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LT1374 Adjustable Dual Output SEPIC


I am designing a high efficiency power supply which requires a clean +-5V rail for a matrix of Op-amps providing a target +60dB gain on an input signal. However I am running into a few issues with getting the dual output SEPIC to work on the LT1374-SYNC as per the application note 'DN100' and the bottom of the datasheet of the 1374. I have also made a small modification by adding a voltage divider between the positive output and the FB pin, this should allow the output voltage to be adjustable (I have tested this with the standard LT1374 eval board), I actually need the SEPIC to have an output voltage around +-6.3 V as I will pass this into dual linear regulators to get the final +-5V. 

I have implemented the circuit on perfboard and I am testing it with 12V (cannot be changed) and it seems to be working quite well, developing the +ve and -ve voltages, however I am not able to get below -5.5 V i.e to my target at -6.3 V and the negative voltage stops changing linearly after -5V. For example when I adjust the potentiometer further the positive voltage continues to adjust up to 10 V however the negative voltage remains stuck in the region of -5 V. When loading the -ve rail with say 10 to 20 Ohms, which should result in 300mA to 600mA being drawn, the voltage on the rail drops drastically to -2 or -3V, however the app note states that the max load should be 1.5A for the 1376 model, so I should expect it to be able more for the 1374 model rated at 3A. 

Initially I thought it was because of the 'C4' capacitor in the app-note being rated at 10V and the output being +-5V that I quickly thought it might be the capacitor limiting the voltage, I installed a 47uF rated at 16V but that didn't make a difference and it would seem that the full voltage range is not imposed on the capacitor anyway. I have used the exact same component values as indicated by the schematic at the bottom of the LT1374 datasheet, though having more and bigger capacitors at the output.

Edit: Changing the input voltage from 12 V upwards, adjusts the magnitude of the negative voltage but doesn't change the positive voltage, if I go to 15V for example I am able to achieve the +-6.3V, it effectively changes this non-linear breakdown point indicated by the black arrow. The fact that I can obtain these voltages tells me that I do not have an issue with any of the voltage ratings on my components. I am still having issues with the negative rail being able to sustain load and I am still not able to get -6.3V with an input voltage of 12V only. Please see the below plot:

Any help or guidance will be appreciated as it is first time designing/working with a system that generates -ve voltages and working with switching power supplies.

Best Regards, Jarez

Attached and embedded the graph properly.
[edited by: Jarez95 at 2:18 PM (GMT -4) on 20 Oct 2020]
  • Hi Jarez,

    LT1374 is a Buck regulator. In this application, it uses a coupled inductor to generate a negative rail. 2 items that can affect the negative output regulation:

    1. Inductor coupling coefficient. It can be measured by shorting windings on either side.
    2. Positive/negative rail load condition. For example if positive rail load is very light and the negative rail is very heavy, the negative rail can’t get enough energy discharged from the coupled inductor.

    Please use LTSpice to simulate this circuit with the same load condition. It will give more understanding of this issue.