We got our first samples back that has our device with the ADI regulator (ADP1607ACPZN-R7) on there. We are seeing something that I don't understand and may make this regulator unusable.
We are trying to get an output voltage of 1.875V with a max voltage is 1.95V and our min voltage is 1.83V. So, we put R1=137k and R2=280k. We used 1% tolerance resistors.
Also, the spec sheet says that Vfb = 1.259 +/- 2%.
The maximum error due to the feedback current is 0.25uA*137k = +34mV.
Adding all these things together, we are supposed to see an output voltage the never exceeds 1.96V and never goes below 1.83V.
We measured 10 parts and here is what we see:
These are all with very light load currents (on the order of 10mA).
What is going on? Why am I seeing 20% being bad in only a sample size of 10? This means in high volume things are going to get way worse.
Your test result is reasonable as the IC your were testing is the ADP1607ACPZN-R7 whose operating mode is AUTO, which means the part operates in PFM ( skipping pulses) at light load and in PWM mode with heavier load. As it says in the datasheet, "In PFM mode, the converter only switches when necessary to keep the output voltage between the PFM comparator high output voltage threshold and the lower sleep mode exit voltage threshold." For more information, please refer to page 11 and figure 24, 25 in the ADP1606/ADP1607 datasheet. At 10mA load, the part is supposed to operate in PFM mode with large output voltage ripple as shown in Figure 24.
Please note that the +/2% tolerate for the VFB is specified for PWM mode operation.
For higher accuracy at the output at light load, please use the PWM only part ADP1607ACPZN001. Thanks.