I would like to power two different components with two different voltages in my circuit.

  • Component 1 : STM32F401 microcontroller (3.3V and 160 mA)
  • Component 2: AD8205 current sensing amplifier (5V and 20 mA)

In my circuit I am using a battery pack of 36V nominal and 42V maximum as a source. First I have an LTC3638 buck converter to get 5V and 250 mA as output. From this I can source my current sensing amplifier. I want to power my microcontroller also so for that I am planning to use an ADP151 voltage regulator to get constant 3.3V to power the microcontroller.

ADP151 has very low power consumption, so it is safe to use.

I am planning to use the output of the buck converter (5V) as input of the voltage regulator to get 3.3V so that I can use 5V for my current sensing amplifier and 3.3V for my microcontroller.

When I simulate both the components separately using LT spice it is working perfectly and giving the expected output voltages. But when I connect the output of the buck converter to the input of the regulator, I am getting regulator output perfectly as 3.3V but my buck converter output is showing as some micro volts.

I think I am connecting it wrong. Please help me to connect it correctly.


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Buck converter output voltage: I need 5V and 250 mA but I am getting some microvolts and microamperes.

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Regulator voltage output is 3.3V but the current is reduced to 100 mA. I need at least 160 mA.

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  • 1
    \$\begingroup\$ You spent far more time writing that than looking at simplest LTC 3638 example schematic on first page of the datasheet. Comparing the differences would show what is wrong. \$\endgroup\$
    – Justme
    Apr 13, 2020 at 12:54

1 Answer 1


I think i am connecting it wrong. kindly help me to connect it correctly

Look what you have done: -

enter image description here

The LTC3638 has no feedback signal from its output hence, it will produce an unregulated output of virtually unknown or unpredictable values AND you have used that FB node to power your 3.3 volt regulator. Now that just isn't right.

Try connecting the OUT node to the IN node and using the correct ratio of feedback resistors from this newly combined node (if needed for 5 volt operation).


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