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LTC3617 Run Problem


I am using two LTC3614 and one LTC3617 in my application. You can see the block diagram of the circuit in the attachment as setup1 file. I am using a 5VDC external power supply for my circuit. In this setup when I switch on the 5VDC supply, the output voltage waveform of 3617 is shown in Figure1 in the attachment. 

When I try to solve this problem, I tried several things and I noticed a situation; the setup for this circuit is shown in setup2 file. In this setup when I switch on the 3 power supplies simultaneously I got an output waveform same as the setup1. But, when i follow the following steps:

-Switch on 5VDC power supply,

-Switch on 1V5DC power supply,

-RUN is pulled down to GND

The output voltage of the 3617 is 0.7VDC.

-Switch on 3V3 RUN power supply

The output voltage of the 3617 is 0.75VDC.

Is there a voltage opening sequence for 3617? Why, the output voltage of the 3617 reaches to 1V in the setup1?

Is there a minimum load requirement for 3617? Why, although the run pin tied the ground the output voltage of the 3617 is 0.7VDC in setup2?

Could you help me with the root cause of the problems I mentioned above?

Thank you and best regards,

  • Hi Onder,

    LTC3617 has internal SS and the VTTR ramps up at a rate of 850mV/ms. A typical startup is shown in datasheet page 5. Depending on your real circuit setup and the load profile, the startup waveform might be a bit different. 

    "Pulling the RUN pin high enables the regulator which allows an internal soft-start to slowly ramp the VTTR pin voltage at a rate of approximately 850mV/ms. During this startup time, the regulator will operate in discontinuous mode until the VTTR pin voltage exceeds approximately 0.45V." 

    Two suggestions.

    1) If you suspect the issue is with the LTC3617, better to narrow down the circuit to it, and see in exactly what condition this happens. 

    2) A detailed schematic will be more helpful. The waveform of the input voltage, switching node voltage, VTTR, output voltage in a single frame will be helpful.  Please feel free to contact our local FAE for assistance. 



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