Hi team,
My customer wants to use large Cext (up to 10uF) for SN74LV123A.
So, they want to know the rough boarder for the Cext which the external clamping diode will be nessesary.
How large Cext can they use without the clamping diode?
BTW, I showed the discription below which is in the datasheet.
10.2.1.1 Power-Down Considerations
Large values of Cext can cause problems when powering down the 'LV123A devices because of the amount of
energy stored in the capacitor. When a system containing this device is powered down, the capacitor can
discharge from VCC through the protection diodes at pin 2 or pin 14. Current through the input protection diodes
must be limited to 30 mA; therefore, the turn-off time of the VCC power supply must not be faster than t = VCC ×
Cext/30 mA. For example, if VCC = 5 V and Cext = 15 pF, the VCC supply must turn off no faster than t = (5 V) ×
(15 pF)/30 mA = 2.5 ns. Usually, this is not a problem because power supplies are heavily filtered and cannot
discharge at this rate. When a more rapid decrease of VCC to zero occurs, the 'LV123A devices can sustain
damage. To avoid this possibility, use external clamping diodes.
Regards,
Ryoma
