Part Number:LM27762
I have built a board with the LM27762, the idea being having +3.3/-3.3 rails from a 5V supply. The positive rail is fine, the negative one quite not... I have 2x4.7µF as input capacitor, the supply seems quite rigid (measured on the cap at less than 3mm from the IC), I only see small transients at about 3kHz (it's working in PFM) and probably they are from the shortest ground wire I have for the scope. The C1 (flying capacitor) is 1µF (tried 470nF didn't get better), charge pump capacitor CP and the output capacitors are 4.7µF as recommended. The 4.7µF are 0805, the others are 0603 so I don't think biasing could ruin the effective capacity... I have 1M+560k for +ve feedback and 820k+470k for -ve feedback. Load is light, in fact for testing I desoldered the opamps being powered so the negative rail is at open load. What do I see: on the flying capacitor there are short bursts (5-7 pulses) at 2 MHz (the main switcher) and then a slow (dis)charging, repeating at 3kHz. I reckon this is the charge pump cycle skipping due to no load being present. The big problem (as I see it) is on the charge pump capacitor (pin 5): I'd expect to see something like -4.7 -4.8V (due to switching losses) instead there is a 1.5kHz (half PFM) from -2.2V and 2.2V. The negative LDO obviously can't work correctly... What could cause the charge pump malfunctioning in this way? Thanks in advance↧