Other Parts Discussed in Thread: OPA209
Hello:
The data sheet for the LM27761 gives 50k as the minimum value for R2. This is one of the two resistors used to set the output voltage of the internal linear regulator in this inverting charge pump switcher. Usually, this type of feedback resistor is given a maximum value so that the current in the resistor is large compared to leakage current into the feedback pin, so that voltage accuracy is not affected by the leakage current.
Is it a typo that R2 is specified as a minimum of 50k?
I do note that many of the performance graphs do show R2 as 500k, but I was wondering if this happened as a result of an initial typo calling for R2 to be greater than 50k. Because, in addition to leakage error with larger resistors, there are two more problems with such large values. One is that the part has an internal RC filter on the feedback pin, and larger resistors will be more noticeable with respect to that internal resistance, and will thus move the filter corner to higher frequency and thus sharply reduce the phase margin of the internal loop. The other problem is the higher thermal noise of the large resistors. The part is specified as 20uVrms error in a 100kHz bandwidth, which is 63nV per root Hz. But, 500k has 89nV of thermal noise, and when this is multiplied to the output voltage this noise is gained up. For example, at -3.3V the thermal noise of R2 alone will be gained to 152nV per root Hz. Then, R1 adds still more thermal noise--in the case of -3.3V it contributes about 117nV. So, these large values are causing large increases in the noise floor of the regulator.
Thanks,
Farron