[SOLVED] HCPL7840 opamp problem

[omid2006]

Hi guys. I faced a problem while I was trying to read current by a shunt resistor and HCPL7840 opamp. the voltage at each of the hcpl7840 input pins is 36V(my supply voltage is 36V). but when I turn my supply on, HCPL punched and burnt terribly. I've tried this with 2 other hcpls but the result was same. I faced a paradox in HCPL datasheet maximum voltage of Vin+,Vin- is VDD+0.5. but It's recommended to use it for motor current sensing application. the motor phase voltage is high not 5V!. so what's wrong. please help me if someone have similar experience.

 

[KlausST]

Hi,

you need to understand that your device is defective. Thus we need to see your schematic and your PCB layout.
Showing a circuit from the datasheet tells us nothing about the problem.

Better post a link to the datasheet (directly at manufacturer internet site)

Klaus

 

[omid2006]

Hi KS yes I've attached but it didn't upload correctly. the circuit is as follows. Sorry

https://docs.broadcom.com/doc/AV02-1289EN

 

[dick_freebird]

You say the "inputs are both 36V" and you show that the front
end supply is 5V (but say "the" supply is 36). That's not gonna
work out well, if that's (5V) the case.

 

[D.A.(Tony)Stewart]

It appears you have 3 grounds which mean you don't control the inputs to stay within 0 to 5V with high voltage outside this range and the Chip will go into SCR latchup mode and explode.

Why did you not follow the datasheet schematic and layout?

problems

You see below C3 is across IC inputs after series R5 surrounded by a primary ground plane.
You put it across Rsense and motor V- is not connected to pin4 so that is a problem.

I feel Broadcom did a poor job in explaining the importance of the lower FET conduction to GND1. (which is missing in your design)
I recommend these changes.

 

[danadakk]

You violating the input CM range ?

Regards, Dana.

 

[FvM]

HCPL7840 is no OPAMP, it's an isolation amplifier. Although input is differential with a certain common mode range, datasheet application circuits are assuming VIN- tied to GND1. Datasheet doesn't even tell operational common mode range, I assume it's similar to drop-in compatible products of other vendors, e.g. TI AMC1300. Absolute maximum ratings have to be kept of course, but don't guarantee amplifier operation. Referring strictly to the datasheet, operation range is VIN- = 0, VIN+ = -0.2..+0.2V.

Tying VIN- to GND1 and limiting VIN+ appropriately also guarantees that absolute maximum ratings can't be exceeded.

 

[D.A.(Tony)Stewart]

Broadcom HCPL-7840 Data Sheet Isolation Amplifier
NOTE: A 0.1-μF bypass capacitor must be connected between pins 1 and 4 and between pins 5 and 8.

In case I wasn't clear before;

1. Move C78 to UC7 p2-3
2. Add 0.1uF to U7 p1-4
3. Short FC -ve to U7-4 GND1 (fan goes between FC+ve and R112)

 

[omid2006]

It appears you have 3 grounds which mean you don't control the inputs to stay within 0 to 5V with high voltage outside this range and the Chip will go into SCR latchup mode and explode.

Why did you not follow the datasheet schematic and layout?

problems

You see below C3 is across IC inputs after series R5 surrounded by a primary ground plane.
You put it across Rsense and motor V- is not connected to pin4 so that is a problem.

View attachment 188693

I feel Broadcom did a poor job in explaining the importance of the lower FET conduction to GND1. (which is missing in your design)
I recommend these changes.
View attachment 188695

thanks tony. yes I neglected that this IC is not a High side current sense amplifier. I've corrected the issues and it's working properly now. thanks for your help