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LTC4411 Datasheet(PDF) 11 Page - Linear Technology |
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LTC4411 Datasheet(HTML) 11 Page - Linear Technology |
11 / 16 page LTC2951 11 2951f µP. This immediately pulls EN low, thus turning off the DC/DC converter. DC/DC Turn Off Blanking When the DC/DC converter is turned off, it can take a sig- nificant amount of time for its output to decay to ground. It is desirable to wait until the output of the DC/DC converter is near ground before allowing the user (via ⎯P⎯B) to restart the converter. This condition guarantees that the µP has always powered down completely before it is restarted. Figure 4 shows the µP turning power off. After a low on ⎯K⎯I⎯L⎯L releases enable, ⎯P⎯B is ignored during the internal 256ms timer period. This is shown as tEN/⎯E⎯N, LOCKOUT in Figure 4. LTC2951-1, LTC2951-2 Versions The LTC2951-1 (high true EN) and LTC2951-2 (low true ⎯E⎯N) differ only by the polarity of the EN/⎯E⎯N pin. Both ver- sions allow the user to extend the amount of time that the ⎯P⎯B must be held low in order to begin a valid power off sequence. An external capacitor placed on the OFFT pin adds additional time to the turn off debounce time. If no capacitor is placed on the OFFT pin, then the turn off de- bounce time is given by an internally fixed 32ms timer. Both versions of the LTC2951 provide extendable ⎯K⎯I⎯L⎯L turn off timer, t ⎯K⎯I⎯L⎯L, OFF DELAY, ADDITIONAL, by connecting an optional external capacitor on the KILLT pin. The default ⎯K⎯I⎯L⎯L turn off delay time is 128ms, t⎯K⎯I⎯L⎯L, OFF DELAY. Figure 3. µP Turns Off Power (LTC2951-1) PB & KILL IGNORED tKILL PB EN DC/DC TURNS OFF 2951 F03 KILL XXX DON’T CARE µP SETS KILL LOW PB & KILL IGNORED PB PB IGNORED EN KILL PB BLANKING XXX DON’T CARE 256ms POWER ON 2951 F04 DC/DC TURNS OFF µP SETS KILL LOW tEN/EN, LOCKOUT Figure 4. DC/DC Turn Off Blanking (LTC2951-1) APPLICATIO S I FOR ATIO and ⎯K⎯I⎯L⎯L (for a turnoff command) while consuming only 6µA of supply current. A high to low transition on ⎯P⎯B (t5) initiates the power off sequence. ⎯P⎯B must stay low continuously (⎯P⎯B high resets debounce timer) for a period controlled by the default 32ms and the external OFFT capacitor (t6–t5). At the completion of the OFFT timing (t6), an interrupt (⎯I⎯N⎯T) is set, signifying that EN will be switched low after the ⎯K⎯I⎯L⎯L turn off (t⎯K⎯I⎯L⎯L,OFFDELAY + t⎯K⎯I⎯L⎯L,OFFDELAY,ADDITIONAL) timeout period. Once a system has finished performing its power down operations, it can set ⎯K⎯I⎯L⎯L low (t7) and thus immediately set EN low, terminating the ⎯K⎯I⎯L⎯L turn off timer. The release of the ⎯P⎯B pin is then debounced with an internal 32ms timer. The system is now in its reset state: where the LTC2951 is in low power mode (6µA). ⎯P⎯B is monitored for a high to low transition. Aborted Power On Sequence The power on sequence is aborted when the ⎯K⎯I⎯L⎯L remains low after the end of the 512ms blanking time. Figure 2 is a simplified version of an aborted power on sequence. At time tABORT, since ⎯K⎯I⎯L⎯L is still low, EN pulls low (thus turning off the DC/DC converter). µP Turns Off Power During Normal Operation Once the system has powered on and is operating nor- mally, the µP can turn off power by setting ⎯K⎯I⎯L⎯L low, as shown in Figure 3. At time t ⎯K⎯I⎯L⎯L, ⎯K⎯I⎯L⎯L is set low by the |
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