LC60: LVD Status at POR and 1.8V Discrepancy

[BasePointer]

Hi,

LVD is activated automatically at any reset. And configured for "Force an MCU reset when LVDF = 1."

Low-voltage detection threshold — low range VLVDL: (LC60 datasheet, page 326)

                min    typ   max

(VDD falling)   1.76   1.8   1.88 V
(VDD rising)    1.93   1.98  2.04 V

If so, how can we expect MCU works at 1.8V properly?

With these values, minimum MCU working voltage should be 2.04V?

Regards,

BP.

[bigmac]

Hello BP,

Yes, your interpretation of the data is basically correct.  Actually, for a worst case situation, the reset would occur when Vdd reached 1.88 volts, but would then not come out of reset until Vdd reached 2.04 volts.

As you have observed, the 1.8 volt nominal threshold is subject to a tolerance.  What battery technology are you using?

Regards,

Mac

[BasePointer]

Hi Mac,

We are using Coil Typed Lithium Battery.

http://www.renata.com/pdf/3vlithium/CR2450N_v06.pdf

It directly supplies VDD of the MCU over a so low leakage diode.

We lived some unexpected POR reset problem at mass production.

So while I research on a solution, I saw this discrepancy.

I still couldn't find the problem. VDD of the MCU never drops below 2.3V.

Regards,

BP.

Message Edited by BasePointer on 2008-04-18 10:58 AM

[bigmac]

Hello BP,

Do you have any short term heavy loads on the battery, perhaps only for milliseconds?  Being quite a small cell, its internal resistance may be significant, especially as the battery approaches its end of life.  If your battery load is "peaky" you might need to consider a largish electrolytic capacitor (or larger than you currently have) at the supply connections to the MCU.

The series diode could also be problematic.  This would need to be a Schottky type for lower voltage drop.  What is the purpose of the diode - to provide reverse polarity protection, or to allow for connection of an external supply voltage?  For a Schottky diode, you would need to allow a voltage drop in the region of 0.4 volts, and this may severely limit the battery life you will achieve.

In your case, another issue may be if the equipment is subject to very low temperatures - the battery voltage will further reduce.

Regards,

Mac

[BasePointer]

Hi Mac,

There are two batteries and an external supplier in the system. We measured (and logged for a long time) Vdd of products that occur POR at all condition, -40C, +85C, at full load. It never droped below 2.3V. We use BAT54ALT1 Schottky Barrier Diodes that are also low leakage to separate supply rails.

We doubt cold joint or flux residues between MCU pins.

Regards,

BP.

[bigmac]

Hello BP,

I take it the two batteries are connected in parallel to increase the current capacity.  What is the worst case peak current draw by all components in the system?

The BAT54A is a small Schottky diode with quite high internal resistance.  According to its datasheet, the forward voltage drop will be about 400mV for a 10mA load, but this will increase to 1 volt for a 100mA load.  I assume you have parallelled both diode sections to halve the effective resistance.

It is also possible that your long term logging of Vdd may miss short term voltage dips of a few milliseconds, but these might be deep enough to cause a reset.

Regards,

Mac

Message Edited by bigmac on 2008-04-22 03:26 AM