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LTC1474CMS8-3.3 Datasheet(PDF) 11 Page - Linear Technology |
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LTC1474CMS8-3.3 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  11 LTC1474/LTC1475 APPLICATIONS INFORMATION LTC1475 Pushbutton On/Off and Microprocessor Interface The LTC1475 provides pushbutton control of power on/off for use with handheld products. A momentary ground on the ON pin sets an internal S/R latch to run mode while a momentary ground on the LBI/OFF pin resets the latch to shutdown mode. See Figure 6 for a comparsion of on/off operation of the LTC1474 and LTC1475 and Figure 7 for a comparison of the circuit implementations. In the LTC1475, the LBI/OFF pin has a dual function as both the shutdown control pin and the low battery com- parator input. Since the “OFF” pushbutton is normally open, it does not affect the normal operation of the low battery comparator. In the unpressed state, the LBI/OFF input is the divided down input voltage from the resistive divider to the internal low battery comparator and will normally be above or just below the trip threshold of 1.23V. When shutdown mode is desired, the LBI/OFF pin is pulled below the 0.7V threshold to invoke shutdown. the depressed switch state is detected by the microcon- troller through its input. The microcontroller then pulls the LBI/OFF pin low with the connection to one of its ouputs. With the LBI/OFF pin low, the LTC1475 powers down turning the microcontroller off. Note that since the I/O pins of most microcontrollers have a reversed bias diode between input and supply, a blocking diode with less than 1 µA leakage is necessary to prevent the powered down microcontroller from pulling down on the ON pin. Figure 19 in the Typical Applications section shows how to use the low battery comparator to provide a low battery lockout on the “ON” switch. The LBO output disconnects the pushbutton from the ON pin when the comparator has tripped, preventing the LTC1475 from attempting to start up again until VIN is increased. RUN RUN SHUTDOWN RUN RUN 1474/75 F06 SHUTDOWN RUN MODE ON OVERRIDES LBI/OFF WHILE ON IS LOW LTC1474 LTC1475 ON LBI/OFF MODE Figure 6. Comparison of LTC1474 and LTC1475 Run/Shutdown Operation The ON pin has precedence over the LBI/OFF pin. As seen in Figure 6, if both pins are grounded simultaneously, run mode wins. Figure 18 in the Typical Applications section shows an example for the use of the LTC1475 to control on/off of a microcontroller with a single pushbutton. With both the microcontroller and LTC1475 off, depressing the pushbutton grounds the LTC1475 ON pin and starts up the LTC1475 regulator which then powers up the microcon- troller. When the pushbutton is depressed a second time, 1474/75 F07 RUN ON OFF RUN ON LBI/OFF 100k LTC1474 LTC1475 100k VIN Figure 7. Simplified Implementation of LTC1474 and LTC1475 On/Off Absolute Maximum Ratings and Latchup Prevention The absolute maximum ratings specify that SW (Pin 5) can never exceed VIN (Pin 7) by more than 0.3V. Normally this situation should never occur. It could, however, if the output is held up while the supply is pulled down. A condition where this could potentially occur is when a battery is supplying power to an LTC1474 or LTC1475 regulator and also to one or more loads in parallel with the the regulator’s VIN. If the battery is disconnected while the LTC1474 or LTC1475 regulator is supplying a light load and one of the parallel circuits is a heavy load, the input capacitor of the LTC1474 or LTC1475 regulator could be pulled down faster than the output capacitor, causing the absolute maximum ratings to be exceeded. The result is often a latchup which can be destructive if VIN is reapplied. Battery disconnect is possible as a result of mechanical stress, bad battery contacts or use of a lithium-ion battery |
Similar Part No. - LTC1474CMS8-3.3 |
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Similar Description - LTC1474CMS8-3.3 |
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