LPC5536 undervoltage VDD_Core

[Max_overvoorde]

Hi,

I've been working on a project working with the LPC5536. I've made a board before where the microcontroller almost worked perfectly out of the box after assembly. No real time was required to debug or fix things.

Now i've been working on a similar board with the same chips. With this, i've replicated the power circuit of the microcontroller as it was before. Unfortunately i haven't been able to communicate with the microcontroller yet. After some digging i found that VDD_Core gets an undervoltage when trying to access the device. As i've used the exact same components and same circuit, the only real difference in my understanding might be the PCB design which can cause an undervoltage.

When adding a 22uF capacitor on the bank (so 44uF in total using the internal DC DC) the undervoltage only drops to 0.9V as it was 0.8V previous. I assume when using multiple of these bulk caps the problem could be solved, but i feel like this isn't fixing the root cause.

My question was what would be causing this undervoltage? And what happens when a connection is being made with the microcontroller? And are there any suggestions to mitigate this VDD_core undervoltage?

[xiangjun_rong]

Hi,

Even if you use internal DC/DC converter with LX connected to an inductor and a large cap, I suppose that the LPC5536 is in LDO mode, in other words, the internal DC/DC converter is NOT used.

You can use the line to switch from LDO mode to internal DC/DC converter mode, assume that you connect the LX pin to inductor and cap.

power_core_pwr_source_t pwrState;

pwrState=POWER_GetCorePowerSource();

__asm("nop");

POWER_CorePowerSourceControl(kPOWER_CoreSrcDCDC,kPOWER_CorePwrEnable);

POWER_SetVoltageForFreq(150000000);

Hope it can help you

BR

XiangJun Rong