MP3394/MPS3394S LED Driver Diagnostic

The earlier MP3394 was replaced by MPS3394S, which is an upgraded version of the same IC, and found in the 17IPS20 power supply board. It is the main heart of the backlighting LED drive and the failure of this IC can cause the TV not to start. This is a voltage step-up IC that takes 5 V to 28 V at its input pin 15 (Vin) and raises it to drive four independent strings of LEDs. It also performs other functions such as voltage regulation between the LED strings, and provides a dimming facility using pulse-width modulation (PWM). An important aspect of this IC is that it has built-in open and short LED protection circuitry. It is also able to sense the current and voltage of the LED matrix load and determine abnormal conditions. That means that if all the LEDs in the matrix go open or short, it will prevent the television from switching ON.

The IC has internal control logic that produces the GATE signal to drive an external MOSFET transistor. This is the transistor that is switched against an inductor to step-up the voltage, and the gate signal is produced only when the internal control logic detects normal conditions. It is able to determine normal/abnormal conditions through internal comparators, and one such is the LED Over-Voltage Protection (LED OVP) comparator that provides Short String Protection (SSP).

When the TV is switched ON, this IC is enabled through pin 2, by a voltage greater than 1.8 V. It first checks the circuit configuration, and then checks the over-voltage protection signal at (pin 12) to determine if the rectifier diode is open and whether the booster MOSFET (driven by the GATE signal) is short to ground. If the OVP voltage at pin 12 is lower than 70 mV (may be different for other board versions), then it disables itself and the TV will not start.

On the 17IPS20 board, the GATE signal is known as GATE_LED, which drives a bi-polar junction transistor (BJT) Q500 (BC858B). This transistor then drives the Q502/Q503 MOSFET booster group of transistors to step-up the voltage (shown in the following pages of this article). In this circuit configuration the inductance coils are L500 and L501. The high voltage produced then goes into a group of rectifier diodes D503 and D504 producing a positive DC voltage for the LEDs. The over voltage protection signal is known as OVP_LED. Whilst all the LED strings take their power from this positive voltage source, they all have separate independent drain pins through the MPS3394S controller IC.

There are also other parameters to consider such as Under-Voltage Lock-Out (UVLO), which is determined by measuring the rising-edge of Vin. This parameter has to be between 3.5 V and 4.4 V, with 3.9 V being the typical value. It is a very clever IC as it can also detect open and short LED strings in the backlight array.

Here is a higher resolution image to help identify some of the SMD components around the MPS3394S backlighting controller. As you can see from the PCB traces, in this design, they do not use the individual string outputs, and instead pins 8, 9, 10, and 11 of the MPS3394S IC are joined together by 0-ohm SMD links, thereby making it a single-string design.

String Protection Diagnostic

Although this diagnostic cannot tell you the condition of each individual diode, it can indicate the condition of each string of diodes in the backlight matrix. The MPS3394S IC (SOIC16 package) drives four separate strings of LEDs through dedicated open drain output pins. LED string 1 is driven by pin 11, LED string 2 by pin 10, LED string 3 by pin 9, and LED string 4 by pin 8. It also monitors the voltages at these pins so that if there was a shorted LED in one of the strings then it would disconnect that string. If the voltage at any of these pins is greater than 6.1 V, then this condition triggers the short circuit protection mechanism for disconnecting the string at that pin. It is also possible to determine an open string, and in this case, this condition is detected when the pin voltage is lower than 196 mV.

Interestingly, this IC is designed such that it will always try to light at least one string if it is working. The only time when the step-up converter shuts-down completely is if all the strings were either open, or short. Therefore, before you rush off to eBay to buy a new chip, measure the voltages at these pins to make sure that the diodes are not faulty. Otherwise, you will end up wasting a lot of time and money!

The Good, Bad, and the Ugly, TV Designs

These days brand names make very little difference as the majority of the televisions are usually manufactured by the VESTEL Corporation. In this section we consider why you should buy a slightly more expensive TV and the benefits of reliability. In the previous section we looked at diagnosing faults on the 17IPS20 power supply board, and the workings of the MPS3394S integrated circuit. We learned that the manufacturer of this IC provides a facility to connect four separate strings of LEDs for the backlight, and if any one string had a fault, then the IC simply disconnects the affected string and the TV continues operating. This excellent fault tolerance capability of the IC is such that the backlight will continue operating even if just one string of LEDs continues operating. Other advantages are that it provides excellent load balancing and voltage regulation for each string. It is also very versatile and can be used in many different ways. For example, when a television has a very large screen, the backlight LED matrix also has to be large in order to produce uniform intensity of backlighting in all areas of the screen. In these situations, more strings would be required and therefore two ICs could also be used for a total of eight strings.

I was surprised to find that in extremely el cheapo televisions, they do not use all the string outputs, and instead they connect all the LEDs of the backlight matrix together into a single string. Therefore, there will be only two wires (usually red and black) coming out from behind the metal backlight screen to the power supply board. A major disadvantage of this configuration, where all the LEDs are on one string, is that if there was a failure of just one diode in the backlight matrix, the whole TV stops working! Because the IC is operating only on one string! Hence the fault tolerance design, the very thing that is meant to keep the TV working when a string fails, is the very thing that stops it from working! Honestly guys, where did you get your degrees from? Of course, I would love to have my own brand of TV, so I could explain to the "Engineers" how to make a better design and specification. Is there anyone out there?

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