1.8 V reference modelling

Hi emoon,

As I understand, the image is for LT6658-1.2 for conditions  VIN = VIN1 = VIN2 = 4.5V.

Supply Current for LT6658-1.2 is 2mA for TA = 25°C. VIN = VIN1 = VIN2 = VOUT1,2_F + 2.5V. Thus for 1.2V output & 3.7V input

From the graph, for an input of 3.7V input, input current will be 1.7mA or 1.8mA which does not match the above condition.

Can you clarify?

And how to convert a value in ppm to voltage and vice versa? For eg 

1. Output Voltage Temperature Coefficient for LT6658A is Typ 3ppm/C : will it be 3uV for every deg of temp change?

2. Load Regulation for Output 1 Sourcing, ΔILOAD = 0mA to 150mA :is 0.25uV/mA : will it be 0.25ppm/mA

Is there a standard when to use in ppm and when to use in voltage?

Thanks,
Uday

Hi Uday,

Thanks for pushing on this.

Regarding the supply current:

The typical curve section does mention VIN for LT6658-1.2 is 4.5V, but that plot sweeps input voltage, so the 4.5V specification doesn't apply.

I agree with your assessment of the graph for 25C; the input current only appears to be 1.8 or 1.7mA. However, the spec in the electrical characteristics table specifies that the spec applies across the full temperature range (see image below).



This leads me to believe the "typ" spec listed on the table is based on the 125C curve, as it is the "worst typical spec" that was measured. Hopefully that clears things up.

Regarding ppm:

PPM refers to parts per million of the output voltage. So, for a 1.0V reference, 1ppm = 1uV (1.0V * 1e-6). For a 5.0V reference, 1ppm = 5uV (5.0V * 1e-6).

For TC, this would seem to imply for that LT6658-1.2, the output voltage would drift 3.6uV per degree C of change (1.2V * 1e-6 * 3 = 3.6uV).

However, temperature coefficient is generally calculated using the "Box Method" which takes the minimum and maximum Vout values over a specified temperature range, and calculates a linear slope based on (Vmax - Vmin) / (Tmax - Tmin). However, Vout vs temp curves are not linear, so you can expect better or worse temperature performance based on "where you are in the curve". I would recommend checking the Output Voltage vs Temperature curves included in the datasheet.

For more info on TC and the Box Method, see AN82.

There is no true standard for when ppm is used versus voltage. Generally, ppm is used when the specs scale with the output voltage of the reference, and uV or V are used when the spec is generally constant across voltage options.

This is because ppm is a ratiometric spec based on the voltage reference's output voltage. Instead of listing a different spec for each voltage option (i.e. specX = 1uV for LT6658-1.0, specX = 2uV for LT6658-2.0, etc.), a single ppm spec can be used (i.e. specX = 1ppm for all LT6658 options).

Let me know if you have any questions on this, AN82 also includes a unit converter and some descriptions of ppm specs.

HI emoon,

Thanks for the detailed explanation. That was really helpful.

Actually I want to generate 1.6V as well. This can be generated by using LT6658-1.2V option with 1:3 resistor combination in feedback. Right?

My query is whether 1.6 generated using this method will meet all the specifications provided by datasheet just like direct 1.2V?

Kindly clarify 

Thanks ,

Uday

Hi Uday,

Yes, a 1:3 resistor combination in feedback will work.

The specifications (noise, initial accuracy, etc.) will depend on the spec of your resistors. I recommend using something like LT5400 if you need a high degree of matching.

Hi emoon,

Thanks for the response.

https://vpgfoilresistors.com/products/surface-mount-resistors/frsm0603/datasheet

I found this SMD resistor with ±0.01% tolerance (same as LT5400) but having TCR ±0.2 ppm/°C which is better against 8ppm/°C of LT5400.

Do you agree with me on this?

Regards,

Uday

The advantage of the LT5400 is it's matching. The matching resistor TC is also 0.2 (since the resistors drift together), and the tolerance refers matching tolerance (0.01%).

It appears the Vishay specs apply to the specified resistance instead.

So, deciding between resistors would be a tradeoff between absolute and matched performance.

The advantage of the LT5400 is it's matching. The matching resistor TC is also 0.2 (since the resistors drift together), and the tolerance refers matching tolerance (0.01%).

It appears the Vishay specs apply to the specified resistance instead.

So, deciding between resistors would be a tradeoff between absolute and matched performance.