Texas Instruments | Simple Backlight Driver Dimming Performance | Application notes | Texas Instruments Simple Backlight Driver Dimming Performance Application notes

Texas Instruments Simple Backlight Driver Dimming Performance Application notes
Application Report
SNVA769 – March 2017
Simple Backlight Driver Dimming Performance
Matthew Lin
ABSTRACT
The TPS61158, TPS61160/61, TPS61162A/63A, TPS61165, and TPS61169 (hereafter "TPS611xx") are
simple backlight drivers commonly used in smartphones and tablets. These devices support PWM
dimming with an input voltage as low as 2.7 V. This application note discusses the requirements for using
the TPS611xx parts at the lowest performance specifications for PWM dimming.
1
2
3
Contents
Minimum PWM On-Time Requirement ................................................................................... 2
Minimum Performance Specifications ..................................................................................... 2
TPS61162A/3A Special Cases............................................................................................. 3
List of Figures
1
Low Duty-Cycle Behavior ................................................................................................... 2
2
Internal Low-Pass Filter to Error Amplifier................................................................................ 3
3
Offset Voltage Model ........................................................................................................ 3
4
TPS61162A/TPS61163A Error Amplifier Block Diagram ............................................................... 3
5
Hysteresis ..................................................................................................................... 4
6
Incrementation Effects on Resolution ..................................................................................... 4
List of Tables
1
Typical Voltage Offset ....................................................................................................... 3
Trademarks
EasyScale is a trademark of Texas Instruments.
SNVA769 – March 2017
Submit Documentation Feedback
Simple Backlight Driver Dimming Performance
Copyright © 2017, Texas Instruments Incorporated
1
Minimum PWM On-Time Requirement
1
www.ti.com
Minimum PWM On-Time Requirement
All devices mentioned except for the TPS61169 are equipped with both PWM dimming and EasyScale™
1-wire control interface; the TPS61169 only has PWM dimming. All devices with 1-wire control have a
minimum PWM input on time of 50 ns. For pulse widths below 50 ns, or equivalently a 0.5% duty cycle at
the maximum frequency, 100 kHz, the internal-glitch rejection on the PWM (or CTRL) input can randomly
detect or ignore logic pulses. As a result, going below 50 ns can inadvertently trigger the device to enter 1wire mode. To ensure proper detected logic levels, keep the pulse width above 50 ns. When 1-wire control
is randomly activated, the default current output is maximum current, causing the device to output
inconsistent output current. Because the TPS61169 does not have 1-wire control, at lower input voltagesthe device does not stop outputting current unless the PWM signal is 0 V. Any pulse width greater than 50
ns at any frequency within the range of each chip, respectively, outputs a stable and steady current.
2
Minimum Performance Specifications
The 100-kHz PWM frequency and 3.3-V input voltage was used to test each device. The maximum
current for all the devices was set to 20 mA.
2.1
Linearity
These devices maintain their linearity as low as 1% duty cycle at all frequencies in range.
2.7
TPS61158
TPS61160/1
TPS61162A/63A
TPS61165
TPS61169
2.4
LED Current (mA)
2.1
1.8
1.5
1.2
0.9
0.6
0.3
0
-0.3
0
0.4
0.8
1.2
1.6
2
2.4
Duty Cycle (%)
2.8
3.2
3.6
4
D001
Figure 1. Low Duty-Cycle Behavior
2.2
Internal Low Pass Filter to Error Amplifier
All devices mentioned have an internal low-pass filter on the control pin. The output of the low-pass filter is
connected to the error amplifier as a DC-reference voltage for the FB pin regulation. Though a PWM
signal is used to control the LED current, the LED current has a varying DC current that is proportional to
PWM input duty cycle. Using this low pass filter eliminates any audible noise due to the output voltage
ripple at the PWM frequency and also allows the regulation voltage to be independent of the PWM logic
voltage, which often has large variations. Figure 2 shows the internal low-pass filter and error amplifier
schematic.
2
Simple Backlight Driver Dimming Performance
Copyright © 2017, Texas Instruments Incorporated
SNVA769 – March 2017
Submit Documentation Feedback
Minimum Performance Specifications
www.ti.com
VBG
204 mV
CTRL
VREF
GM
Amplifier
EA Output
FB
Figure 2. Internal Low-Pass Filter to Error Amplifier
2.3
Offset Voltage in Error Amplifier
Offset voltage is a non-zero value of a differential input created when the inputs of the GM amplifier are
not exactly matched. It can be modeled as shown in Figure 3.
VOFFSET
±
+
+
VREF
±
ISET
GM Amplifier
FB
RSET
GND
Figure 3. Offset Voltage Model
Table 1. Typical Voltage Offset
3
Device
TPS61158
TPS61160/61
TPS61162A/63A
TPS61165
TPS61169
Voltage
offset (mV)
0.039
0.68
1.27
0.83
8.6
TPS61162A/3A Special Cases
The TPS61162A/63A are the only devices listed in Table 1 that use a 9-bit ADC to convert the low-pass
filtered PWM signal-to-digital current values. Figure 4 is the block diagram of the front end before the error
amplifier. This creates special cases as described below.
Input PWM
LPF
ADC
AD
DAC
VI_SET
+
±
Error Amplifier
ISET
FB
RSET
GND
Figure 4. TPS61162A/TPS61163A Error Amplifier Block Diagram
SNVA769 – March 2017
Submit Documentation Feedback
Simple Backlight Driver Dimming Performance
Copyright © 2017, Texas Instruments Incorporated
3
TPS61162A/3A Special Cases
3.1
www.ti.com
Hysteresis
Hysteresis is the system dependence on the history of the previous current. It prevents any PWM jitter
from feeding through to the LED current by preventing duty-cycle-to-LED current changes until the duty
cycle overcomes 2 LSBs after a change in direction.
The minimum current the TPS61162A/TPS61163A can supply is 3 × IFULL SCALE / 512 and supplies this
current as soon as a measurable PWM signal is detected. Furthermore, when using the device at either
maximum or minimum ADC values to output 3 × IFULL SCALE / 512 or IFULL SCALE, the hysteresis holds these
values for longer, typically about 3 LSBs, as opposed the 2 LSBs of hysteresis in the intermediate steps
(see Figure 5).
3x
..
.
LED Current
VISET_full
RSET
VISET_full
512*RSET
...
Code
6
3
Code Maximum
Duty Cycle (%)
Figure 5. Hysteresis
3.2
Resolution
The TPS61162A/63A has a 9-bit ADC, which provides for a minimum of IFULL SCALE / 512 increment in LED
current. Any PWM signal with greater than 9-bit resolution steps still results in 9-bit incremented current
steps.
0.7
9-bit Resolution
0.65
0.6
LED Current (mA)
0.55
0.5
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0
0.5
1
1.5
2
2.5
Duty Cycle (%)
3
3.5
4
D002
Figure 6. Incrementation Effects on Resolution
4
Simple Backlight Driver Dimming Performance
Copyright © 2017, Texas Instruments Incorporated
SNVA769 – March 2017
Submit Documentation Feedback
IMPORTANT NOTICE FOR TI DESIGN INFORMATION AND RESOURCES
Texas Instruments Incorporated (‘TI”) technical, application or other design advice, services or information, including, but not limited to,
reference designs and materials relating to evaluation modules, (collectively, “TI Resources”) are intended to assist designers who are
developing applications that incorporate TI products; by downloading, accessing or using any particular TI Resource in any way, you
(individually or, if you are acting on behalf of a company, your company) agree to use it solely for this purpose and subject to the terms of
this Notice.
TI’s provision of TI Resources does not expand or otherwise alter TI’s applicable published warranties or warranty disclaimers for TI
products, and no additional obligations or liabilities arise from TI providing such TI Resources. TI reserves the right to make corrections,
enhancements, improvements and other changes to its TI Resources.
You understand and agree that you remain responsible for using your independent analysis, evaluation and judgment in designing your
applications and that you have full and exclusive responsibility to assure the safety of your applications and compliance of your applications
(and of all TI products used in or for your applications) with all applicable regulations, laws and other applicable requirements. You
represent that, with respect to your applications, you have all the necessary expertise to create and implement safeguards that (1)
anticipate dangerous consequences of failures, (2) monitor failures and their consequences, and (3) lessen the likelihood of failures that
might cause harm and take appropriate actions. You agree that prior to using or distributing any applications that include TI products, you
will thoroughly test such applications and the functionality of such TI products as used in such applications. TI has not conducted any
testing other than that specifically described in the published documentation for a particular TI Resource.
You are authorized to use, copy and modify any individual TI Resource only in connection with the development of applications that include
the TI product(s) identified in such TI Resource. NO OTHER LICENSE, EXPRESS OR IMPLIED, BY ESTOPPEL OR OTHERWISE TO
ANY OTHER TI INTELLECTUAL PROPERTY RIGHT, AND NO LICENSE TO ANY TECHNOLOGY OR INTELLECTUAL PROPERTY
RIGHT OF TI OR ANY THIRD PARTY IS GRANTED HEREIN, including but not limited to any patent right, copyright, mask work right, or
other intellectual property right relating to any combination, machine, or process in which TI products or services are used. Information
regarding or referencing third-party products or services does not constitute a license to use such products or services, or a warranty or
endorsement thereof. Use of TI Resources may require a license from a third party under the patents or other intellectual property of the
third party, or a license from TI under the patents or other intellectual property of TI.
TI RESOURCES ARE PROVIDED “AS IS” AND WITH ALL FAULTS. TI DISCLAIMS ALL OTHER WARRANTIES OR
REPRESENTATIONS, EXPRESS OR IMPLIED, REGARDING TI RESOURCES OR USE THEREOF, INCLUDING BUT NOT LIMITED TO
ACCURACY OR COMPLETENESS, TITLE, ANY EPIDEMIC FAILURE WARRANTY AND ANY IMPLIED WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, AND NON-INFRINGEMENT OF ANY THIRD PARTY INTELLECTUAL
PROPERTY RIGHTS.
TI SHALL NOT BE LIABLE FOR AND SHALL NOT DEFEND OR INDEMNIFY YOU AGAINST ANY CLAIM, INCLUDING BUT NOT
LIMITED TO ANY INFRINGEMENT CLAIM THAT RELATES TO OR IS BASED ON ANY COMBINATION OF PRODUCTS EVEN IF
DESCRIBED IN TI RESOURCES OR OTHERWISE. IN NO EVENT SHALL TI BE LIABLE FOR ANY ACTUAL, DIRECT, SPECIAL,
COLLATERAL, INDIRECT, PUNITIVE, INCIDENTAL, CONSEQUENTIAL OR EXEMPLARY DAMAGES IN CONNECTION WITH OR
ARISING OUT OF TI RESOURCES OR USE THEREOF, AND REGARDLESS OF WHETHER TI HAS BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGES.
You agree to fully indemnify TI and its representatives against any damages, costs, losses, and/or liabilities arising out of your noncompliance with the terms and provisions of this Notice.
This Notice applies to TI Resources. Additional terms apply to the use and purchase of certain types of materials, TI products and services.
These include; without limitation, TI’s standard terms for semiconductor products http://www.ti.com/sc/docs/stdterms.htm), evaluation
modules, and samples (http://www.ti.com/sc/docs/sampterms.htm).
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265
Copyright © 2017, Texas Instruments Incorporated
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertising