Tps51125 help needed in uderstanding this.

[Gary B]

Hi I am having trouble in understanding this regulator circuit. Basically I have 3v on pin 8 vreg3 and 5v on pin 17 vreg5 but I am not getting any 3v or 5v output across l3 or l6. There is no 3v on the Sio or powerbutton so the unit is dead. What is needed for this to start outputting the 3v needed from the powerbutton and SIO?.

The circuit is from a Toshiba l300

Many thanks.

[spleenharvester]

Re: Tps51125 help needed in uderstanding this.

The orientation on the schematic is not always the same as the orientation on the chip's surface. I got caught out by that once. Could that be the issue?

I do have an L300 board here I could take some readings off if you want. Though I can't do anything too precise because of how shaky my hands are.

[Gary B]

Re: Tps51125 help needed in uderstanding this.

The orientation on the schematic is not always the same as the orientation on the chip's surface. I got caught out by that once. Could that be the issue?

I do have an L300 board here I could take some readings off if you want. Though I can't do anything too precise because of how shaky my hands are.

No that's fine but thank you. I am just curious as to what makes this Ic start outputting. I have seen other regulators and understand them but this one Im confused as normally the 3v is present on the Sio and powerbutton without powering it on. But this tps51125 is confusing me.

[spleenharvester]

Re: Tps51125 help needed in uderstanding this.

Yeah I could never get my head round them either. With a TPS51120 I was messing about with I think I figured it was something to do with the PWRGOOD_3 signal but I never figured it out.

[fddizon]

Re: Tps51125 help needed in uderstanding this.

check pin 16 which is VIN

[rievax_60]

Re: Tps51125 help needed in uderstanding this.

Also pin 13. I think it needs to be more than 2.4v to turn on all rails.
It is a confusing IC.
You should check the missing rails with a CRO while power is first applied in case these rails are trying to come on then shutting down.

[rievax_60]

Re: Tps51125 help needed in uderstanding this.

Pins 1 and 6 if pulled low will turn off their respective SMPS rails. They need to be about 0.4v or higher.

[Francisco.s]

Re: Tps51125 help needed in uderstanding this.

A photo will explain better than words. (of course it is not an easy circuit)

[Gary B]

Re: Tps51125 help needed in uderstanding this.

Thank you all for the input unfortunately Im still no nearer to understanding this circuit. Most other 3.3v regulators I have came across automatically release the 3.3v to power the sio and the powerbutton but with this circuit I cant see how I have vreg3 but no 3v on the sio or powerbutton? if its switched then how as the sio has no power to control the switching. I changed the Tps and was only getting .7v on the vin as opposed to 19v looks like it may have been faulty as well as I pulled from scrap board now the fet on the charging circuit is not being switched on so only 19v on one side of it..

[fddizon]

Re: Tps51125 help needed in uderstanding this.

trace back to where that vin came from.

[Francisco.s]

Re: Tps51125 help needed in uderstanding this.

the charging circuit is not being switched on so only 19v on one side of it..

What laptop are you trying to repair? (mobo code)

[Francisco.s]

Re: Tps51125 help needed in uderstanding this.

If you don´t have 19v. in +VBAT, check for short-circuit in this line to gnd.
Also check ACDET voltage, (pin 5 U503) could be that R524 is open, or Q503 with leakage current. U503 could be also responsible of not being supply the correct voltage in pin 2 (ACDRV#)

[Gary B]

Re: Tps51125 help needed in uderstanding this.

trace back to where that vin came from.

Vin comes from Adapter into mosfet (q504) that is switched by the Charger IC using the Ac detect signal, this then immediately goes through current limiting resistor and onto the 3v/5v (u511) regulator, 99% of laptop mobos work the exact same way. This Vin 19v voltage is used as the vin onto the 3v/5v regulator. Usually as long as you have the vin to the regulator the regulator will output 3.3v onto Sio and powerbutton. The powerbutton when brought to ground then starts the Sio working.

On this set I had the Vin=19, Vreg3=3v, veg5=5v. Now after regulator change (exact same chip) I don't have vreg3 or vreg5 because theres is no Vin. The aforementioned mosfet has 19v (q504) on one side and .7v on the other its not being switched on (it might be faulty) I will change it. I have attached the schematic.

Many thanks to everybody for their time

[rievax_60]

Re: Tps51125 help needed in uderstanding this.

R546 and R547 bleed a small current to the VBAT rail which would normally cause
some voltage rise which is detected somehow before Q504 is allowed to be turned on.
Your VBAT rail might have low resistance to GND fault.

[rievax_60]

Re: Tps51125 help needed in uderstanding this.

If the VBAT rail stays low because of a short to GND, D515 will hold ACDET(pin5) low.