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# LMP8645: Can the chip limit the current below 7mA

Part Number: LMP8645
Other Parts Discussed in Thread: INA190

The load is powered by dcdc,and the  maximum current of the load is limited to 7mA,so can LMP8645 limit the current below 7mA? And what the accuracy is?

• Suyi,

Can you clarify your question for our engineers here? How are you wanting to limit current here? Are you wanting current in the line to be held less than 7mA, or are you wanting the device to flag above 7mA and notify the system of this? A little more explanation here would help a great deal.

• The design i prefer to limit the peak current of DCDC output,considering the design is for 4-20mA loop .

In detail,the load is powered by 5V DC,and converted from 12V with dcdc.But,the peak current to the load should be limited less than 7mA.

1.I doubt that how to design with ti chips.

2.can LMP8645 limit the peak current less than 7mA ?

• Hello Suyi,

The lower measurement bound of our devices is typically determined by the input bias current and input offset voltage specifications.  For the LMP8645, the input bias current max is specified around 20uA and varies with common mode voltage.  For less than 1% measurement error from just input bias current your minimum current measurement should be at least 2mA.  While input offset can be related to input bias current, it too can provide additional error.  For less than 1% error from input offset we recommend your minimum current of interest generate a sense voltage 100x greater than the input offset voltage.  Larger shunt resistors can guarantee that at the price of more power dissipated across them.  As the input offset can be as large as 1mV, you would need minimum sense voltage of 100mV to have 1% or better offset error or you would need to perform a calibration.  Without calibration, if you started caring about the current at 2mA, you would need a shunt value of Vsense=100mV/2mA=50Ω.  I presume this might be a bit on the high end for you.

Alternatively you might consider a device like our INA190, which has 3nA input bias current and 15uV input offset voltage specs.  However, im not sure whether its bandwidth may be adequate for you.  When you say peak current are you implementing a form of peak mode control for your dcdc converter?  If so, what is your switching frequency?