Need help with TPS68000 CCFL driver

Hello Fausto,

Please excuse the late response.

First of all: is there the possibility for you to exchange the CCFL by LED backlight? The application is a lot less dependent on parasitic effects in the housing than with CCFL. If there is the possibility, I would encourage you to go this easier way.

There are 2 possible root causes leading to the STC capacitor not getting charged:
1. One of the protection features is activated, like overvoltage or overcurrent protection, so please observe the voltage on OCP, VSEN, and CSEN at least.
2. The part did try to start, but did not detect any current for a certain time and therefore stopped immediately. Please check the gate voltages if the part tries to start and is not able to.

The part is dependent on the parasitics in the system, so the setup is as well influencing the performance of the system. Please check your setup for unnecessary parasitics like long cables or so.

Best regards,
Brigitte

Brigitte:

thank you for your reply

Would it be possible that I can talk to you for 5min? (if so, please email me your contact info)

My is Fausto - 416 449 3084

I need to fix this problem very soon.

I can't change to an LED since this application requires a CCFL lamp

The fault signal is not active.

The documentation is very poor on this device, I would like to get a copy of a more detailed document about the settings if possible.

I am trying to set it up on analog dimming.

SET resistor = 249k, fq = 39khz

STC = 1uf,

PH & BBR = 5V

BC = ground

BF = 0.047uf

I have copied the schematic from the eval board TPS68000EVM-161, page 11

I have several questions:

1.- CSEN current is the average value of the positive side (half wave rectified) of the current wave form only?

2.- VSEN voltage is the average value of the positive side of the voltage wave form only?

3.- OCP voltage the average value of the positive side of the voltage wave form only?

4.-Can you describe in detail how the start up sequence works? Please add a time line if possible.

i.e. The frequency sweeps from 2xf to f/2, when does it stop?

when VSEN has reached VREG?

when CSEN has reached VABR?

5.- There are also TSW & TW, what are the differences between them?

6.- After the TSW & TW are completed, does only CSEN regulate the lamp brightness?

Any suggestions will be greatly appreciated.

Thank you

Fausto Bartra, PEng

Assistant Vice President of R&D

Tekran Instruments Corp.

330 Nantucket Boulevard

Toronto, Ontario

M1P-2P4

CANADA

P: 416-449-3084 x 287

F: 416-449-9298

fbartra@tekran.com

Brigitte:

did you have a chance to look at my request?

Thank you

Fausto Bartra

Brigitte:

what are the VOL & VOH for the GA, GB, GC & GD pins?

Thank you

Fausto

Hello Fausto,

Please check the functional block diagram on page 7 of the datasheet. GA drives between V5A and SA, GC drives between V5C and SC and GB and GD drive between V5 and GGND.

The transistors are NMOS therefore the lower voltage turns off and the higher voltage turns on.

Best regards,
Brigitte

Hello Fausto,

1.- CSEN current is the average value of the positive side (half wave rectified) of the current wave form only?
The value that you can measure on CSEN is AC voltage that is equivalent to the AC current through the lamp.

2.- VSEN voltage is the average value of the positive side of the voltage wave form only?
The voltage you can measure on VSEN is a divided AC voltage. The capacitive divider connected in parallel to the lamp gives this AC value.

3.- OCP voltage the average value of the positive side of the voltage wave form only?
The input voltage on the OCP pin is in the standard application a equivalent to the current through secondary side of the transformer and therefore as well an AC signal.

4.-Can you describe in detail how the start up sequence works? Please add a time line if possible.

i.e. The frequency sweeps from 2xf to f/2, when does it stop?

when VSEN has reached VREG?

when CSEN has reached VABR?
The frequency sweeps from 2*f to f/2 and it stops at the moment when the lamp ignites or the voltage regulator starts to regulate.

The frequency starts to sweep and latest stops to sweep when tSW has passed, if the lamp does not ignite and the voltage did not rise to a value where the voltage regulation had to start.

If the voltage regulation started, the frequency sweep is stopped as well and longest tW is waited until the system is stopped because the lamp did not ignite.

5.- There are also TSW & TW, what are the differences between them?
I think the above answer answered this.

6.- After the TSW & TW are completed, does only CSEN regulate the lamp brightness?
Neither tSW nor tW should pass. Normally within tSW the lamp should ignite.

Please check as well the voltage on the ABR, if this is too low, the current through the lamp is too low and then ignition does not happen at all. For ignition, the voltage here should be at the maximum and can then be reduced.

Best regards,
Brigitte

Brigitte:

Our application is very unusual, this is not a backlight for a display

We require more information (if possible) to take advantage of your device

Please if you could help me with these questions:

1. CSEN, VSEn & OCP (internally) are half wave rectified signals?
2. How is this voltage actually calculated internally?
3. When you say "the lamp has ignited" is this means it has reached CSEn? VSEN? please explain
4. During power up, as it sweeps frequencies, what percentage of phase shift is used ?

If you have any app notes on this device, it will be very helpful

Thank you

Fausto Bartra