TPS25940-Q1: Confusion in using R(IMON) equation

Hi Danilo,

1. The difference between eq5 and eq13 is that eq5 ensures that relation between current through device  and voltage at IMON pin is linear. So it is suggesting the max voltage at IMON for max current using numerator.  Eq13 is focusing on system that is using ADC.  Eq13 tries to ensure that for maximum current through device(that is current limit ) the voltage at IMON pin is not exceeding the ADC input voltage range.  So if you are using ADC you can use equation 13 and for linear output ensure that ADC max input voltage is less than 6V.

2. Now circuit 36 is for a special circuit configuration. If you want to set a flag when there is an overcurrent event then you can short IMON to PGTH pin. Here you are not using IMON for current monitor. So you will not use eq 5 or eq 13. Here we are selecting RIMON such that when current is above a particular value then PGTH will give voltage above 0.99(PGTH rising threshold) . When PGTH is above 0.99 then then PG will go high. So when current is above a certain value then PG will go high. This particular value of current can be same as current limit. When it is same as current limit PG will go high when your system is in overcurrent.

So if you want to monitor current and you are using ADC then use equation 13.

Regards

Kunal Goel

Hi Kunal,

Thank you for your response. According to the customer,

1. If I were to use INA3221 (operating range = 0 - 26V) to measure the voltage across IMON pin, I still have to restrict myself to less than 6V? Because of this 6V restriction, I would still use Equation 5 then?

2. Thank you for the clarification. My concern for this circuit configuration is that PGOOD will go high during overcurrent. It only goes high to inform over current. It does not automatically shutdown the device. Am I correct in saying so, yes?

Another thing: you said, "This particular value of current can be same as current limit." But I don't see R(ILIM) as part of the equation. What if I use R(ILIM) such that the current limit from that is different than the current limit from R(IMON) in equation 36?

Ideally for my application, I would like the eFUSE to set a flag during overcurrent, regulate the overcurrent to converge to current limit, shutdown during short circuit current I(FASTTRIP), and activate thermal shutdown if necessary.

Regards,

Danilo

Hi Danilo,

1. Please follow equation 5

2. No it can not shutdown device. 

3. Circuit for equation 36 is a special configuration. It is used for setting PG during overcurrent. Through eq 36 you can chose a value of current at which your system should think that there is overcurrent. You put this value in eq36 to get RIMON. Practically you will want that overcurrent value in eq36 is same as current limit. But if you don't want you can keep them different. Device internal fet will current limit based on value set using RLIM.

This device does not have functionality to set flag during overcurrent, but you can achieve that using circuit for eq 36 . Are you using INA3221 for overcurrent detection or to continuously monitor the current through device?

If you need automotive qualified part then please see TPS25200-Q1, it has inbuilt  flag to detect overcurrent . Also we have new device TPS25947 that can also work in your application.

Regards

Kunal Goel

Hi Kunel,

We have received further inquiries from the customer as follows.

1. Ok, I will follow Eq. 5.
2. Understood.
3. I think I have a better understanding now of what you are saying. I think I will make overcurrent value in eq36 is same as current limit. If I do this, then Sec. 9.3.4.1 and 9.3.4.2 will be employed. By this way, a flag will be asserted (FLT pin).

Tentatively, I am thinking of using INA3221 for IMON pin and I will use the eFUSE product for overcurrent detection. Please do not confuse the two.

4. Also I am a bit confused with the "Short-to-battery protection" and "Short-to-ground protection" features. Could you please elaborate on this? Thanks

5. Also, what is the minimum value for I(LIM) in equation 4? Is it true that the lowest value is 0.5A? If yes, could you share to me an eFUSE with I(LIM) in the mA level? thanks

TPS25940 is rated "AEC-Q100". Why did you suggest TPS25200-Q1 if TPS25940 is also automotive qualified?

I am reading TPS25947 and it looks simplier and more convenient than TPS25940 

Regards,

Danilo

Hi Danilo,

1.FLT pin of TPS25940-Q1 device will be asserted when there is undervoltage, overvoltage, reverse voltage-current and thermal shutdown conditions. FLT pin will not assert on overcurrent. If customer want that a flag should be asserted during overcurrent then they can use configuration in 10.2.4.3 Overload Detection Using TPS25940xx-Q1 where IMON is shorted to PGTH to use PG pin as a flag for overcurrent. Either they connect IMON to PG or not device will current limit and fast trip based on RLIM value.   

2. Short to battery means that for example in an automotive system your battery is of 12v and you want to protect your USB 5V rail. Suppose efuse input is 5v and output side voltage becomes 12v due to short to battery. This will cause reverse current flow from out to in. So TPS25940 device blocks this reverse current by turning off.

Short to ground means that when output shorts to ground then their is huge current flow which can damage load,supply,board. So this device has short circuit protection that turns off device quickly in such event.

3.Minimum value of ILIM is around 0.6A. Please see TPS2596 for lower current limit.

4. I thought customer only need automotive qualified part, so I recommended TPS25200-q1 because this device asserts fault flag on overcurrent and customer doesn't need to use special configuration of shorting IMON to PG in TPS25940-Q1.

Does customer need automotive qualified part? If they can tell more about application, end equipment , protection needs I can recommend an eFuse. 

Regards

Kunal Goel

Hi Kunal,

Here is the response of the customer.

This is my application. In a single event latch up, an overcurrent will surge thru the VDD pin of G1. G1 is a schmitt inverter (BU4S584G2-TR which has 10mA max current in VDD) used for our satellite reset circuit. To prevent the overcurrent from the SEL, I will put the eFUSE on the VDD pin as shown by the green arrow. Hopefully, the eFUSE will regulate overcurrent if current is below I(FASTTRIP) or the eFUSE will shutdown / open circuit the power rail if the overcurrent is above I(FASTTRIP).

Seeing the low 10mA max current of VDD, it looks like the eFUSE with the smallest ILIM is TPS25200-Q1. Correct?

And yes, I would need an Automotive-grade eFUSE or something better.

If your product specialist can recommend more protection, please do share them.

Regards,

Danilo

Hi Danilo,

Does customer want to set current limit as 10mA? I think you forgot to attach a block diagram as customer is referring to a figure in above post.

Regards

Kunal Goel

Hi Kunal,

I apologize that I was not able to attached the schematic diagram. Here is the copy.

Regards,

Danilo

Hi Danilo,

Maximum current that can go into VDD pin is 10mA ? They want to limit current below 10mA?

Regards

Kunal Goel

Hi Kunal,

Here is the customer's response.

HAHA yes, 10mA. IDEALLY 10mA, since 10mA is the maximum absolute rating for the VDD pin of the schmitt inverter.

I know it kinda sounds ridiculous to limit below 10mA, so I think I will just use TPS25200-Q1. Using this eFUSE, I will limit the current down to 40mA. In this way, any overcurrent due to CMOS latchup shouldn't go thru the VDD pin and destroy the CMOS.

Regards,

Danilo

Hi Danilo,

TPS25200-Q1 device minimum current limit is 85mA. Upto this range we  guarantee current limit accuracy and also test it and mention in datasheet. If they set current limit below 85mA,then they have to verify this on their system. Let me know if I can close the thread or their are other queries.

Regards

Kunal Goel