Hi,
I know THVD1420 has built in failsafe function.
Is it okay if the customer connects redundant external fail-safe resistors?
The following resistors are connected to the bus terminals of the existing circuit.
A: Pull-up 4.7k
B: Pull-down 4.7k
I don't see a problem with redundant failsafe resistors, but if it's better to remove them, please let me know why.
Best regards,
Hiroshi
Hi Hiroshi-san,
Clemens is correct - adding additional external failsafe resistors improve the noise margin at the cost of additional power consumption.
It is generally advised to just add one pair of external fail-safe resistors - i.e. one pull down at B and one pull up on A as these will help bias the entire bus.
The only thing that should be taken into consideration when sizing the resistors is the common mode loading - i.e if you short every voltage source except the driver and remove the termination resistor - the common mode loading is the impedance between A or B and GND. This value should be no less than 375 Ohms. The THVD1420 will also load the bus - since this is a 1/8th unit load device we can approximate its input impedance as 96K Ohms
This ends up just being the formula :
Where R_TRX = 96k
R_FS = 4.7k
N = the number of THVD1420 IC's on the bus
M = the total number of fail-safe resistors / 2 --> so the suggested value of M = 1 for external fail-safes as that is 1 resistor on A and 1 on B - if you increase that to 2 on each line than M will increase to 2.
This equation assumes all fail-safe resistors will be the same size - which is recommended.
If RCM >= 375 then there is no concern.
Please let me know if you have any other questions!
Best,
Parker Dodson
