Post Go back to editing

Configuration of the output voltage on the ADP123-EVALZ


We have the project wearable device that is powered by charged battery, has an LCD, touch, and buzzer. 

Before that, my PCB worked from Buck-Boost type DC-DC at 3.3V. I can't work with DC-DC because of high current consumption. I want to use linear regulator ADP123. For this reason, I bought the ADP123-EVALZ board. According to the datasheet the output voltage is adjustable and can be calculated by the next formula:

VOUT = 0.5 V(1 + R1/R2) + (ADJI-BIAS)(R1)

assuming a typical ADJ pin bias current of 15 nA at 25°C.

On the ADP123-EVALZ board, I changed the R2 resistor to 20K to get a 3V on VOUT. After this, I have connected the battery connection to the data logger to check system power consumption. After connection, I see a very strange situation: Power consumption is falling down all time. For example, 

1st check: start operation at 77mA and within 12 minutes falling to 66mA.

2nd check: (5 seconds after the end of the first check) start operation at 66.2mA and within 12 minutes falling to 62.1mA.

3rd check: start operation at 66.4mA and continue falling down.

When I have was using DC-DC, the operation currency was about 96mA.

What could be the problem with this behavior of the system? Power down to 3 volts?

What values of the resistors R1 and R2 should be in order to get exactly 3.3V at the output?



Added detailed information
[edited by: evgeni_k at 11:35 AM (GMT -4) on 28 Jul 2021]