PWM Dimming ......................................................................... 5
2
3
Dimming Linearity With Analog Dimming ................................................................................. 6
4
Efficiency With Analog and PWM Dimming .............................................................................. 7
Trademarks
All trademarks are the property of their respective owners.
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1
Schematic
1
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Schematic
Figure 1 shows a TPS62150 low-power, dc-dc buck converter application that drives one high-current LED
and includes the capability to dim the LED light output. The resistor values in Figure 1 were calculated
using the equations within this application report and the IC’s data sheet. For this design, the maximum
desired output current is 1 A, and the current may be dimmed from 1 A to 0 A by either analog dimming or
PWM dimming. Analog dimming is accomplished by injecting a dc voltage (from the output of a DAC, for
example) at the SS/TR pin and PWM dimming is realized by applying a PWM signal to the EN pin. If PWM
dimming is not used, simply tie the EN pin to VIN. The LED used is Osram’s LW W5SN Golden DRAGON
and exhibits a forward voltage of about 3.6 V at 1 A.
Figure 1. TPS62150 LED Driver Schematic
2
Design Procedure
2.1
LED Current Set
The value of the current-sense resistor, RCS, sets the output current from Equation 1:
VFB
RCS =
IOUT
(1)
IOUT = Output current through LED
VFB = FB pin voltage, determined from Equation 3
The power dissipation of this resistor is calculated in Equation 2 and must be considered when choosing
the package size:
2
PDIS =
VFB
R CS
(2)
This power dissipated is a loss in the system and lowers the converting efficiency and battery life. The
loss can be reduced by lowering the VFB voltage through the SS/TR pin. In a standard buck converter, the
FB voltage is equal to 0.8 V. However, this voltage is proportional to the voltage on the SS/TR pin, until
the SS/TR pin reaches approximately 1.25 V. Above this SS/TR voltage, the FB pin voltage remains at 0.8
V, even though the SS/TR voltage continues to increase. When the SS/TR pin voltage is less than 1.25 V,
the FB pin voltage is related to the SS/TR pin voltage by the gain factor 0.8/1.25:
0.8
V FB = VSS ´
1.25
(3)
2
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Design Procedure
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VSS = SS/TR pin voltage
The SS/TR pin is simply a constant-current source of typical 2.5 µA and is meant to charge a capacitor. If
a resistor (RSS) is placed at the SS/TR pin to AGND, a constant voltage is present at SS/TR due to the
constant-current output (ISS). This voltage can be found using Equation 4:
V SS = I SS ´ RSS
(4)
Combining Equation 1, Equation 3, and Equation 4, RSS can be calculated using Equation 5 with a given
IOUT and RCS:
RCS ´ IOUT
1.25
RSS =
´
I SS
0.80
(5)
To set the output current, RCS is chosen first due to the limited values available for current-sense resistors.
It is chosen such that VFB is in a recommended range of 200 mV to 250 mV with the desired full output
current. This VFB voltage range is chosen as a good tradeoff of efficiency versus accuracy of the LED
current. Lower VFB voltages are more efficient but have less LED current accuracy, because smaller
variations in the resistance of the sense resistor (from self-heating effects, etc.) become a larger
percentage of the total resistance. With a chosen value for RCS, RSS is found using Equation 5.
For example, using Equation 1 with the designed 1-A output current, RCS = 0.25 Ω can be used which
requires VFB = 250 mV.
V FB = RCS ´ I OUT = 0.25Ω ´ 1A = 0.250 V
(6)
Using Equation 5, RSS to the nearest standard value is 154 kΩ.
RCS ´ IOUT 1.25
0.25Ω ´ 1A 1.25
RS S =
=
= 156 kΩ » 154 kΩ
I SS
0.80
2.5 μA
0.80
(7)
It is important to note that the PG pin is not valid in this application where the FB pin voltage is being
lowered through the SS/TR pin. It is only allowed to go high once VSS is above its 1.25-V threshold. Also to
limit inrush current at start up a capacitor can be placed on the SS/TR pin to GND to slow down the start
up.
2.2
Output Voltage
The supplied output voltage of the LED driver circuit is approximated by:
V OUT = NL ED ´ VLE D + V FB
Where:
NLED = number of LEDs in series
V = forward voltage drop of each LED at the maximum desired LED current
VFB = the FB pin voltage, from Equation 3
(8)
LED
This design drives a single LED with VLED = 3.6 V and VFB = 0.25 V. The output voltage is calculated at
3.85 V.
For the TPS62150, usually only one high-current LED can be driven because the maximum allowed
output voltage is 6 V. However, it is possible to drive two LEDs in series if the FB pin voltage is designed
to be low enough such that the total output voltage is kept within the IC’s ratings. This usually involves
reducing the current through the LEDs to reduce their forward voltage.
The VOS pin must still connect to the anode of the LED(s), as this is the VOUT that the IC is regulating. The
ground of the sense resistor, RCS, is a power ground return that must handle the entire output current. This
point must be connected to the PGND node at the output capacitor.
2.3
Output Inductor
To maintain loop stability, an inductor must be selected that is within the TPS62150’s recommended
range. In addition, the inductor value has a direct effect on the ripple current, which affects the maximum
achievable output current. To keep the IC from reaching its 1.4-A current limit during normal operation, the
recommended 2.2-µH inductor is used in this design.
Once the inductor is chosen, the inductor ripple current (ΔIL) can be calculated using Equation 9.
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Design Procedure
ΔIL =
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(VINMAX -
VO UT ´
VO UT )
VINMAX ´ FSW ´ L
(9)
VOUT = Output voltage, from Equation 8
VINMAX = Maximum input voltage
FSW = Switching frequency of 2.5 MHz typical
L = Output Inductance
The selected inductor has to have sufficient RMS current and saturation current ratings for the application.
The inductor’s RMS current and peak current can be calculated using Equation 10 and Equation 11.
I LRMS =
IOUT
I LP = I OUT +
2
æ VO UT ´ ( VINMAX - VO UT ) ö
+
´ çç
÷÷
12
VINMAX ´ FSW ´ L
è
ø
2
1
(10)
æ VOUT ´ (VINMAX - V OUT ) ö
1
´ ç
÷÷
ç
2
VINMAX ´ F SW ´ L
è
ø
(11)
Using the parameters from this design, VOUT = 3.85 V, VINMAX = 17 V and IOUT = 1 A, the calculated peak-topeak inductor current, RMS current and peak current are 536 mA, 1.01 A and 1.27 A, respectively. The
inductor used is a 2.2 µH TDK VLF3012ST-2R2M1R4, which has an RMS current rating of 1.4 A and
saturation current rating of 1.9 A.
2.4
Output Capacitor
To maintain loop stability, an output capacitor must be selected within the recommended range. This
application uses the recommended 22-µF ceramic capacitor.
The output capacitor must also be chosen to limit the ripple current in the LED. To find this, first calculate
the dynamic resistance of the LED using Equation 12.
ΔVF
RLED =
ΔI OUT
(12)
This is the slope of the V/I curve at the operating point and can be found using the LED’s data sheet. In
this application, the set current (IOUT) is 1 A, and the corresponding forward voltage of the LED (VF) is 3.6
V. When the forward voltage changes from 3.5 V to 3.6 V, ΔVF is 0.1 V and ΔIF is 0.16 A. Therefore, the
LED’s dynamic resistance is approximately equal to 0.625 Ω.
The output ripple current of the converter is the output inductor peak-to-peak current (ΔIL). This ripple
current is shared by the LED and output capacitor. The impedance of the output capacitor is calculated
using Equation 13.
1
æ
ö
ZCOUT = R2 ES R + ç
÷
è 2p ´ F SW ´ CO UT ø
2
(13)
RESR = Equivalent series resistance of output capacitor
Assuming an ESR of 3 mΩ, ZCOUT is about 4 mΩ for the ceramic capacitor. Because the impedances of
the output capacitor and LED are in parallel, the ripple current is shared between the two. The ripple
current flowing in the LED is:
ZCOUT
ΔIL ED = ΔIL ´
Z COUT + RLE D + RCS
(14)
For this design, the ripple current in the LED is 3 mA. This shows most of ripple current is shunted through
the output capacitor as desired. This is most always the case when using ceramic output capacitors.
2.5
Input Capacitor
The recommended ceramic input capacitor value of 10 µF is used for CIN. The input capacitor value
determines the input voltage ripple of the regulator. The input voltage ripple can be calculated using
Equation 15:
4
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Design Procedure
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ΔVIN =
IOUT ´ 0.25
CIN ´ FSW
(15)
For this design, the voltage ripple is 10 mV. The voltage rating of the input capacitor must be greater than
the maximum input voltage and the dielectric must be a quality X5R or X7R. A 10-µF, 25-V, X5R ceramic
capacitor is used.
2.6
PWM Dimming
The EN pin is typically used for disabling and enabling the device. By applying a PWM signal to EN, it can
be used to dim the LED. With a PWM signal applied directly to the EN pin, the LED current is turned on
when the PWM signal is high and off when the PWM signal is pulled low. Changing the PWM duty cycle
changes the LED brightness. The PWM signal used must have a high level greater than the minimum
VEN_H level of 0.9 V and a low level less than the maximum VEN_L level of 0.3 V.
When dimming with the EN pin, the frequency of the PWM signal is important. It must be kept low enough
such that the turnon and turnoff delays of the TPS62150 do not significantly affect the dimming linearity. It
also must be high enough to keep the LED flicker from being noticeable to the human eye. A dimming
frequency of 100 Hz is recommended. At this low frequency, the turnon and turnoff times, of less than 100
µs total, only slightly impact the dimming linearity.
A 100-Hz PWM signal was applied to the EN pin and the duty cycle was adjusted from 1% to 99% on the
circuit in Figure 1, giving the example LED current regulation shown in Figure 2.
1100
1000
VIN = 17 V
900
LED Current - mA
800
700
VIN = 4 V
600
500
400
300
200
100
0
0
10
20
30
40
50
60
PWM Duty Cycle - %
70
80
90
100
Figure 2. Dimming Linearity With 100-Hz PWM Dimming
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Design Procedure
2.7
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Analog Dimming
Analog dimming is done by applying a voltage to the SS/TR pin, for example, from the output of a DAC.
As shown in Equation 3, this directly adjusts VFB which then changes IOUT.
Figure 3 shows the analog dimming linearity of the circuit in Figure 1 when a voltage is applied to the
SS/TR pin.
1100
1000
VIN = 4 V
900
LED Current - mA
800
700
600
VIN = 17 V
500
400
300
200
100
0
0
50
100
150
200
VSS - mV
250
300
350
400
Figure 3. Dimming Linearity With Analog Dimming
2.8
Comparison Between Dimming Methods
Efficiency data through the entire output current range was taken using the two dimming methods at both
the maximum and minimum input voltage. This is shown in Figure 4.
6
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Extending Battery Life with TPS62150 LED Driver
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100
VIN = 4 V, Analog
90
VIN = 17 V, Analog
80
Efficiency - %
70
60
VIN = 17 V, PWM
50
VIN = 4 V, PWM
40
30
20
10
0
0
100
200
300
400
500
600
LED Current - mA
700
800
900
1000
Figure 4. Efficiency With Analog and PWM Dimming
As can be seen in Figure 2, Figure 3, and Figure 4, the main tradeoff is between efficiency and dimming
linearity. PWM dimming gives an accurate dimming linearity throughout the entire range of currents but is
significantly less efficient at low LED brightness. Analog dimming, on the other hand, has a higher
efficiency that stays relatively constant throughout the range. The increased efficiency is due to lower
forward voltage of the LEDs. However, it has worse dimming linearity especially at very low LED currents.
3
Extending Battery Life with TPS62150 LED Driver
Additional methods to size RCS to minimize power dissipation are described in the application report
SLEA004.
4
Conclusion
This application report describes how to choose the external components to implement the TPS62150 as
a small and simple high-brightness LED driver. This is done by regulating the voltage across a known
resistance RCS. Efficiency is improved and battery life is increased by lowering VFB through the SS/TR pin
and properly sizing RCS. Analog dimming is realized by applying a voltage to the SS/TR pin and PWM
dimming is performed through the EN pin.
5
References
1. 3-17V 1A Step-Down Converter In 3X3 QFN Package, Datasheet (SLVSAL5)
2. Extending Battery Life With the TPS61040 White Light LED Driver (SLEA004)
3. TPS54160 60-V, Step-Down LED Driver Design Guide (SLVA374)
Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Original (November 2011) to A Revision ................................................................................................ Page
•
Changed title of application report.
.....................................................................................................
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