Hi Folks,
I'm trying to troubleshoot the following two issues in a section of a DAC power supply. I don't have schematics. The DAC is working but I'm concerned about the heat. There is a row of 6 LT1763 chips that are part of power supply. All 6 LT1763s are connected to Vin = 6Volts, and the same (shutdown?) circuit. The shutdown circuit consists of Vin, a 10Kohm resistor and a switching diode labelled 4148C which might be MMBD4148CA or MMBD4148CC, and possibly other components to be mapped out though I'm not sure how important that is in troubleshooting the two issues.
1. IC42 is running much much hotter than the other LT1763s at 97C. Is this expected when Pin 2(ADJ) is connected to Pin 1 (Vout)? I have verified Vout is 1.22Volts, as labelled on the PCB, and expected from the datasheet.
2. IC11 seems to be defective since Vout is 0.22 Volts - the label is 3.3 Volts. IC11 appears identical to IC15 and IC40, both output around 3 Volts. I haven't been able to verify if any of the surface mount capacitors or resistors are failing, except that they are not shorted or open.
All other LT1763 Vout values are as expected: IC17 (Vout = ~4Volts), IC41 (Vout = ~2 Volts)
LT1763 datasheet: https://www.analog.com/media/en/technical-documentation/data-sheets/1763fh.pdf
the datasheet shows Tjmax is 150C.
I verified the following on IC42:
Pin 8 (Vin) = 6 Volts
Pin 1 (Vout) = 1.22 Volts
Pin 2 (ADJ) Connected to 1
Pin 3,6 and 7 (GND) are connected to GND
Pin 5 (SHDN) is connected to the shutdown circuit described about.
Pin 4 (BYP) is connected to a 103 (10000pF) Capacitor, which is connected to Vout.
I am thinking of ordering two LT1763s but these are quite expensive at $6 a piece.
I'm trying to troubleshoot the following two issues in a section of a DAC power supply. I don't have schematics. The DAC is working but I'm concerned about the heat. There is a row of 6 LT1763 chips that are part of power supply. All 6 LT1763s are connected to Vin = 6Volts, and the same (shutdown?) circuit. The shutdown circuit consists of Vin, a 10Kohm resistor and a switching diode labelled 4148C which might be MMBD4148CA or MMBD4148CC, and possibly other components to be mapped out though I'm not sure how important that is in troubleshooting the two issues.
1. IC42 is running much much hotter than the other LT1763s at 97C. Is this expected when Pin 2(ADJ) is connected to Pin 1 (Vout)? I have verified Vout is 1.22Volts, as labelled on the PCB, and expected from the datasheet.
2. IC11 seems to be defective since Vout is 0.22 Volts - the label is 3.3 Volts. IC11 appears identical to IC15 and IC40, both output around 3 Volts. I haven't been able to verify if any of the surface mount capacitors or resistors are failing, except that they are not shorted or open.
All other LT1763 Vout values are as expected: IC17 (Vout = ~4Volts), IC41 (Vout = ~2 Volts)
LT1763 datasheet: https://www.analog.com/media/en/technical-documentation/data-sheets/1763fh.pdf
the datasheet shows Tjmax is 150C.
I verified the following on IC42:
Pin 8 (Vin) = 6 Volts
Pin 1 (Vout) = 1.22 Volts
Pin 2 (ADJ) Connected to 1
Pin 3,6 and 7 (GND) are connected to GND
Pin 5 (SHDN) is connected to the shutdown circuit described about.
Pin 4 (BYP) is connected to a 103 (10000pF) Capacitor, which is connected to Vout.
I am thinking of ordering two LT1763s but these are quite expensive at $6 a piece.
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Thank you, it looks like at least one is defective. if I’m lifting the pins then might as well have the spare LT1763s, resistors and capacitors available. 4148c is mmbd4148cc. Will be sure to clean up that area of the board.
Update: IC11 seems to be working fine. It turns on (Vout=3.4volts) when the USB input is selected. IC12 is part of the USB power supply. And Vout drops down to 0.22 volts when selecting the other inputs. Planning to replace IC42 and IC11 after measuring pin 1 voltage without a load. trigger happy on ordering components 😉. Still 95C seems suspect, right?
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No change after replacing IC42. Forgot to lift pin 1 and measure without a load. IC11 remains on after updating the DAC firmware. Turns out IC11 Vout=0.22 Volts because shutdown mode was triggered (Pin 5 - Vshdn=0 Volts).
Found this article:
https://ez.analog.com/power/f/q-a/564798/lt1763-output-oscillation-issue
Could these type of oscillations be the cause of overheating?
Found this article:
https://ez.analog.com/power/f/q-a/564798/lt1763-output-oscillation-issue
Could these type of oscillations be the cause of overheating?
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