[SOLVED] Current Sense with an Analog Isolation Amplifier?

[calenosa]

Hi guys,

In my motor control project I use HCPL-7520, an isolated linear sensing IC from Avago, and a shunt resistor to measure the current of a brushed DC motor (the motor is driven by an integrated H-Bridge MOSFET driver).

In the datasheet of HCPL-7520, there are two pins Vdd1 and Vdd2. My question is can I connect these two pins to the same voltage supply?

thanks in advance!

 

[FvM]

can I connect these two pins to the same voltage supply?

You can't, if you want to keep the isolation. Unfortunately, you need a floating supply for the input side of the isolator.

 

[calenosa]

You can't, if you want to keep the isolation. Unfortunately, you need a floating supply for the input side of the isolator.

Well, let me express my problem more clearly. My motor driver IC is DRV8432 from TI. This IC needs 2 power supplies. So, with the components I am using, I think I need 3 power supplies in total:

Supply A for Microcontroller and Vdd2 of the amplifier.
Supply B for the gate drive circuitry of the motor driver and Vdd1 of the amplifier.
Supply C (high power) for the power stage of the motor driver.

However, from the datasheet of the motor driver, there is no need to isolate the MCU and the motor driver (like what we used to do with traditional mosfets). Therefore I wonder whether we can just 'merge' supply A and supply B into a single voltage supply? In other words, I want to use only 2 power supplies for my system. Is this possible?

 

[FvM]

think I need 3 power supplies in total:

Supply A for Microcontroller and Vdd2 of the amplifier.
Supply B for the gate drive circuitry of the motor driver and Vdd1 of the amplifier.
Supply C (high power) for the power stage of the motor driver.

Now you are mixing two questions:
- How to supply the processor and power electronics?
- How to supply the isolation amplifier

For the first point, you should refer to the DRV8432 datasheet. It's designed for a 12 V supply, an undervoltage lockout doesn't allo to use considerable lower voltage levels. So supply B is mandatory. Because the processor (and HCPL-7520 secondary side) needs 5V, a separate supply A can't be avoided. In most cases, the motor power supply will be separate, too. So far about DRV8432.

The other point is HCPL-7520 primary side supply. I see, that you still didn't get the problem. The primary side supply has to be 5V, but the analog input voltage must be within the valid common mode range. Because it's connected to a shunt, it spans the full range of power supply C. As a consequence, Vdd1 supply must be floating, you'll need power supply "D" isolated from supply A - C. In my opinion, this requirement seriously questions the use of HCPL-7520 for the design.

 

[calenosa]

For the first point, you should refer to the DRV8432 datasheet. It's designed for a 12 V supply, an undervoltage lockout doesn't allo to use considerable lower voltage levels. So supply B is mandatory. Because the processor (and HCPL-7520 secondary side) needs 5V, a separate supply A can't be avoided. In most cases, the motor power supply will be separate, too. So far about DRV8432.

Supply B can provide a 12V voltage for the logic circuit of the driver, and it can also power the MCU (and HCPL-7520 secondary side) with the help of an 7805 voltage regulator: supply B (12V) -> 7805 -> MCU. Is this approach ok?

The other point is HCPL-7520 primary side supply. I see, that you still didn't get the problem. The primary side supply has to be 5V, but the analog input voltage must be within the valid common mode range. Because it's connected to a shunt, it spans the full range of power supply C. As a consequence, Vdd1 supply must be floating, you'll need power supply "D" isolated from supply A - C. In my opinion, this requirement seriously questions the use of HCPL-7520 for the design.

Thanks for this explanation. I'm a little bit clearer. However, as shown by DRV8432's datasheet, the motor driver has a built-in floating supply voltage, which is used to drive the high-side MOSFETs.

As you can see from the above picture, the floating voltage supply is accommodated by built-in bootstrap circuitry (requiring external bootstrap capacitor). So can I just use one of BST_X pins as the floating voltage supply for the primary side of HCPL-7520?

 

[FvM]

So can I just use one of BST_X pins as the floating voltage supply for the primary side of HCPL-7520?

Basically yes. It would need another 5V regulator, as shown in the HCPL-7520 datasheet. An you should consider, that there's an inductor Loc between driver output and sense resistor. The respective bootstrap capacitor may need to be increased.

 

[calenosa]

Basically yes. It would need another 5V regulator, as shown in the HCPL-7520 datasheet. An you should consider, that there's an inductor Loc between driver output and sense resistor. The respective bootstrap capacitor may need to be increased.

Hi FvM, thank you for your helpful answer ^^

As for the other problem, do you think my solution will be ok?

Supply B can provide a 12V voltage for the logic circuit of the driver, and it can also power the MCU (and HCPL-7520 secondary side) with the help of an 7805 voltage regulator: supply B (12V) -> 7805 -> MCU. Is this approach ok?

 

[FvM]

Yes, of course. Saying, you need supply A and B doesn't exclude to derive one from the other, as long as no isolation is required. If a linear regulator is suitable depends on the current consumption and acceptable losses. But in this case, it seems to be the obvious solution.