LTC2956 OPERATION OVERVIEW tOFF (typically 1s). Once the tOFF timer completes, the The LTC2956 is a micropower, highly configurable wake-up LTC2956 transitions to the Off state, the EN pin is pulled timer with pushbutton control. It controls power to a con- low and all timers are disabled, and the quiescent current nected system to allow that system to perform a periodic drops to about 0.3μA. task. After completing the task, the LTC2956 turns the When the LTC2956 returns to the RUN mode from the system off to conserve power. Figure 1 shows a simpli- SHUTDOWN mode, it first enters the From Off state to fied state diagram of the LTC2956. The LTC2956 has two allow the connected system to run initialization tasks. This modes of operation: RUN mode and SHUTDOWN mode. is indicated by the ONALERT pin pulling low. The EN pin In RUN mode, the LTC2956 cycles between the Awake is pulled high to turn the connected system on. state and the Sleep state with a period of tPERIOD. During The LTC2956 turns the connected system on and off the Awake state the EN pin is pulled high to turn the sys- via the EN pin. The LTC2956-1's active high EN pin can tem on, and the tONMAX timer runs. The LTC2956 exits the be connected to the system supply RUN pin while the Awake state when a falling edge is detected on the SLEEP LTC2956-2's active low EN pin can directly control the pin or if the tONMAX timer expires. The LTC2956 ignores system supply through a P-channel MOSFET. any SLEEP pin transitions for tAWAKE_MIN (typically 8ms) The wake-up timer period can be adjusted from 250ms to after entering the Awake state to ensure that the minimum 39 days via resistors at the PERIOD and RANGE pins. The on time is met. timing of the long pushbutton press and SLEEP pulse can In the Sleep state, the EN output stays low, and the con- be adjusted via resistors connected to the LONG pin. To nected system turns off. The LTC2956 returns to the Awake minimize the quiescent current, the RANGE, LONG, and state if the tPERIOD time is reached, if the pushbutton PERIOD pins are biased only when the LTC2956 performs (connected to the PB pin) is pressed, or if a logic high a configuration cycle. is detected on the SLEEP pin. The minimum duration for Timer configuration is performed once every 66 seconds the Sleep state is tSLEEP_MIN (typically 128ms), and any while the part is operating in RUN mode, and it is also input or tPERIOD transitions during this time are ignored in performed following power-up and whenever the LTC2956 order to allow the system supply to sufficiently discharge. exits the SHUTDOWN mode. During configuration, the SHUTDOWN mode is a very low power mode with the timer LTC2956 biases and reads the voltages at the RANGE disabled, entered through a long press of the pushbutton and LONG pins that are defined by external configuration or a long high pulse at the SLEEP input. This is useful for resistors. See Configuration section for more details. products that are shipped with their batteries installed. A short press on the pushbutton brings the LTC2956 back Operations with SLEEP Tied Low (Passive System) into RUN mode. The LTC2956 can be configured to start The LTC2956 is designed to control the power of both in either RUN or SHUTDOWN mode at power-up via the active and passive systems. With a passive system where LONG pin (see the Applications Information section). the SLEEP pin is not managed by a microcontroller, the SHUTDOWN mode consists of the Shutting Off state and SLEEP pin can be tied low and a capacitor connected to the Off state. The Shutting Off state occurs first during the ONMAX pin determines the awake time. The minimum the transition from RUN mode to SHUTDOWN mode, and tONMAX must be longer than the maximum expected time allows time for the system to perform any shutdown tasks the system takes to complete its routine tasks. before the EN pin goes low in the Off state. This state is indicated by the OFFALERT pin pulling low and lasts for 2956fa 10 For more information www.linear.com/LTC2956
