LPC54618 VBAT leakage.

[StephenYeh]

Hi Sir, 

Our design is using LPC54618 with VBAT power by coin battery.

MCU_VDD is power by system power 3.3V. When our system enter power down mode, and the MCU_VDD will turn-off. MCU enter deep power down mode, but there ls the leakage because VBAT and VDD will been different level. Our design reset pin will pull up 100K to MCU_VDD and got leakage issue.

I check the datasheet, if the VBAT over VDD, the external reset pin must be floating to prevent leakage. 

If our design the reset function must be required, how about to modify pull up 100K to higher and prevent leakage more lower?Would you have any recommand value about the pull-up resister?

Or Could you have any solution to prevent leakage issue?

[StephenYeh]

Hi Sir,

Sorry, I summrize our design status.

Our design is to use reset IC for MCU reset funion. But we check that will have leakage issue. from reset pin.

Then, we refer to the EVK design, and also have leakage issue.

If we keep RESET IC design, how to improve the leakage.

[xiangjun_rong]

Hi,

If you want to keep Reset IC and want to remove the leakage current from reset pin, you can use a relay device, use the VDD_MCU as the power supply pin for the relay. When the VDD_MCU exists, the relay works, the reset pin of LPC is connected to RC and Reset IC. When the VDD_MCU does exist, the relay does work, the reset pin of LPC is disconnected to RC and Reset IC, in the case, the reset pin is float. But you have to trade off between the relay power consumption and the leakage current.

Hope it can help you

BR

XiangJun Rong

[xiangjun_rong]

Hi,

In low power mode, I think you only want to connect VBAT pin to a battery so that only RTC works, all the other VDD are not powered, the LPC54618 does not work.

In order to save battery power, the Reset pin has to be float in above case, I think there is not official recommended circuit so that the Reset pin is float. But I think you can use the two schemes to make the Reset pin float.

The scheme1 use a P MOSFET power device, when the MCU_VDD is 3.3V, the P MOSFET turns on, this is a normal RC circuit. When the MCU_VDD is power off, the MCYU_VDD is 0V, P MOSFET is off, the VBAT power supply can not leakage current to GND. so I think the Reset pin is float. But the scheme increases the cost.

The scheme 2 uses just a diode, I think it also works.

Pls have a try.

BR

XiangJun Rong

[StephenYeh]

This is my schematic.
If I want to keep Reset IC design, what 's the resister(R1) that I can choose? (200K or more)

[xiangjun_rong]

Hi, Stephen,

As you know that the leakage current is nA level, which is very small for VBAT pin.  For the diode scheme, the diode itself has leakage current, which is dependent the diode type, temperature and the reverse input voltage. I think the Reset IC also has leakage current spec in power off.

Regarding the RC circuit for Reset, if you increase the resistor R value, the Power On Reset will become long, it may lead to the result that the LPC54618 can not start from power-on. Moreover, because the leakage current is nA level, it appear that it takes less effect to increase the resistor value.

Hope it can help you

BR

XiangJun Rong

[frank_m]

What is your C114 ?

Electrolytic capacitors have relatively low parasitic resistance, i.e. cause leakage.

Are you sure the reset IC does not cause leakage when unpowered ?