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LM32 Datasheet(PDF) 7 Page - National Semiconductor (TI) |
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LM32 Datasheet(HTML) 7 Page - National Semiconductor (TI) |
7 / 14 page Pin Function Description (Continued) for instance cannot be held for more than 4 seconds These levels are based on 40 C maximum initial chip tem- perature When driving inductive loads always use a clamp diode to protect the transistor from inductive kick-back A boost pin is provided on the LM122 to increase the speed of the internal comparator The comparator is normally op- erated at low current levels for lowest possible input current For timing periods less than 1 ms where low input current is not needed comparator operating current can be increased several orders of magnitude Shorting the boost terminal to V a increases the emitter current of the vertical PNP drivers in the differential stage from 25 nA to 5 mA This pin is not available on the LM3905 With the timer in the unboosted state timing periods are accurate down to about 1 ms In the boosted mode loss of accuracy due to comparator speed is only about 800 ns so timing periods of several microseconds can be used The 800 ns error is relatively insensitive to temperature so tem- perature coefficient of pulse width is still good The Logic pin is used to reverse the signal appearing at the output transistor An open or ‘‘high’’ condition on the logic pin programs the output transistor to be ‘‘off’’ during the timing period and ‘‘on’’ all other times Grounding the logic pin reverses the sequence to make the transistor ‘‘on’’ dur- ing the timing period Threshold for the logic pin is typically 100 mV with 150 mA flowing out of the terminal If an active drive to the logic pin is desired a saturated transistor drive is recommended either with a discrete transistor or the open collector output of integrated logic A maximum VSAT of 25 mV at 200 mA is required Minimum and maximum voltages that may appear on the logic pin are 0 and a50 respectively Typical Applications Basic Timers Figure 1 is a basic timer using the collector output Rt and Ct set the time interval with RL as the load During the timing interval the output may be either high or low depending on the connection of the logic pin Timing waveforms are shown in the sketch along side Figure 1 Note that the trig- ger pulse may be either shorter or longer than the output pulse width Figure 2 is again a basic timer but with the output taken from the emitter of the output transistor As with the collec- tor output either a high or low condition may be obtained during the timing period Simulating a Thermal Delay Relay Figure 3 is an application where the LM122 is used to simu- late a thermal delay relay which prevents power from being applied to other circuitry until the supply has been on for some time The relay remains de-energized for Rt Ct sec- onds after VCC is applied then closes and stays energized until VCC is turned off Figure 4 is a similar circuit except that the relay is energized as soon as VCC is applied Rt Ct sec- onds later the relay is de-energized and stays off until the VCC supply is recycled TLH7768 – 11 FIGURE 1 Basic Timer-Collector Output and Timing Chart TLH7768 – 12 FIGURE 2 Basic Timer-Emitter Output and Timing Chart TLH7768 – 13 FIGURE 3 Time Out on Power Up (Relay Energized Rt Ct Seconds after VCC is Applied) a 5V Supply Driving 28V Relay Figure 5 shows the timer interfacing 5V logic to a high volt- age relay Although the Va terminal could be tied to the a 28V supply this may be an unnecessary waste of power in the IC or require extra wiring if the LM122 is on a logic card In either case the threshold for the trigger is 16V 7 |
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