Although there are more stringent tolerance and efficiency requirement and the power management circuit in cellular phones, even the simple circuit like below can be used to obtain the negative voltage in cellular phones.
Negative Voltage in Cellular Phones
Microcontroller drives the DAC which generates low positive voltage. This output is inverted and amplified by XC221A, which produces a triangular wave output. This is used to obtain 0 to -5V output from 0 to +2.5V.
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