CD74HC4538-Q1: Rapid-Power-Down Protection Circuit

Basil

Part Number: CD74HC4538-Q1
Other Parts Discussed in Thread: TINA-TI

Dear Team,

could you please support with following question:

As per DS, Fig.6, adding a current limiting resistor in series to Cx can be used to protect the device against quick capacitor discharge into the device.

What is the formula to calculate the impact of this resistor on the output pulse timing?

Please let me know if you need more details.

Thanks and regards,

over 6 years ago

0 Emrys Maier over 6 years ago

TI__Guru* 96591 points

The resistor will have multiple impacts to the circuit (cap discharge rate, charge rate, and offset due to current through the reistor), but if your timing resistor value is at least 2 orders of magnitude larger than the 51 Ω current limiting resistor (ie 5.1 kΩ or larger), then the impact of the current limiting resistor will be minimal.

I don't have an equation to easily calculate the impact of this resistor - the best method for determining its effect is to build a test board with your expected values.