Datasheet LTC2950-1, LTC2950-2 (Linear Technology) - 9

LTC2950-1/LTC2950-2
applicaTions inForMaTion Description
Note that the KILL input can be pulled low (thereby releasing The LTC2950 is a low power (6µA), wide input voltage the enable output) at any time after tKILL, ON BLANK period. range (2.7V to 26.4V), pushbutton on/off controller that can interface to a µP and a power supply. The turn-on and
Simplified Power On/Off Sequence
turn-off debounce times are extendable using optional Figure 1 shows a simplified LTC2950-1 power on and power external capacitors. A simple interface (INT output, KILL off sequence. A high to low transition on PB (t1) initiates input) allows a system to power on and power off in a the power on sequence. This diagram does not show any controlled manner. bounce on PB. In order to assert the enable output, the PB pin must stay low continuously (PB high resets timers)
Turn On
for a time controlled by the default 32ms and the external When power is first applied to the LTC2950, the part ini- ONT capacitor (t2–t1). Once EN goes high (t2), an internal tializes the output pins. Any DC/DC converters connected 512ms blanking timer is started. This blanking timer is to the EN/EN pin will therefore be held off. To assert the designed to give sufficient time for the DC/DC converter enable output, PB must be held low for a minimum of to reach its final voltage, and to allow the µP enough time 32ms (t to perform power on tasks. DB, ON). The LTC2950 provides additional turn on debounce time via an optional capacitor connected to the The KILL pin must be pulled high within 512ms of the EN ONT pin (tONT). The following equation describes the ad- pin going high. Failure to do so results in the EN pin going ditional time that PB must be held low before asserting low 512ms after it went high. (EN = low, see Figure 2). the enable output. CONT is the ONT external capacitor: Note that the LTC2950 does not sample KILL and PB until C after the 512ms internal timer has expired. The reason PB ONT = 1.56E-4 [µF/ms] • (tONT – 1ms) is ignored is to ensure that the system is not forced off Once the enable output is asserted, any DC/DC converters while powering on. Once the 512ms timer expires (t4), the connected to this pin are turned on. The KILL input from release of the PB pin is then debounced with an internal the µP is ignored during a succeeding 512ms blanking 32ms timer. The system has now properly powered on time (tKILL, ON BLANK). This blanking time represents the and the LTC2950 monitors PB and KILL (for a turnoff maximum time required to power up the DC/DC converter command) while consuming only 6µA of supply current. and the µP. If KILL is not brought high during this 512ms time window, the enable output is released. The assumption A high to low transition on PB (t5) initiates the power off is that 512ms is sufficient time for the system to power up. sequence. PB must stay low continuously (PB high resets debounce timer) for a period controlled by the default 32ms
Turn Off
and the external OFFT capacitor (t6–t5). At the completion of the OFFT timing (t To initiate a power off sequence, PB must be held low for a 6), an interrupt (INT) is set, signifying that EN will be switched low in 1024ms. Once a system minimum of 32ms (tDB, OFF). Additional turn off debounce has finished performing its power down operations, it time may be added via an optional capacitor connected can set KILL low (t to the OFFT pin (t 7) and thus immediately set EN low), OFFT). The following equation describes terminating the internal 1024ms timer. The release of the the additional time that PB must be held low to initiate a PB pin is then debounced with an internal 32ms timer. power off sequence. COFFT is the OFFT external capacitor: The system is now in its reset state: where the LTC2950 COFFT = 1.56E-4 [µF/ms] • (tOFFT – 1ms) is in low power mode (6µA). PB is monitored for a high Once PB has been validly pressed, INT is switched low. This to low transition. alerts the µP to perform its power down and housekeeping tasks. The power down time given to the µP is 1024ms. 295012fd 9 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Timing Diagrams Applications Information Typical Applications Package Description Revision History Typical Application Related Parts