TPS92512HV: IADJ pin voltage

Hi Clint,

Thank you for your support.

Our customer supplied 1.8V to IADJ terminal of EVM. (The terminal is connected to IADJ pin of IC via 1kohm.)

Even the connection, the LED current is saturated to 1.7V.

(Please see the following graph. Y axis is the current. X axis is the voltage of IADJ terminal.)

Is this because the supplied by external?

Could you give us any advice for this?

Regards,

Koji Hamamoto

Hello Koji,

That is odd. The same data is in the user's guide and goes up to the full 1.8V. I am not sure what you are driving IADJ with but have you tried removing the 10Mohm resistor? Also, are you measuring the voltage directly at the IADJ pin?

It would be strange to have such variation at room temperature and it might be a good idea to try another IC in case that one somehow got damaged, but in reality it is possibly within spec. If you look at the VISENSE spec using the clamp it has a typ of 300mV and a minimum of 285mV. So that is -5%, and -5% on 1.8V is 1.71V. So it's possible what you are seeing is real, but it would be on the edge and likely would not have passed the test program.

Regards,

Clint

Hi Clint,

I suggested to remove 1Mohm to our customer. Then they measured the linearity of the current adjustment by IADJ again.

However there is no changes. And I found the IADJ function result on TI EVM application note as following.

http://www.tij.co.jp/jp/lit/ug/sluubb2/sluubb2.pdf

This is just same as customer's result. Why the current is saturated at over 1.7V?

If this is real characteristics, how much is the IADJ range to keep linearity of the current? Our customer would like the linearity for the LED current.

Regards,

Koji Hamamoto

   

Hello Koji,

As inferred in the datasheet spec the clamp could be as low as 1.71V, and for best accuracy you would not want to rely on the clamp range when analog dimming. So it is recommended for best accuracy and linearity to have a max IADJ voltage lower than 1.71V when driving IADJ externally. I would choose a max of either 1.6V or 1.7V and set the ISENSE resistor accordingly for your max output current.

Regards,

Clint

Hi Clint,

Thank you for your support.

I am sorry. The graph which was posted above was the data for TPS92513HV (TPS92513EVM). (It was not for TPS92512.)

The data for TPS92512(left side) is the following graph.

At the above graph, The data of TPS92512 looks better than TPS92513 at the high IADJ voltage but it looks not good accuracy around IADJ=1.8V.

Is the current characteristic of TPS92512 same as TPS92513? (Is that same circuit?)

On the other hands, at customer's measurements the LED current was obtained 1096mA at IADJ=1.2V. (current limit is 1.5A) That accuracy is about +9.6%.

That data was obtain at TI-EVM (TPS92512EVM). In my understanding the accuracy will be within +/-5%. However it was over 5%.
Do you think that data is correct? If it is correct, how much is the accuracy?

As I mentioned at previous post, our customer would like high accuracy and linearity. So that , we would like to know the range for the high linearity and how much is the accuracy at that condition.

Regards,

Koji Hamamoto

Hi Koji,

They both use the same clamp, and the clamp can have a soft knee. That is likely the only difference. But as I mentioned, for best accuracy I would use 1.6V maximum to avoid the clamp. Below that you can see the linearity is very good. But to double check are they measuring the voltage directly at the IADJ pin? With a soft knee there may be some voltage drop across the 1k resistor.

As far as the 1096mA, are you sure that is being measured correctly and accurately? Is the voltage being measured directly at the IADJ pin here as well? It could also depend on the conditions. What is the input voltage and LED stack voltage? Has there been any other changes done to the EVM? Thanks.

Regards,

Clint

Hi Clinton,

Thank you for your support.

I confirmed to our customer the condition and how they measured . They measured IADJ voltage at the both IADJ terminal of TI-EVM and IADJ pin of IC. However there were only about 10mV differences. (I double-checked IADJ voltage by myself at TPS92512EVM when I supplied 1.8V to IADJ terminal. )

Please let me make sure how can we calculate the accuracy of ILED by IADJ voltage.
(On the datasheet.)
VSENSE is typ:200mV(min:191mV, max:210mV). * -4.5%~+5% @IADJ=1.2V
VSENSE is typ:30mV (min:21.4mV, max:40mV). *-29%~+33%@IADJ=0.12V

So, the accuracy will be about +/- 5% at least if IADJ is 1.2V. If the accuracy is more than 5%, it is depends on measurement condition.
Is my understanding correct?

If it is correct, I will need to check the condition of measurement on the TPS92512EVM.
What is any parameter to affect the accuracy?

Regards,
Koji Hamamoto

Hello Koji,

You are correct. The accuracy is determined by the error amp offsets, and they will become a larger % as the current sense voltage is lowered. This is typical with all LED drivers during analog dimming. If you need more accuracy you could consider a device with lower offsets such as the TPS92640 (+/-600uV maximum).

Regards,

Clint