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Part Number: TLC555-Q1
I am using TLC555-Q1 and my schematic is similar to Missing-Pulse Detector in datasheet 9.2.1.
As far as I know, absolute maximum current of DISCH pin is 150mA which is written in datasheet 7.1.
When discharge switch is on, discharge capacitor is shorted with small on resistance and ESR.
If capacitance is big, high current goes out from DISCH pin.
If the current is larger than 150mA, do I need to add resistance in the loop?
Hello Junwoo, welcome to the e2e forums.
The current peaks can be high however the pin 7 internal transistor as well as the PNP transistor are large structures and can easily absorb the energy of the capacitor without any external resistance added. Energy is ½*C*V2 per discharge and is a small energy value. The timer will rarely do the discharge as the external transistor usually does it. Power is just frequency multiplied by energy. The abs max value is based on a longer duration up to continuous.
In summary resistors are not needed, but you can add them if you prefer. Adding a series resistor, Rx, will limit peak current less than (2 * Vdd) / (3 * Rx)
Regards,Ronald MichallickLinear Applications
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In reply to Ron Michallick:
Thank you for your reply.
I understand it is very short time.
But, as far as I know, absolute maximum rating mean the value which must not reach even for a short time?
In the datasheet, there is only current specification in Absolute maximum rating table. (not energy)
When I simulated my schematic, DISCH pin current is over than 150mA.
If so, do I need to add resistor?
In reply to Junwoo Kim:
The AMR table wants the designer not to exceed the rating. If the device, on it own choice, seems to violate the spec then that is usually acceptable. The data sheet shows the normal example has a capacitor directly on the discharge pin. So the data sheet recommends having this capacitor directly attached and makes no mention about calculating peak current or adding resistance. In this case the AMR limit would not apply to the capacitor.
In summary, you do not need to add a resistor.
Thank you for the reply.
I have one more question.
In the previous reply, the internal transistor is large structures and can easily absorb the energy of the capacitor without any external resistance added.
Do you have any data for energy specification of internal transistor?
I would like to compare 1/2*C*V^2. (In my schematic, C = 1uF, V = 5V*2/3 = 3.33V, E = 5.5 uJ)
I have no data for energy rating for the transistor. This question is becoming more popular, so I expect it will be in the next revision of the data sheet. In the meantime, 5.5uJ is a small number. This could be accepted as obliviously safe. Alternatively a resistor could be added just to be safer.
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