SN74LVC1G123: leakage currents of Cext

Hans

Part Number: SN74LVC1G123
Other Parts Discussed in Thread: SN74LV123A

Hello Team,

according data sheet the current into Rext/Cext pin is specified with max +/-0.25µA.

Theoretically a leakage current of Cext would slow down the charging of Cext and therefore I would expect a longer pulse duration, correct? But the pSPICE simulation shows a shorter pulse with higher leakage current.

Do we have to add or subtract the 0.25µA to the charging current of Cext?

Thanks and Best Regards, Hans

over 3 years ago

0 Sebastian Muriel over 3 years ago

TI__Mastermind 20060 points

Hi Hans,

What PSpice model are you using? We do not have any updated models for monostable multivibrators and many of the older models only work for a particular use case. If you are trying to size the external resistor, the leakage current will limit the resistor size. The resistor should be between 10KΩ and 1MΩ to keep the voltage drop at the output minimal.

Regards,

Sebastian

0 Hans over 3 years ago in reply to Sebastian Muriel

TI__Genius 12865 points

Hi Sebastian, which resistor are you taking about? I don't understand how Rext can impact any voltage drop at the output signal.

In this post Maximum REXT and CEXT values that can be used with 74LVC1G123 a TI person stated "There is no upper limit on REXT or CEXT".

For Vcc=5V and Rext=20M the specified input current at Rext/Cext pin is 0.25µA. Does that mean Cext cannot be charged as shown below?

I used the SN74LV123A pSPICE model from ti.com.

Thanks and Best Regards, Hans

0 Sebastian Muriel over 3 years ago in reply to Hans

TI__Mastermind 20060 points

Hi Hans,

I am referring to R_ext.

That large of a resistor value can cause a voltage drop from V_cc that is too large for the output switching to occur. C_ext won’t get to charge up to a high enough voltage fast enough considering the large resistor drop.

Regards,

Sebastian

0 Hans over 3 years ago in reply to Sebastian Muriel

TI__Genius 12865 points

Hi Sebastian,

thank you. So the recommendation is Rext,max =1MOhm which makes sense :-)

Best Regards, Hans

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SN74LVC1G123: leakage currents of Cext