SM72295: Current Sense Accuracy Issue

Hi Ashish,

thanks to reaching out to us on this one! I would like to check your sch to make sure everything is ok. For ex the current sense inputs are sensitive and needs bypass cap in the right location for noise and to keep the inputs from exceeding spec.

-where is the equation below coming from?
Vout (Iout Pin) = 3 Vdc

-is BOUT clamped to VDD? the load current might be exceeding the VDD clamp in the area of interest (for ex at your peak current BOUT should be less than VDD)

-also can you confirm the diff voltage of SIB/SOB is within +-500mV?

-are you using the IIN current sense amp as well or just IOUT? unused pins should be tied to AGND.

-for your excel - where are you measuring Vsense from exactly? what about ADC?

Thanks,

Hello Jeff,

1. Please find the schematic for your reference. 

Note: please not the changes in reference designator for Capacitor / Resistor (eg. R14 = 0.005E, R29=R31=470E, R34=8.25K, C44=100nF, C34=10nF)

-where is the equation below coming from?
Vout (Iout Pin) = 3 Vdc

[AG]: Iout Pin voltage range was with Gain=100, Rsense=0.005E, Load Current = 6 A (max)

0.005 * 6 * 100 = 3 Vdc

-is BOUT clamped to VDD? the load current might be exceeding the VDD clamp in the area of interest (for ex at your peak current BOUT should be less than VDD)

[AG] No, I checked BOUT voltage, its very less compared to VDD and similar to Iout.

-also can you confirm the diff voltage of SIB/SOB is within +-500mV? 

[AG] I checked the voltage with multimeter its in 2mV range between pin-13 (SOB) and pin-14 (SIB)

-are you using the IIN current sense amp as well or just IOUT? unused pins should be tied to AGND.

[AG] We are using both IIN and IOUT but not using BIN/BOUT.

-for your excel - where are you measuring Vsense from exactly? what about ADC?

[AG] Vense is the voltage measured across Current Sense Resistor R14=0.005E

Also, VADC is voltage measured at I_OUT net (i.e. IOUT pin of SM72295)

Thanks,

--Ashish

Hi Ashish,

Thanks for your detailed response. It sounds like for Case1b everything works ok until you apply input pwm, whats the application?

SIB/SOB is only 2mV ? these pins should be a similar voltage to Vsense right?

What is the value of C41? this is VDD cap and more is better, also the logic supply cap should be close to the IC to support current sense circuitry as well as the pwm input comparators.

I also have seen that C44 can be 10n or 100n. what is your purpose of choosing 10n vs 100n for the differential csa input cap?
You can also add a common mode cap from SOA to AGND to hold this voltage steady.

What is C34 doing, you mention its 10nf?

Thanks,

Good Morning Jeff,

---It sounds like for Case1b everything works ok until you apply input pwm, whats the application?

[AG]: Its for EV charger.

---SIB/SOB is only 2mV ? these pins should be a similar voltage to Vsense right?

[AG]: I feel that should be fine as during closed loop negative feedback system SIB and SOB should be very close to each other. So, Vsense would be seen across R29.

---What is the value of C41? this is VDD cap and more is better, also the logic supply cap should be close to the IC to support current sense circuitry as well as the pwm input comparators.

[AG]: C41=0.1uF, Yes, it is close to IC VDD pin.

---I also have seen that C44 can be 10n or 100n. what is your purpose of choosing 10n vs 100n for the differential csa input cap? 

[AG]: I kept the cutoff freq as 1/(2*pi*C44*R29) = 1/(2*pi*100n*120) = 13.2KHz, I wanted to keep C44 even higher than this, to decrease the cutoff freq, but couldn't find any other recommended value other than 10nF or 100nF in application/datasheet notes.

---You can also add a common mode cap from SOA to AGND to hold this voltage steady.

[AG]: you mean SOB (not SOA) right? That would need new board revision.

---What is C34 doing, you mention its 10nf?

[AG]: you mean C47=10nF? with R34=12K => cutoff freq is 1.32KHz. And also 10nF was recommended in application notes. 

Do you have detailed Current Amplifier specs of SM72295? I feel data sheet has very limited information (Ex. compared to INA213 series Current Sense Amplifier ICs)

Thanks,

--Ashish