how to design kw41z ble system with Lithium-ion battery?

[xieguangbao]

hi all!

KW41Z supports operation with battery voltage from 2.1 V to 4.2 V in buck mode, but PSWITCH cannot be shorted to VBAT in Lithium-ion battery configurations,  so DCDC can't  automatic and continuous turn on， why is that? 

how to design it and have any reference design with lithium-ion battery?

thanks.

[gerardo_rodriguez]

Hi xie guangbao,

The reason you can't short PSWITCH to VBAT in Lithium-ion battery configurations is because PSWITCH is a digital pin and the MCU tolerance is 3.6V. Lithium-ion batteries can reach up to 4.2V and this voltage could damage the MCU. You have to make sure that PSWITCH voltage is less than 3.6V. 

UPDATE:

The max voltage of the DCDC IO pins (DCDC_CFG and PSWITCH) is VDCDC_IN (4.25V max in buck mode). From the datasheet:

The reason that PSWITCH is not recommended to be shorted to VDCDC_IN in Li-ion configurations is to avoid over discharge, as this my damage this kind of battery and reduce its life-time. When PSWITCH is tied high, the DCDC will always try to turn on, so when using Li-ion rechargeable battery, we recommend to use PSWITCH to turn on the DCDC. The software then can monitor the VDCDC_IN periodically and shut off the DCDC if the battery voltage becomes to low.

Regards,

Gerardo