Re: 19v adapter to 16v?

Whoa man! Yes, I agree that the TL431 is a voltage reference not a regulator - that was a slip. However, that resistor math will work with almost all adapters that use the TL431 in this configuration. I hope you saw the attached PDF file. Just to let you know, I am a manufacturer of laptop adapters. Almost all adapters these days use a very similar output circuit design with a TL431 or similar voltage reference for output voltage control. The PWM part of the circuit, does have a lot of variations but the overall design concept is still common.
I noticed...

Re: 19v adapter to 16v?

ben7 has the right answer. Almost all Laptop adapters use a TL431 or similar voltage regulator which uses 2 resistors as a voltage divider to sense the output voltage. see the attached pdf file. R13 and R14 are the two resistors. The values of the 2 resistors are not critical; what is important is the ratio. The ratio should be such that for the required output the voltage at pin 1 of TL431 is 2.5. The formula for calculating R13 and R14 is as follows:
R13 = R14(V-2.5)/2.5 where V is the required output voltage
If you fix the output as 16V,...