Negative Output Voltage Ripples of Bipolar DC–DC Converter LM27762 near Maximum Input Voltage

The paper presents the results of studies of the operation of the inverting DC–DC converter with charge pump and LDO, which are part of the combined bipolar secondary power supply LM27762, at the near to maximum input voltage of 5.5 V and an output voltage of –4.9 V. The ripple voltages were measured at various load currents from 10 to 250 mA at the positive pole of the flying capacitor, at the output of the charge pump system and at the output of the microcircuit. It was shown for the first time on the basis of the obtained information that at low load currents up to about 107 mA the charge pump system operates in the burst mode, and at currents greater than 109 mA – in the charge pump mode with a constant frequency. The results do not confirm the information in the documentation on the microcircuit that, at themaximum input voltage of 5.5 V, the charge pump can enter the PWM mode in hot conditions. When working in burst mode, the presence of LDO in the LM27762 chip reduces the ripples of the negative voltage at the output. However, they significantly exceed the values given in the documentation on the chip. During switching to the constant–frequency mode, the level of negative voltage ripples at the output of the microcircuit decreases sharply, but it increases with further increase of the load current and exceeds the values given in the documentation.