peculiar input current problems

[abq_guy]

Hello,

I am using a DC-DC converter from DATEL ( BWR-15/670-D12A). The inputs are +15V and the outputs are +15V and -15V. The converter can give upto 670mA of output current on both +15V and -15V individually.

I am using these output voltages to power 2 Linear LDO regulators (LT1962 and LT1964). These regulators take in the +15V and the -15V from the DC-DC converter and convert them to +14.85V and -14.85V respectively.

I use these +14.85V and -14.85V to power 5 LTC2704 (DAC's ), 5 OP400 op-amps, and 5 LT1885 op-amps.

When I turn the power On, the DC-DC converter starts taking in about 0.375A of current, but it rises to 1.0142A, at which point the +14.85V goes down to 1.62V and the input current drops to 0.250A.

Why is this happening ? Any explanations.

Thanks,

abq_guy

 

[audioguru]

I think the regulators are oscillating because maybe their output capacitor is not correct.

 

[abq_guy]

Any other suggestions??

Thanks,

abq_guy

 

[crutschow]

Does the Datel work if you disconnect its loads?

 

[abq_guy]

Does the Datel work if you disconnect its loads?

Hello Crutschow,

Yes the datel works without the loads. I also removed the 2 LDO (LT1962 and LT1964) and powered the remaining circuit directly through the DATEL converter.
I noticed that when all the 4 DAC's were turned ON, and the total current drawn by the DATEL converter from the OUTSIDE power supply was about 1.1A

I then connected the LDO's back but removed the output filter caps on the DATEL, and this time the total current drawn by the DATEL from the OUTSIDE power supply was about 0.485A.

All 4 DAC's were set to +10V during these tests. I don't understand, why the Input current would change with respect to the LDO's being there or not being there.

Thanks,

abq_guy

 

[crutschow]

Does sound like some sort of oscillation problem, perhaps with the LDOs.

Do the LDOs have the data sheet recommended filter capacitors connected close to their input and output leads? Skimping on those can cause oscillations.

Do you have an oscilloscope to observe the various voltages?

 

[abq_guy]

Does sound like some sort of oscillation problem, perhaps with the LDOs.

Do the LDOs have the data sheet recommended filter capacitors connected close to their input and output leads? Skimping on those can cause oscillations.

Do you have an oscilloscope to observe the various voltages?

Hello Carl,

I looked at all the voltages and none of them seem to be oscillating. I am using a 1uF, ceramic, X7R capacitor on the input of the LDO and a 10uF Tantalum, Low ESR (450 mOhms ) at the output.

I don't understand, why the Input current difference exists when the LDO is present in the circuit and when its not.

Input current without LDO is about 0.7A and with LDo it is 0.490A.

Thanks
Abq_guy

 

[crutschow]

Hello Carl,

I looked at all the voltages and none of them seem to be oscillating. I am using a 1uF, ceramic, X7R capacitor on the input of the LDO and a 10uF Tantalum, Low ESR (450 mOhms ) at the output.

I don't understand, why the Input current difference exists when the LDO is present in the circuit and when its not.

Input current without LDO is about 0.7A and with LDo it is 0.490A.

Thanks
Abq_guy

It certainly is a puzzle. The only thing I can think to do at this point is to connect an ammeter is series with each of the Datel outputs and see if it changes for the two conditions.