Hi,

I am using a 9V to 5V switching regulator, and a 5V to 3V3 LDO in my circuit. The NXP LPC series microcontroller on board is powered by this 3V3 supply. Sometimes, when I switch on the system, the microcontroller doesn't initialize, and the external 12MHz crystal for oscillator doesn't give any pulse at that time. If I switch off and switch on immediately, it works fine. In all cases, 3V3 and 5V appears properly. The external power on reset pules is also same in both cases. Please help me to find out why this happens

 

What kind of 3V3 LDO?
Using O'scope to measure the 3.3V power up rising time and the reset time of the uC.

 

Its a fixed 3V3 output LDO. 800mA rated current.
TLV1777

I've seen 3V3 rise time and reset with oscilloscope, it's fine. I have a few boards which do not have this issue. Compared the rise time and reset with them, i couldnt make out any difference

 

Sorry, TLV1117

 

You have three different parts, the first one is 9V to 5V and the second one is 5V to 3.3V, the last one is the board of uC, so your problem is on the board of uC or on the power side?

 

This is where I am confused. All three sections are on a single PCB. The PCB is powered from an adaptor (9V, 2.5A rated). Adaptor is fine, this has been verified with other systems. What happens is, the microcontroller doesn't initialize, as I already told. And I directly connected 5V supply from another adaptor, bypassing the 9V to 5V buck regulator circuit, and in this case evrything is fine. So I doubt something is wrong with 9V to 5V concverter section. But the interesting thing is that the regulator always generates 5V. I don't know how to blame it then.

 

Modern adaptor made by switching mode, it will generating the noise, so it may happens something we don't know.

If you have the coil with core about 22~47uH, then you can in series it between adaptor 5V and the input of TLV1117.

Sometimes the GND also need to in series with the coil with core, it's the same, the GND is between adaptor 5V and the input of TLV1117.

If your Adaptor is not a switching mode then you could in parallel one 47uf and 0.1uf on the output of 5V adaptor.

 

Seems the issue only is apparent when you use this "9V to 5V buck regulator circuit." Have you looked at the turn on closely, with an oscilloscope (digital or storage) to capture the waveform when a bad start condition occurs? *Something* is causing the problem; sounds like some sort of power sequencing or race condition. One shotgun approach is to add a power on clear monitor chip: a 3 pin device that sits between the power rails and on power up generates a per-determined reset pulse for the micro.