How to use coin cell (not rechargeable) for RTC with protection diodes

[nahoko]

Hello,

I would like to populate coin cell (not rechargeable) for Real Time Clock on my board which I am designing with i.MX6Q. First I thought single 3.0V coin cell was enough for i.MX6Q. But I can not solve the problem of recharging protection.

-VDD_SNVS_IN is required from 2.8 - 3.3V (i.MX6Q datasheet)

-Coin cell voltage (e.g. CR2032) is normally 3.0V.

-Coin cell needs recharge protection. In UL standard, a low leakage diode is recommended as the protection device. The following picture is copied from Panasonic website.

When I put protection diode between coin cell and i.MX6Q, the voltage at VDD_SNVS_IN drops approx. 0.3-0.6V because of V(Forward), so the voltage made by single coin cell does not meet 2.8V.( >3.0V-0.3V)

Does i.MX6Q only intend to use with rechargeable coin cell which may not need a protection diode?

I would really appreciate it if you could give me an advice.

Thank you.

[davecbluechip]

Hi nahoko,

We have a similar issue, and believe there may be an error in the datasheet.

Looking at the datasheet for the MMPF0100 it specifies a valid range of 1.8V to 3.3V for the LICELL input (see Table 109 or Figure 26). Since this is designed to power the i.MX6 SNVS supply, we suspect that the 2.8V to 3.3V on the i.MX6 datasheet may be a typo. (Apart from anything else, 2.8V to 3.3V seems like a very narrow range for an RTC).

Anyone from Freescale care to comment?

This post talks around the issue a bit, but doesn't seem to come to any definite conclusions:

Q&A: i.MX6S minimum level of VDDSNVS_IN for keeping RTC running

Dave C

[jamesbone]

Freescale PMIC PMPF0100 VSNVS regulator is limited to 400 μA output current.

Concerning i.MX6:

•When VDD_SNVS_IN = VDDHIGH_IN, SNVS domain current is drawn from both equally.

• When VDDHIGH_IN > VDD_SNVS_IN, VDDHIGH_IN

supplies all SNVS domain current and current flows into VDD_SNVS_IN to charge a coin cell battery.

•When VDD_SNVS_IN > VDDHIGH_IN, VDD_SNVS_IN supplies current to SNVS, and some current flows into VDDHIGH_IN.

Note: VDDHIGH_In must be valid (above the internal detector threshold) for the current flow to occur. Thus, current flow only happens when VDDHIGH is powered to a level below VDDSNVS_IN. If VDDHIGH_IN is off or low, no extra current is drawn

from VDDSNVS_IN. The whole circuit assumes it is charging a coin cell and starts charging when

VDDHIGH_IN is valid. If you are driving VDDSNVS_IN with a non-battery power source, it

must be at the same level as VDDHIGH_IN or current will flow between them.

•When VDD_SNVS_IN is not powered by a battery, it is recommended that VDD_SNVS_IN ≥ VDDHIGH_IN.

If VDD_SNVS_IN is tied to a battery, the battery eventually discharges to a value equal to that of VDDHIGH_IN and never subsequently charges above VDDHIGH_IN.

The battery chemistry may add restrictions to

VDDHIGH_IN’s voltage range. External charging components should be based on the battery

manufacturer's specifications