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| may 2009 doc id 14893 rev 1 1/17 AN2810 application note 6-row 85 ma led driver with boost converter for lcd panel backlighting introduction the led7707 led driver from stmicroelectroni cs consists of a high-efficiency monolithic boost converter and six controlled current generat ors (rows), specifically designed to supply led arrays used in the backlighting of lcd panels. the device can manage an output voltage up to 36 v (i.e. ten white-leds per row). the generators can be externally programmed to sink up to 85 ma and can be dimmed via a pwm signal (1% of minimum dimming duty-cycle at 1 khz can be managed). the device allows detection and management of open and shorted led faults, and permits unused rows to be left floating. basic protections (output overvoltage, internal mosfet overcurrent and thermal shutdown) are provided. figure 1. led7707 demonstration board www.st.com
contents AN2810 2/17 doc id 14893 rev 1 contents 1 led7707 main features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1 boost section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2 backlight driver section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2 led7707 demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3 component list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4 component assembly and layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 5 i/o interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 6 recommended equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 7 configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 7.1 sw1 fixed or adjustable switching frequency (fsw pin) . . . . . . . . . . . . . . 8 7.2 sw2 fault management mode (mode pin) . . . . . . . . . . . . . . . . . . . . . . . . 9 7.3 sw3 enable function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 8 test setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 9 getting started . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 9.1 quick startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 9.2 open and shorted wleds fault testing . . . . . . . . . . . . . . . . . . . . . . . . . . 13 9.3 device synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 9.4 efficiency measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 10 revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 AN2810 list of figures doc id 14893 rev 1 3/17 list of figures figure 1. led7707 demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 figure 2. led7707 demonstration board schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 figure 3. top side component placement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 figure 4. bottom side test points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 figure 5. sw1 (f sw ) setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 figure 6. sw2 (mode) setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 figure 7. led7707 demonstration board and white led test board assembly . . . . . . . . . . . . . . . . . 11 figure 8. led7707 demonstration board test setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 figure 9. led7707 synchronization setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 figure 10. efficiency measurement setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 figure 11. efficiency vs. dim duty cycle, v in =12 v, 6 rows, 10 white leds in series, i out = 360 ma 15 figure 12. efficiency vs. dim duty cycle, v in =24 v, 6 rows, 10 white leds in series, i out = 360 ma 15 led7707 main features AN2810 4/17 doc id 14893 rev 1 1 led7707 main features 1.1 boost section 4.5 v 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 250 khz to 1 mhz adjustable switching frequency external sync for multi-device application pulse-skip power saving mode at light load programmable soft-start programmable ovp (overvoltage protection) single ceramic output capacitor non-latched thermal shutdown 1.2 backlight driver section six rows with 85 ma maximum current capabilit y (adjustable) up to 10 wleds per row parallelable rows for higher current row disable option less than 10 s minimum dimming time 2% current matching between rows led failure (open and short-circuit) detection AN2810 led7707 demonstration board doc id 14893 rev 1 5/17 2 led7707 demonstration board the led7707 demonstration board has been designed to manage six strings of 8 to 10 white leds each. ta bl e 1 summarizes the board features and figure 2 shows the led7707 demonstration board schematic. the input voltage range is limited to 32 v because of the 35 v rated input capacitor. an extended operating input voltage range (up to 36 v) can be achieved by using a 50 v-rated mlcc. figure 2. led7707 demonstration board schematic table 1. led7707 performance summary parameter conditions value minimum input voltage 6 v maximum input voltage 32 v output voltage v in AN2810 component assembly and layout doc id 14893 rev 1 7/17 4 component assembly and layout figure 3. top side component placement figure 4. bottom side test points qty componen t description package part-number mfr value 1 j8 header 8 sil 8 standard 1 sw1, sw2 jumper 3 sil 3 standard 1 sw3 pushbutton 6x6mm fsm4jsmat tyco table 2. led7706 demonstration board component list (continued) i/o interface AN2810 8/17 doc id 14893 rev 1 5 i/o interface the led7707 demonstration board is equipped with the test points described in ta bl e 3 . 6 recommended equipment 4.5 v to 32 v, 2 a capable power supply digital multi-meters 20 mhz oscilloscope signal generators for pwm dimming and synchronization clock (optional) 7 configuration the led7707 demonstration board allows the user to choose the desired mode of operation using the sw1 and sw2 selectors (refer to the configuration description in the following paragraphs). a red led is connected to the fault pin to easily monitor its status; if this option is not required, the monitor led ca n be disconnected 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 250 khz to 1 mhz (see figure 5 ). when placed in the ?down? position, the fixed switching frequency is selected. table 3. led7707 demonstration board test points description test point description vin+ input voltage, positive terminal vin- input voltage, negative terminal gnd reference ground row1 to row6 current generator 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 AN2810 configuration doc id 14893 rev 1 9/17 if sw1 is in the ?up? position, the switching frequency is given by: equation 1 figure 5. sw1 (f sw ) setting the r5 resistor is set to 330 k (f sw = 825 khz) and can be changed by the user. 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 ta b l e 4 . otherwise, when sw2 is set to the ?down? position, the mode pin is connected to avcc and the corresponding fault management is summarized in the second column of ta b l e 4 . figure 6. sw2 (mode) setting 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 ! - 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 configuration AN2810 10/17 doc id 14893 rev 1 7.3 sw3 enable function the terminals of 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. pressing the sw3 pin connects the en pin to ground. when sw3 is released, the device re-starts (a soft-start is performed). the sw3 switch can be activated whenever a new startup is required or to escape a latched condition. table 4. fault management summary fault mode to gnd mode to vcc internal mosfet overcurrent fault pin high power mosfet turned off output overvoltage fault pin low power mosfet turned off (hysteretic regulation) thermal shutdown fault pin low. device turned off. automatic restart after 30 c temperature drop. led short-circuit fault pin low device turned off (100 a masking time), latched condition (v th =4.0 v) open row(s) fault pin low device turned off at first occurrence, latched condition fault pin high faulty row(s) disconnected (100 s masking time) AN2810 test setup doc id 14893 rev 1 11/17 8 test setup an appropriate white led array is required as a load to correctly evaluate the led7706. figure 7 shows a possible assembly of the led7707 with a wled test board. this demonstration board includes 60 white leds (150 ma), switches, jumpers and test points which can be used to easily perform functional testing of the led7707. figure 7. led7707 demonstration board and white led test board assembly figure 8 shows the complete test setup. figure 8. led7707 demonstration board test setup pwm generator 6v ? 32v @ 2a power supply vboost vin+ vin- gnd dim + a-meter ch2 ch1 scope v-meter row1 6x10 ? 75ma wleds array pwm generator 6v ? 32v @ 2a power supply vboost vin+ vin- gnd dim + a-meter ch2 ch1 scope v-meter row1 6x10 ? 75ma wleds array getting started AN2810 12/17 doc id 14893 rev 1 9 getting started the following step-by-step instructions are provided as a guide for quick evaluation of the performance of the led7707 demonstration board. 9.1 quick startup 1. working in an esd-protected environment is highly recommended. check all wrist straps and esd mat earth connections before handling the led7707 board 2. connect the power supply to the led7707 board and insert the a-meter as shown in figure 8 . connect a v-meter between vboost and ground to monitor the output voltage 3. connect the white leds array to the row1-row6 and vboost terminals of the led7707 board 4. set the pwm signal (500 hz, 5% duty-cycle, 3.3 v cmos logic levels) on a signal generator and connect it to the dim input 5. set sw1 and sw2 in the ?down? position (fixed frequency and mode to avcc). do not change jumpers settings when the board is powered 6. set the input voltage to 12 v 7. turn-on the pwm generator 8. turn-on the vin supply. the device turns on 9. vary the input voltage within the range 6 v - 32 v 10. set the input voltage to 12 v 11. vary the dimming duty-cycle from 1% to 100% 12. check the shape of the row current at 10 us dimming on-time note: when used for rowx current measurement, some autoranging a-meters can trigger open- row or shorted-led fault detection during the automatic scale selection procedure. caution: disabling the auto-ranging option on the a-meter is recommended. AN2810 getting started doc id 14893 rev 1 13/17 9.2 open and shorted wleds fault testing 1. set the input voltage to 12 v 2. set the dimming duty-cycle to 20% 3. set sw1 and sw2 in the ?up? position 4. turn-on the pwm generator and the supply in sequence 5. disconnect the rows in sequence and compare the behavior of the led7707 to ta b l e 4 6. restore all row connections and press the sw3 pushbutton on the led7707 board to reset the device 7. short one or more wleds and compare the led7707 behavior to ta b l e 4 8. press the sw3 pushbutton on the led7707 board 9. turn-off the power supply and set the sw2 selector to the ?up? position (mode to ground) 10. turn-on the power supply and repeat steps 5 through 8 11. remove all shorted wleds and leave row1 and row2 floating 12. turn-on the power supply. the floating rows are ignored 13. turn-off the pwm generator 14. 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 700 khz clock generator (0 v-1 v logic levels, 30% 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 getting started AN2810 14/17 doc id 14893 rev 1 figure 9. led7707 synchronization setup 9.4 efficiency measurements figure 10 shows the setup used to perform the efficiency measurements. the efficiency in this device is typically defined as the ratio be tween 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 way 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: equation 2 where v string_i is the voltage across the leds in row i, whereas i string_i is the current flowing through the row i. to facilitate th e measurement, the voltage drop of all the generators is equalized by connecting them together. in this condition, the power provided to the leds is simple to calculate: equation 3 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 pwm generator 4.5v ? 32v @ 2a power supply 6x10 ? 75ma leds array vboost vin+ vin- gnd dim + ch2 ch1 scope 700khz ? 30% clock generator fsw sync pwm generator 4.5v ? 32v @ 2a power supply 6x10 ? 75ma leds array vboost vin+ vin- gnd dim + ch2 ch1 scope 700khz ? 30% clock generator 700khz ? 30% clock generator fsw sync p load v stringi i1 = 6 i stringi ? = p load v string i string ? = AN2810 getting started doc id 14893 rev 1 15/17 channels are in parallel, a single string of 700 ma-rated leds is required as load ( figure 10 ). figure 10. efficiency measurement setup figure 11 and 12 shows two efficiency measurements versus the duty cycle of the dimming signal at two different input voltages. figure 11. efficiency vs. dim duty cycle, v in =12 v, 6 rows, 10 white leds in series, i out = 3 6 0 m a figure 12. efficiency vs. dim duty cycle, v in =24 v, 6 rows, 10 white leds in series, i out = 360 ma pwm generator 4.5v ? 32v @ 2a power supply 10 wleds 700ma 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 700ma array vboost vin+ vin- gnd dim + a-meter v-meter row1 v-meter a-meter revision history AN2810 16/17 doc id 14893 rev 1 10 revision history table 5. document revision history date revision changes 26-may-2009 1 initial release AN2810 doc id 14893 rev 1 17/17 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 - philippines - singapore - spain - sweden - switzerland - united kingdom - united states of america www.st.com |
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