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LM122 データシート(PDF) 6 Page - National Semiconductor (TI) |
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LM122 データシート(HTML) 6 Page - National Semiconductor (TI) |
6 / 14 page Pin Function Description (Continued) trollers Typical temperature drift of the reference is less than 001% C The trigger terminal is used to start a timing cycle (see functional diagram) Initially Q1 is saturated Ct is dis- charged and the latching buffer output (V1) is latched high A trigger pulse unlatches the buffer V1 goes low and turns Q1 off The timing capacitor Ct connected from RC to GND will begin to charge When the voltage at the RC terminal reaches the 20V threshold of the comparator the compara- tor toggles latching the buffer output (V1) in the high state This turns on Q1 discharges the capacitor Ct and the cycle is ready to begin again If the trigger is held high as the timing period ends the comparator will toggle and V1 will go high exactly as before However V1 will not be latched and the capacitor will not discharge until the trigger again goes low When the trigger goes low V1 remains high but is now latched Trigger threshold is typically 16V at 25 C and has a tem- perature dependence of b50 mV C Current drawn from the trigger source is typically 20 mA at threshold rising to 600 mA at 30V then leveling off due to FET action of the series resistor R5 For negative input trigger voltages the only current drawn is leakage in the nA region The trigger can be driven from supplies as high as g40V even when device supply voltage is only 5V The RC pin is tied to the non-inverting side of the compara- tor and to the collector of Q1 Timing ends when the voltage on this pin reaches 20V (1 RC time constant referenced to the 315V regulator) Q1 turns on only if the trigger voltage has dropped below threshold In comparator or regulator applications of the timer the trigger is held permanently high and the RC pin acts just like the input to an ordinary comparator The maximum voltages which can be applied to this pin are a55V and b07V Current from the RC pin is typically 300 pA when the voltage is negative with respect to the VADJ terminal For higher voltages the current drops to leakage levels In the boosted mode input current is typical- ly 30 nA Gain of the comparator is very high 200000 or more depending on the state of the logic reverse pin and the connection of the output transistor The ground pin of the LM122 need not necessarily be tied to system ground It can be connected to any positive or negative voltage as long as the supply is negative with re- spect to the Va terminal Level shifting may be necessary for the input trigger if the trigger voltage is referred to sys- tem ground This can be done by capacitive coupling or by actual resistive or active level shifting One point must be kept in mind the emitter output must not be held above the ground terminal with a low source impedance This could occur for instance if the emitter were grounded when the ground pin of the LM122 was tied to a negative supply The terminal labled VADJ is tied to one side of the compara- tor and to a voltage divider between VREF and ground The divider voltage is set at 632% of VREF with respect to groundexactly one RC time constant The impedance of the divider is increased to about 30k with a series resistor to present a minimum load on external signals tied to VADJ This resistor is a pinched type with a typical variation in nominal value of b50% a100% and a TC of 07% C For this reason external signals (typically a pot between VREF and ground) connected to VADJ should have a source re- sistance as low as possible For small changes in VADJ up to several kX is all right but for large variations 250X or less should be maintained This can be accomplished with a 1k pot since the maximum impedance from the wiper is 250X If a voltage is forced on VADJ from a hard source voltage should be limited to b05 and a50V or current limited to g10 mA This includes capacitively coupled sig- nals because even small values of capacitors contain enough energy to degrade the input stage if the capacitor is driven with a large fast slewing signal The VADJ pin may be used to abort the timing cycle Grounding this pin during the timing period causes the timer to react just as if the capaci- tor voltage had reached its normal RC trigger point the capacitor discharges and the output charges state An ex- ception to this occurs if the trigger pin is held high when the VADJ pin is grounded In this case the output changes state but the capacitor does not discharge If the trigger drops while VADJ is being held low discharge will occur immediately and the cycle will be over If the trig- ger is still high when VADJ is released the output may or may not change state depending on the voltage across the timing capacitor For voltages below 20V across the timing capacitor the output will change state immediately then once more as the voltage rises past 20V For voltages above 20V no change will occur in the output This pin is not available on the LM2905LM3905 In noisy environments or in comparator-type applications a bypass capacitor on the VADJ terminal may be needed to eliminate spurious outputs because it is high impedance point The size of the cap will depend on the frequency and energy content of the noise A 01 mF will generally suffice for spike suppression but several mF may be used if the timer is subjected to high level 60 Hz EMI The emitter and the collector outputs of the timer can be treated just as if they were an ordinary transistor with 40V minimum collector-emitter breakdown voltage Normally the emitter is tied to the ground pin and the signal is taken from the collector orthe collector is tied to Va and the signal is taken from the emitter Variations on these basic connections are possible The collector can be tied to any positive voltage up to 40V when the signal is taken from the emitter However the emitter will not be pulled higher than the supply voltage on the Va pin Connecting the collector to a voltage less than the Va voltage is allowed The emit- ter should not be connected to a low impedance load other than that to which the ground pin is tied The transistor has built-in current limiting with a typical knee current of 120 mA Temporary short circuits are allowed even with collector- emitter voltages up to 40V The power x time product how- ever must not exceed 15 watt-seconds for power levels above the maximum rating of the package A short to 30V 6 |
同様の部品番号 - LM122 |
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同様の説明 - LM122 |
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