I am currently planning a battery-powered project, where one of the tasks involves reading Desfire EV1 tags. Since the cheap RC522 based NFC shields are reported not to work with the Desfire EV1 tags, I wanted to make an attempt with a PN532 based shield. Due to offering the fastest shipping option, I ordered an Aptotec PN532 shield, although there are many other, and likely cheaper options out there as well. The Aptotec shield seems identical to the Seeed NFC Shield V2.0.
Communicating with the Desfire EV1 tags works as expected, but I am not understanding the shield's power consumption. If I understand the PN532 datasheet correctly, the chip itself should only draw 45µA when in standby mode, some 30mA when active, but not transmitting and additionally up to 150mA more during actual transmission.
After sending an 'InRelease all targets' command to the PN532, the shield however still draws about 50mA, although the PN532 should be in stand by mode. Using the RFConfiguration command to turn the RF field off completely still does not decrease the shield's power consumption. Activating the RF field e.g. by issuing an InListPassiveTarget command, the current draw increases to about 80mA, which matches with my assumption that the PN532 now draws 30mA more than when idle. There seems however to be something else on the shield causing a 'base line' 50mA current draw, which is not caused by powering the PN532 chip.
Does someone know where this power is lost, if it is possible to software-based turn off whatever is causing the power draw or if other PN532 shields show the same behaviour? Since my project must run on a battery, the 50mA unexplained current draw is not acceptable.