Hi,
thanks for your reply. i am using LM1117-5.0V Regulator. input to LDO is 12V and O/P is 5V. can you explain why if i give 12V input to LDO from externally it is working fine.
I will download the datasheet to have a better idea how to answer you ;-)
It seems that the high frequency ripple of the switching regulator introduces a state of instability in the internal circuitry of the LDO. The 10uF at the input may not be effective at this frequency. Perhaps if increased to 100uF, it will be able to attenuate this ripple to a good extent.
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Edited:
But loading the output of a switching supply by a relatively large capacitor may disturb its function.
And if the ripple frequency is relatively high, it is better to add at the LDO input 100nF and perhaps 10nF to bypass it.
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Of course, even a small resistor between the 12V and the LDO input can better attenuate the ripple.
In any case, since your input is now 12V and your load is 400mA, your LDO has to dissipate 7*0.4 = 2.8 W. This needs a good heatsink. I personally give my LDO a hand to be cooler by inserting at its input a resistor to take some heat from it ;-)
Added:
The length of wires between the 12V and LDO if long enough may cause a state of resonance at the LDO input. Obviously when one has an oscilloscope most problems can be seen and solved more quickly.
Kerim