SMPS will cause spike current ?

May 24, 2007 2 Replies
B

Hi All,



Our wireless short range communication is in standby mode now, however, the software is working, should be no anything to do. ( I want to indicate the s/w level is still alive, but nothing to do )



I got spike current in the duration of standby mode, I can't make sure that is because NOP command ( or other not really important commands) in the s/w level or h/w components effect it.



Any idea about read / write a EEPROM value will cause spike current about 7mA, is it possible ?



Any idea about change a PIO will cause a spike current about 7mA ?


The module work at 1.8V at core chip, however, it accept 4.2V battery voltage, it should has its own regulator. The PIO output I got is 3V. The normal normal standby current is 1.5mA range.



Please let me know if I have to input more info.



Thanks!



Best regards, Boki.



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B

At least some switched mode power supplies operate in the "intermittent mode" when lightly loaded - they draw a signifcant current for a short time, charge up their output reservoir capacitor to the upper threshold output voltage then wait until it discharges discharges to the lower threshold.

A really cheap crude switching step-down regulator would always operate this way - the 4.2V battery charges up the reservoir capacitor to a certain voltage which is less than the highest output voltage, storing a certain amount of energy in the inductor in the process. Once the reservoir capacitor hits this voltage, the input side of the inductor is switched from the 4.2V down to ground, and the current through the inductor proceeds to decrease to zero, charging up the resevoir capacitor even further in the process.

At this point the system designer could elect to open circuit the input side of the inductor (but this needs a sensor for the current through the inductor, which costs money) and wait until the reservoir capacitor has discharged itself to the lower output voltage threshold before starting to recharge the inductor.

The cheaper solution is to let the current through the inductor go negative and start discharging the output capacitor until it hits a threshold voltage somewhat higher than the lowest acceptable output voltage, and then reconnect the input side of the inductor to the battery. It will take a while for the battery to increase the current through the inductor back to zero, and the voltage across the resevoir capacitor continues to decrease untl you get to this point.

-- Bill Sloman, Nijmegen

-- Bill Sloman, Nijmegen

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W

Any part numbers for us to evaluate, boki?

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