Rev C of the data sheet for LM27761 says in section 8.2.2.5 for Equation 5 that VOUT = –1.22 V × (R1 + R2) / R2. This seems to be in error as (R1+R2)/R2 will always be greater than 1 which would result in VOUT always being larger than V.
Figure 22 in that document shows VIN = 5.5 V, VOUT = –5 V, R1 = 1.54 MΩ, R2 = 500 kΩ but using those values in the above equation does not produce -5V OUT.
Using VOUT = –1.22 V × R1/(R1 + R2) does produce the stated VOUT.
Using WEBENCH Designer with those same VIN=5.5V and VOUT=-5V produces those same R1 = 1.54 MΩ, R2 = 500 kΩ (499K actually).
But reducing VIN to 5.2 V still yields the same R1 and R2 values, which cannot be correct since VIN has changed.
Also, Figure 12 of the data sheet shows less than 30 mV dropout V for 100 mA. Yet WEBENCH will not allow VIN to be reduced below 5.16 V when VOUT=-5V and Iout=100 mA. That implies a dropout V of 160 mV.
