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| february 2009 rev 2 1/22 AN2809 application note 6-row / 30-ma led driver with boost converter for the backlight of lcd panels introduction the led7706 consists of a high-efficiency monolithic boost converter and six controlled current generators (rows) specifically designed to supply the led arrays used in the backlights of lcd panels. the device can manage an output voltage of up to 36 v (ten white leds per row). the generators can be externally-programmed to sink up to 30 ma and can be dimmed via a pwm signal (a 1% dimming duty-cycle at 20 khz can be managed). the device detects and manages the open and shorted led faults and leaves unused rows floating. basic protections (output over-voltage, internal mosfet over-current and thermal shutdown) are provided. figure 1. led7706 demonstration board www.st.com
contents AN2809 2/22 contents 1 led7706 main features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1 boost section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.2 backlight driver section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2 led7706 demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3 component list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4 components assembly and layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 5 i/o interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 6 recomended equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 7 configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 7.1 sw1 fixed or adjustable switching frequency (fsw pin) . . . . . . . . . . . . . 10 7.2 sw2 fault management mode (mode pin) . . . . . . . . . . . . . . . . . . . . . . . 11 7.3 sw3 enable function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 8 test setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 9 getting started . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 9.1 quick startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 9.2 open and shorted wleds fault testing . . . . . . . . . . . . . . . . . . . . . . . . . . 14 9.3 device synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 9.4 efficiency measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 10 wleds test board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 11 revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 AN2809 list of figures 3/22 list of figures figure 1. led7706 demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 figure 2. led7706 board schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 figure 3. top side component placement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 figure 4. bottom side test points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 figure 5. sw1 (f sw ) setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 figure 6. sw2 (mode) setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 figure 7. led7706 demonstration board and white leds test board assembly . . . . . . . . . . . . . . . . 12 figure 8. led7706 board test setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 figure 9. led7706 synchronization setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 figure 10. efficiency measurements setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 figure 11. efficiency vs dim duty cycle, v in =12v, 10 white leds in series, i out = 120ma . . . . . . . . 17 figure 12. efficiency vs dim duty cycle, v in =24v, 10 white leds in series, i out = 120ma . . . . . . . . 17 figure 13. wleds test board. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 figure 14. wleds test board schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 list of tables AN2809 4/22 list of tables table 1. led7706 performance summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 table 2. led7706 component list. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 table 3. led7706 demonstration board test points description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 table 4. faults management summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 table 5. test board jumpers function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 table 6. test boards switches functions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 table 7. test board test-points function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 table 8. document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 AN2809 led7706 - main features 5/22 1 led7706 - main features 1.1 boost section 4.5- to 36-v input voltage range internal power mosfet internal +5 v ldo for device supply up to 36-v output voltage constant frequency peak current-mode control 200-khz to 1-mhz adjustable switching frequency external sync for multi-device application pulse-skip power-saving mode at light loads programmable soft-start programmable ovp protection single ceramic output capacitor non-latched thermal shutdown 1.2 backlight driver section six rows with 30 ma maximum current capabilit y (adjustable) up to 10 white leds per row row disabling option less than 500 ns minimum dimming time (1% minimum dimming duty-cycle at 20 khz dimming frequency) 2.0% current matching between rows led failure (open and short circuit) detection led7706 demonstration board AN2809 6/22 2 led7706 demonstration board the led7706 demonstration board has been designed to manage six strings of 8 to 10 white leds each. table 1 summarizes the board features and figure 2 shows the schematic of the led7706 demonstration board. the input voltage range is limited to 32 v because of the 35 v rated input capacitor. extended operating input voltage ranges (up to 36 v) can be achieved by using a 50-v rated mlcc. figure 2. led7706 board schematic table 1. led7706 performance summary parameter conditions value minimum input voltage 4.5 v maximum input voltage 32 v output voltage v in component assembly and layout AN2809 8/22 4 component assembly and layout figure 3. top side component placement figure 4. bottom side test points AN2809 i/o interface 9/22 5 i/o interface the led7706 demonstration board has the following test points. table 3. description of led7706 demonstration board test points test point description vin+ input voltage, positive terminal vin- input voltage, negative terminal gnd reference ground row1 to row6 current generators output vboost boost regulator output voltage dim pwm dimming input en enable input (active high) sync synchronization output fsw synchronization input fault fault signal, active low recommended equipment AN2809 10/22 6 recommended equipment 4.5?32 v, 2 a capable power supply. digital multimeters. 200 mhz oscilloscope. signal generators for pwm dimming and synchronization clock (optional). 7 configuration the led7706 demonstration board allows the user to select the desired mode of operation using the sw1 and sw2 selectors. refer to the following configuration description. a red led is connected to the fault pin to easily monitor its status; if this option is not desired, the monitor led can be disconnected by opening the j2 jumper. 7.1 sw1 fixed or adjustable switching frequency (fsw pin) the sw1 selector is used to choose between the fixed switching frequency (660 khz) and a user-defined switching frequency in the range of 200 khz to 1 mhz. when connected in the lower position, the fixed switching frequency is selected. if sw1 is in the upper position, the switching frequency is given by: figure 5. sw1 (f sw ) setting the r5 resistor is set to 330 k ? (f sw = 825 khz). 7.2 sw2 fault management mode (mode pin) the sw2 selector is used to connect the mode to avcc or ground. when the jumper is set to the upper position, the mode pin is con nected to ground and the corresponding fault management is summarized in the first column of table 4 . f sw 2.5 r 5 ? = & i x e d 3 w i t c h i n g & r e q u e n c y d e f a u l t p o s i t i o n ! d j u s t a b l e 3 w i t c h i n g f r e q u e n c y & i x e d 3 w i t c h i n g & r e q u e n c y d e f a u l t p o s i t i o n ! d j u s t a b l e 3 w i t c h i n g f r e q u e n c y ! - v AN2809 configuration 11/22 otherwise, when sw2 is set to the lower posi tion, the mode pin is connected to avcc and the corresponding fault management is summarized in the second column of table 4 . figure 6. sw2 (mode) setting 7.3 sw3 enable function the terminals of the switch sw3 are connected on one side to the en pin and on the other side to ground. therefore, when the switch is not pressed, the en pin is floating, which implies that the device is working. when the sw3 pi n is pressed, the en pin is connected to ground. when the sw3 is released, the device re-starts (the soft-start is performed). the sw3 switch can be activated whenever a new start-up is required or to escape a latched condition. table 4. fault management summary fault mode to gnd mode to vcc internal mosfet over current fault pin high power mosfet turned off output overvoltage fault pin low device turned off, latched condition thermal shutdown fault pin low. device turned off. automatic restart after 30c temperature drop. led short circuit fault pin low device turned off at first occurrence, latched condition (v th =3.4v) fault pin low faulty row(s) disconnected. device keeps on working with the remaining row(s) (v th =6v) open row(s) fault pin low device turned off at first occurrence, latched condition fault pin high faulty row(s) disconnected. device keeps on working with the remaining row(s) ! - v 0 2 ' ( s l q w r $ 9 & & v h h 7 d e o h q g f r o x p q 0 2 ' ( s l q w r j u r x q g v h h 7 d e o h v w f r o x p q 0 2 ' ( s l q w r $ 9 & & v h h 7 d e o h q g f r o x p q 0 2 ' ( s l q w r j u r x q g v h h 7 d e o h v w f r o x p q test setup AN2809 12/22 8 test setup a proper wled array is required as load to correctly evaluate the led7706. figure 7 shows a possible assembly of led7706 with a wleds test board. this demonstration board includes 60 white leds (20 ma), switches, jumpers and test points used to easily perform the functional tests of the led7706. chapter 10 provides a brief description of the test board and its schematic, which can be used as reference for any customized board. figure 7. led7706 demonstration board and white leds test board assembly figure 8 shows the complete test setup. figure 8. led7706 board test setup pwm generator 4.5v ? 32v @ 2a power supply 6x10 ? 20ma wleds test board vboost vin+ vin- gnd dim + a-meter ch2 ch1 scope v-meter row1 pwm generator 4.5v ? 32v @ 2a power supply 6x10 ? 20ma wleds test board vboost vin+ vin- gnd dim + a-meter ch2 ch1 scope v-meter row1 AN2809 getting started 13/22 9 getting started the following step-by-step sequences are provid ed as a guideline to quickly evaluate the performance of the led7706 board. 9.1 quick startup note: working in a esd-protected environment is hi ghly recommended. first check all wrist straps and mat earth connections before handling the led7706 board. 1. connect the power supply to the led7706 board and insert the a-meter as shown in figure 8 . connect a v-meter between vboost and ground to monitor the output voltage. 2. connect the proper wleds array to the j8 connector or to the row1-row6 and vboost terminals of the led7706 board. 3. set the pwm signal (20 khz, 5% duty-cycle, 3.3 v cmos logic levels) on a signal generator and provide it to the dim input. 4. set sw1 and sw2 to the lower position (fixed frequency and mode to avcc). do not change jumper settings when the board is switched on . 5. set the input voltage to 12 v. 6. turn on the pwm generator. 7. turn on the vin supply: the device turns on. 8. vary the input voltage in the range 4.5?32 v. 9. set the input voltage to 12 v. 10. vary the dimming duty-cycle from 1 to 100%. 11. check the shape of the rows? current at a 1% dimming duty-cycle. note: when measuring the current of row x, an auto-ranging a-meter can trigger the open-row or shorted-led fault detection during the automatic scale selection procedure. disabling the auto-ranging option on the a-meter is recommended. getting started AN2809 14/22 9.2 open and shorted wled fault testing 1. set the input voltage to 12 v. 2. set the dimming duty-cycle to 20%. 3. disconnect the rows in sequence and compare the behavior of the led7706 to table 4 . 4. restore all row connections and force the en input to ground. 5. release the en input. 6. short one or more wleds and compare the behavior of the led7706 to table 4 . 7. press the sw3 push button of the led7706 board to reset the device. 8. turn off the power supply and set the sw2 selector to the upper position (mode to ground). 9. turn on the power supply and repeat steps 3 to 7. 10. remove all shorted wleds and leave row1 and row2 floating. 11. turn on the power supply: the floating rows are ignored. 12. turn off the pwm generator. 13. turn off the power supply. 9.3 device synchronization 1. set the pwm dimming signal to 100%. 2. remove the jumper from the sw1 selector to leave the fsw pin floating. 3. connect an external 600 khz clock generator (0 to 1 v logic levels, 40% duty-cycle) between the fsw test point and ground. refer to figure 9 . 4. turn on the pwm generator. 5. turn on the power supply: the device remains off until the fsw pin is low. 6. turn on the clock generator: the device turns on. 7. monitor the sync output and verify the synchronization (the sync output is a replica of the fsw signal). 8. turn off the pwm generator. 9. turn off the clock generator. 10. turn off the power supply. AN2809 getting started 15/22 figure 9. led7706 synchronization setup 9.4 efficiency measurements figure 10 shows the set-up used to perform efficiency measurements. the efficiency in this device is typically defined as the ratio between the power provided to the load (current flowing through the leds multiplied by the voltage across the leds) and the total input power. the power dissipated in the current generators is correctly considered as a power loss. this method of calculating the efficiency implies that the voltage across the leds is the same for all the strings. however, this is not true. the power delivered to the load should be calculated as follows. where v string_i is the voltage across the leds in row i, whereas i string_i is the current flowing through row i . in order to ease the measurement, the voltage drop of all the generators is equalized by connecting them together. pwm generator 4.5v ? 32v @ 2a power supply 6x10 ? 20ma wleds test board vboost vin+ vin- gnd dim + ch2 ch1 scope 600khz ? 40% clock generator fsw sync pwm generator 4.5v ? 32v @ 2a power supply 6x10 ? 20ma wleds test board vboost vin+ vin- gnd dim + ch2 ch1 scope 600khz ? 40% clock generator fsw sync p load v stringi i1 = 6 i stringi ? = getting started AN2809 16/22 in this condition, the power provided to the leds is simply calculated as: where v string is the voltage across the parallelized channels, whereas i string is the total current delivered to the load (the sum of the current of the six channels). since all the channels are in parallel (120 ma total current), a single string of 150 ma-rated leds is required as load ( figure 10 ). figure 10. efficiency measurement setup figure 11 and figure 12 show two efficiency measurements against the duty-cycle of the dimming signal at two different input voltages. p load v string i string ? = pwm generator 4.5v ? 32v @ 2a power supply 10 wleds 150ma array vboost vin+ vin- gnd dim + a-meter v-meter row1 v-meter a-meter pwm generator 4.5v ? 32v @ 2a power supply 10 wleds 150ma array vboost vin+ vin- gnd dim + a-meter v-meter row1 v-meter a-meter AN2809 getting started 17/22 figure 11. efficiency vs dim duty-cycle, v in = 12 v, 10 white leds in series, i out = 1 2 0 m a figure 12. efficiency vs dim duty-cycle, v in = 24 v, 10 white leds in series, i out = 120 ma wleds test board AN2809 18/22 10 wleds test board the wleds test board here described mounts sixty vertical white leds (size 0603, 20 ma) arranged in a 6 x 10 matrix. figure 13 shows an image of the board, whereas figure 14 provides the schematic which can be used as reference to realize a customized board. several jumpers, switches and test points are provided to cover most of the test configurations. figure 13. wleds test board AN2809 wleds test board 19/22 figure 14. wleds test board schematic d 6 0 d 5 9 d 5 8 d 5 7 t p 1 4 1 d 5 6 d 5 5 d 5 4 d 5 3 d 5 2 d 5 1 t p 1 5 1 d 5 0 d 4 9 d 4 8 d 4 7 t p 1 6 1 d 4 6 d 4 5 d 4 4 d 4 3 d 4 2 d 4 1 t p 1 7 1 d 4 0 d 3 9 t p 1 8 1 d 3 8 d 3 7 d 3 6 d 3 5 d 3 4 d 3 3 d 3 2 d 3 1 d 3 0 d 2 9 t p 1 9 1 d 2 8 d 2 7 d 2 6 d 2 5 d 2 4 d 2 3 d 2 2 d 2 1 t p 2 0 1 j 3 1 2 t p 4 1 d 2 0 d 1 9 d 1 8 d 1 7 d 1 6 d 1 5 d 1 4 d 1 3 t p 5 1 d 1 2 d 1 1 d 1 0 d 9 d 8 d 7 d 6 d 5 d 4 j 1 4 1 2 d 3 d 2 t p 6 1 d 1 t p 7 1 j 1 3 1 2 t p 8 1 j 1 2 1 2 t p 9 1 j 1 1 1 2 j 4 1 2 t p 1 0 1 j 1 0 1 2 j 5 1 2 j 2 3 1 2 j 2 2 1 2 j 2 1 1 2 j 2 0 1 2 j 2 7 1 2 j 2 6 1 2 j 2 5 1 2 j 6 1 2 j 2 4 1 2 j 7 1 2 j 3 1 1 2 j 3 0 1 2 j 2 9 1 2 j 2 8 1 2 j 3 5 1 2 j 3 4 1 2 j 3 3 1 2 j 3 2 1 2 s w 3 s w d i p - 6 j 3 9 1 2 j 3 8 1 2 j 3 7 1 2 j 3 6 1 2 t p 1 1 t p 1 1 1 j 1 1 2 3 j 2 1 2 3 s w 1 s w 2 j 9 1 2 t p 2 1 1 t p 1 2 1 t p 1 3 1 t p 2 1 t p 3 1 j 1 5 1 2 j 8 h e a d e r 8 x 2 2 4 6 8 1 0 1 2 1 4 1 6 1 3 5 7 9 1 1 1 3 1 5 j 1 9 1 2 j 1 8 1 2 j 1 7 1 2 j 1 6 1 2 am0067 3 v1 wleds test board AN2809 20/22 table 5 , table 6 and table 7 respectively describe the board?s jumper functions, switches and test-points. table 5. test board jumper functions jumper function default position j1 two-position selector. used in co njunction with sw1 to simulate a fault on shorted leds on row1. when set to position 1, the d10 led can be shorted by pressing sw1. when set to position 2, both d9 and d10 leds can be shorted by pressing sw1. 1 j2 two-position selector. same function as j1 but related to row6 (d59 and d60 leds). 1 j3 to j7 pcb tin-drop jumpers, bottom-sided. used to parallelize the desired number of rows. open j8 8-terminal connector. used to interface the led7706 demonstration board. - j9 pcb tin-drop jumper, bottom-sided . used to provide the output voltage of the boost section of led7706 to the low-current (d1- d60), top-sided wleds array. shorted j10 to j15 two-pin jumpers, top-sided. used to access the led strings to perform row current monitoring and voltage threshold measurement. shorted j16 to j39 pcb tin-drop jumper, bottom-sided. used to reduce the number of active leds of each row by shorting unused diodes. open table 6. test board switch functions switch function default position sw1 used in conjunction with j1 to simulate a shorted led fault condition. see j1 function. open (released) sw2 used in conjunction with j2 to simulate a shorted led fault condition. see j2 function. open (released) sw3 dip switch. used to individually break row1 through row6. used to simulate open led fault or unused (floating) rows. on table 7. test board test-point functions test point function tp1 to tp10 pcb test points, top-sided. used to easily access each led of row1 (d1-d10). tp11 to tp20 pcb test points, top-sided. used to easily access each led of row6 (d51-d60). tp21 pcb test point, top-sided. auxilia ry access to the output voltage. AN2809 revision history 21/22 11 revision history table 8. document revision history date revision changes 10-jan-2009 1 initial release. 11-feb-2009 2 updated table 2: led7706 component list and table 4: fault management summary AN2809 22/22 please read carefully: information in this document is provided solely in connection with st products. stmicroelectronics nv and its subsidiaries (?st ?) reserve the right to make changes, corrections, modifications or improvements, to this document, and the products and services described he rein at any time, without notice. all st products are sold pursuant to st?s terms and conditions of sale. purchasers are solely responsible for the choice, selection and use of the st products and services described herein, and st as sumes no liability whatsoever relating to the choice, selection or use of the st products and services described herein. no license, express or implied, by estoppel or otherwise, to any intellectual property rights is granted under this document. i f any part of this document refers to any third party products or services it shall not be deemed a license grant by st for the use of such third party products or services, or any intellectual property contained therein or considered as a warranty covering the use in any manner whatsoev er of such third party products or services or any intellectual property contained therein. unless otherwise set forth in st?s terms and conditions of sale st disclaims any express or implied warranty with respect to the use and/or sale of st products including without limitation implied warranties of merchantability, fitness for a parti cular purpose (and their equivalents under the laws of any jurisdiction), or infringement of any patent, copyright or other intellectual property right. unless expressly approved in writing by an authorized st representative, st products are not recommended, authorized or warranted for use in milita ry, air craft, space, life saving, or life sustaining applications, nor in products or systems where failure or malfunction may result in personal injury, death, or severe property or environmental damage. st products which are not specified as "automotive grade" may only be used in automotive applications at user?s own risk. resale of st products with provisions different from the statements and/or technical features set forth in this document shall immediately void any warranty granted by st for the st product or service described herein and shall not create or extend in any manner whatsoev er, any liability of st. st and the st logo are trademarks or registered trademarks of st in various countries. information in this document supersedes and replaces all information previously supplied. the st logo is a registered trademark of stmicroelectronics. all other names are the property of their respective owners. ? 2009 stmicroelectronics - all rights reserved stmicroelectronics group of companies australia - belgium - brazil - canada - china - czech republic - finland - france - germany - hong kong - india - israel - ital y - japan - malaysia - malta - morocco - singapore - spain - sweden - switzerland - united kingdom - united states of america www.st.com |
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