TFT backlight question - Page 1
Forum section: electronics / beginners

author: nicksydney
author_rank: Regular Contributor
timestamp: April 18, 2014, 03:59:46 am
content: Hi,I was looking through schematics for a TFT controller RA8875 to drive a 5-7inch display and one of the requirement for the TFT is power to the backlight. The attached schematics (not mine obtained from internet) shows the AIC1896 supplies VLED+ and VLED- which will connect to the backlight of the TFT pin. What I'm confused about is the use of + and - in VLED does that mean that the AIC1896 able to produce +13V and also -13V for the backlight ? or how does it work normally ?Thanks for the help.

author: han
author_rank: Frequent Contributor
timestamp: April 18, 2014, 04:09:50 am
content: Quote from: nicksydney on April 18, 2014, 03:59:46 amnot +13V and -13V but only +13Vit's a boost converter, the -Vled in only connected to ground via resistor to current limit (feed back to the boost converter).

author: nicksydney
author_rank: Regular Contributor
timestamp: April 19, 2014, 04:47:17 pm
content: Thanks Han.I have few questions related to the step-up schematics that I downloaded from this linkhttp://elecfreaks.com/store/download/datasheet/lcd/TFT/7_Material.zip.. it's for a 7inch TFT. I've attached the schematic in this post and what I'm bit lost is how does the circuit able to get voltage of 10.4V via Q1 and how does the circuit works that it was able to generate the -7VI read the TPS61040 (http://www.ti.com/lit/ds/slvs413f/slvs413f.pdf) datasheet which has got similar schematics for positive and negative output LCD Bias Supply (page 15) but it is different when compared to the one from elecfreaks.Thanks for the help

author: han
author_rank: Frequent Contributor
timestamp: April 20, 2014, 08:01:26 am
content: Simple,The DC-DC converter IC is switch on/off and the output waveform is like ac voltage, but rather than alternate +/-. Its alternate Gnd and V+.But since there is an inductor that cannot stop current flow immediately, the circuit can boost voltage greater than Vin, and same with the negative cycle, that can lower than GND..the diode determine the positive/negative voltage that can be taken to the circuit...

author: nicksydney
author_rank: Regular Contributor
timestamp: April 20, 2014, 12:27:00 pm
content: Quote from: han on April 20, 2014, 08:01:26 amLet me see if I can understand this:1) The function of D3 (with D5) is to conduct it to GND when D6 is in 'ON' state ?2) D6 is a Zener Diode and looking at the schematic it is 9.1v which means that 16-9.1 = 7v (approx) and because the Cathode side of the Zener is pointing to (-) it is able to generate -7v3) I understand that Q1 voltage drop gives us 11-0.6v = 10.4v but I'm bit lost where the 11v comes in ? is that the function of D7 ?Thanks for the help

author: han
author_rank: Frequent Contributor
timestamp: April 21, 2014, 01:47:41 am
content: 1) The function of D3 (with D5) is to conduct it to GND when D6 is in 'ON' state ?- D3 is for "balancing" the average voltage so the ac current can pass trough C19- D5 is for "harvesting the negative voltage from the negative cycle came from C192) D6 is a Zener Diode and looking at the schematic it is 9.1v which means that 16-9.1 = 7v (approx) and because the Cathode side of the Zener is pointing to (-) it is able to generate -7v3) I understand that Q1 voltage drop gives us 11-0.6v = 10.4v but I'm bit lost where the 11v comes in ? is that the function of D7 ?-Yes, the D7 is a zener that "clamp" the voltage at 11V (the diode start to conduct at 11V and limiting the voltage to 11V"

author: nicksydney
author_rank: Regular Contributor
timestamp: May 07, 2014, 01:46:51 pm
content: Hi,I've created the schematics for the TFT power supply and want to ask whether the diode placement that I've attached the picture in this post is laid out correctly (D1,D2 and D3). The datasheet for the schottkyhttp://www.onsemi.com/pub_link/Collateral/MBR0530T1-D.PDFThanks