how to make the timer of nrf52840 low power?

hello everyone, I need a low power product, so I am using nrf52840. There is a problem now, When I add "app_timer_init()"(I never change the code in this method) in my code, the current is about 450ua,and when I deleted this line,the current is about 7ua. Oh , I use the example "ble_app_pwr_profiling". why I cannot lower the power about the timer?  Thanks.