High Voltage Circuit Problems

Fuses seldom just "fail" (same for the doubler component you replaced). Usually there's some reason why they gave their life to protect your circuit.

If this is a circuit that has worked before (it is), and it is not working now, there are two possibilities: 1) One or more components is not meeting its specifications; or 2) The *effective* schematic of the circuit is not what you think it is--otherwise the circuit would work again.

You should have--but I don't want to assume--checked each component of the circuit as well as the *effective* schematic of the physical circuit. That is, do you have reason to believe the LT1768 is good? And are all the other components connected to it good? And is their connection with the other parts what it should be? No opens? No shorts? No components dead or whose value has been changed due to whatever blew the fuse?

Here are three things I'd look at:

1) Is the external MOSFET dragging down the GATE output? A quick test would be to open the line from GATE to the MOSFET and replace it with a reasonably high-value resistor tied to the point where the 'FET should connect (so that you don't corrupt the feedback circuit that goes to the SENSE input), and see if you now get appropriate output. If you do, the transistor is pulling down the GATE line. Replace it.

If that looks good, and the Vin looks clean (good DC level and acceptible AC ripple/noise), two control pins might matter.

One is PROG which apparently is used to control the device output for the purpose of dimming the backlight. If PROG is wrong, it may command the control chip to *not* enable the MOSFET.

A second important input is the SENSE pin. If the input there is incorrect it also might cause the LT1768 to shut down the GATE output.

I call problems like this doing "the wrong thing for the right reason". The control chip outputs a wrong value on GATE because the inputs to it are lying.

So make sure you understand the theory of operation of the LT1768 in the configuration you have and look for some pin or part of the circuit to *not* be like it should. Check all the pins, like Vref, Rmin, PWM and not-SHDN if you use it, and even if your circuit does not use it, check not-FAULT to see what it says. If the LT1768 shows FAULT then GATE should not be driven.