adjustable voltage regulator

In the board I am designing I am using a LT1761 adjustable voltage regulator to generator 3.5V for my AVDD.

This is what the datasheet says

The adjustable version of the LT1761 has an output voltage range of 1.22V to 20V. The output voltage is set by the ratio of two external resistors as shown in Figure 1. The device servos the output to maintain the ADJ pin voltage at 1.22V referenced to ground. The current in R1 is then equal to 1.22V/R1 and the current in R2 is the currentin R1 plus the ADJ pin bias current. The ADJ pin bias current, 30nA at 25°C, flows through R2 into the ADJ pin. The output voltage can be calculated using the formula in Figure 1.

The value of R1 should be no greater than 250k to minimize errors in the output voltage caused by the ADJ pin bias current. The adjustable device is tested and specified with the ADJ pin tied to the OUT pin for an output voltage of 1.22V. Specifications for output voltages greater than 1.22V will be proportional to the ratio of the desired output voltageto 1.22V: VOUT/1.22V.

Attatching the figure,

This is simple, but I want to check if this is right , I need a output of

V1 = 3.5V I use R1 = 10k and R2 = 18.7K
V2 = 2.4V I use R1 = 10K and R2 = 9.7K

or should I choose like 100K and 97.6K,

Here is the link to the datasheet

https://www.emesystems.com/pdfs/parts/LT1761.pdf


And how/where do I provide the heat sink for this chip because it only tells me

For surface mount devices, heat sinking is accomplished by using the heat spreading capabilities of the PC board and its copper traces. Copper board stiffeners and plated through-holes can also be used to spread the heat generated by power devices.

Circuit,

You have answered all questions by yourself .. (this is called self-confidence).

As far as R2 and R1 values are concerned, I prefer these lower values (10kΩ and 18.7kΩ) as you dont need to think about Iadj at all ..

regards ..

Thanks IanP I really lack self confidence... Regards the heat sink for the chip, can I just provide unconnected copper under the chip and probably extend a little bit on the outside as well. what is the best way to do this ?

Use thick truc for the Vin and Vout but the most of cooling you can transmit through the GND pin, so design a copper area underneath he IC and extend it in both directions. Pins such as ADJ and SHDN do not matter very much ..

regards ..

so you mean this H kind of arrangement of copper that I put under the chip and outside should be grounded along with pin 2.. this is my first pcb design/layout, so forgive my innocence

That is right as the most of heat you can transmitt from IC like this is through their pins, and GND pin in most cases is the best option, unless you have special tab as for example in D-PAKs, SOT223 , .. which is designed for good heat transmission, and in case of voltage reguators it is connected to GND pin, in case of transistors it is C(ollector) or D(rain) ..

the unconnected copper on the board under the component and extending on both sides should not be exposed copper right, it should have epoxy on top of it...only at the pad (which is ground) I have bare copper ?

Please have a look on page 14 and 15. There is the information of the heatsink you nee.