Vizio E320VL Mainboard DC/DC converter

[MagicSmoke]

Working on a Vizio (Amtran) mainboard 3632-1122-0150 in a e320vl.

There is a 1MHz 5v/2a synchronous DC/DC step down converter at U6 labeled LA8004PG that gets instantly too hot to touch when 5v stby power is applied.

Data sheet here

Fairly certain its bad if it instantly gets hot right? Cannot find an exact replacement P/N. I did find this one AP3435 that I think is compatible.

Datasheet here

Most of the specs similar if I am reading them correctly. (why are all data sheets laid out differently?)

Could someone more knowledgeable look them over too see if the AP3435 will work in place of the LA8004? Thanks.

[tom66]

Re: Vizio E320VL Mainboard DC/DC converter

That looks like it will work. The chip could also be getting hot if the load is short circuited or excessive, so check this too.

[MagicSmoke]

Re: Vizio E320VL Mainboard DC/DC converter

Will check load circuit also. Tested U6 while stby 5v only on.

1.VCC 5.28v
2.NC 0v
3.GND 0v
4.FB .19v
5.Vin 5.3v
6.SW .40v
7.Pgnd 0v
8.EN 5.23v

Looks like 1 5 & 8 are shorted together? 8. EN is supposed to be the on/off switch. 1&5 are supposed to be connected.

Pin 6 SW does show short but I have not removed U6 to see if it is the cause or if problem is elsewhere.
SW should be output? Am I correct?

[tom66]

Re: Vizio E320VL Mainboard DC/DC converter

Sometimes it's normal for EN to be shorted to VCC, this is by design as it will keep the chip switched on whenever power is available. Typically would be used for a rail always active in standby.

The way this converter works, in a simplified way of thought, is the duty cycle on SW varies so that it's a fraction of the output voltage. The wave is a 1MHz square wave where the on and off times vary to keep the output in regulation. The chip measures the voltage on FB to determine how close it is to the output voltage. When operating, FB nearly always measures 0.8V (as that is the reference voltage for this IC.)

You can see that in this case the FB voltage is 0.2V which is 1/4 of 0.8V which means the output voltage is probably 1/4 of what it should be, all else being the same (such as input current into FB being low)

Note: The FB pin is connected through a resistor divider, not directly to the output. So a fault on FB pin could cause low FB voltage.

On a good trueRMS meter, the SW pin should measure ~approx the output voltage.

My guess is the output voltage should be about 1.5V given FB measuring 1/4 (this is derived by 4 x SW voltage)

You should still check if the output is shorted.

Internally in the chip there are a pair of MOSFETs, one from VCC to SW (high side) and one from SW to GND (low side.) So check from VCC to SW and SW to GND as well.

[Andrew F. Ali]

Re: Vizio E320VL Mainboard DC/DC converter

Some very very useful information in Electronic Engineering.

[MagicSmoke]

Re: Vizio E320VL Mainboard DC/DC converter

If I tested it correctly the high side fet reads on diode mode

Black on pin 1
Red on pin 6 486 ohm

reversed 1146 0hm

Low side fet readings

Black on pin 3
Red on pin 6 000 ohm

reversed is the same 000 ohm.

Problem is I cant remove it from the circuit to see if it is shorted or something on the output. It has a pad underneath it soldered to the ground plane of the board.

[tom66]

Re: Vizio E320VL Mainboard DC/DC converter

You will need to use hot air to remove these devices.

[MagicSmoke]

Re: Vizio E320VL Mainboard DC/DC converter

Finally got a hot air station and removed the converter. It was not the problem, output rail still shorted.

Testing further found that the Right audio channel IN + is shorted to ground, the R channel audio OUT is not shorted.

So I guess the processor is shorted. All traces from R audio IN + go to CPU, and the output of the DC converter ends underneath the processor.