The first method to generate a negative power supply from positive power supply is to use a charger pump. The principle of negative power supply charge pump is shown in Figure 2-1 . The voltage inverter portion of the charger pump contains four large CMOS switches which are switched in sequence to invert the input supply voltage. Energy transfer and storage are provided by external capacitor C1. In the first time internal, the switches S2 and S4 are open, as Figure 2-1 shows. S1 and S3 is closed. Then input energy is transferred to external capacitor C1. In the second time interval, S1 and S3 are open. At the same time, S2 and S4 are closed, and C1 is charging COUT. After a number of cycles, the voltage across COUT is pumped into VIN. Because the anode of COUT is connected to ground, the output at the cathode of COUT equals –(V_IN) when there is no load current. When a load is added the output voltage drop is determined by the parasitic resistance (R_{DS_ON} of the MOSFET switches and the equivalent series resistance (ESR) of the capacitors) and the charge transfer loss between the capacitors. In this way, the charger pump converter applies a positive input voltage to negative output voltage. Obviously, the amplitude of output voltage is equal input voltage if there is no load.

Figure 2-1 Principles of Positive Voltage Input and Negative Output Charger Pump

The output characteristic of the charger pump can be approximated by a voltage source in series with resistance. The voltage source equals –(V_IN). The output resistance R_OUT is a function of the ON resistance of the internal MOSFET switches, the oscillator frequency, the capacitance, and the ESR of C1 and COUT. Because the switching current charging and discharging C1 is approximately twice as the output current, the effect of the ESR of the pumping capacitor C1 is multiplied by four in the output resistance. The charge-pump output capacitor COUT is charging and discharging at a current approximately equal to the output current; therefore, the ESR only counts once in the output resistance. A good approximation of charge-pump R_OUT is shown in Equation 1:

where R_SW is the sum of the ON resistance of the internal MOSFET switches. High capacitance and low-ESR ceramic capacitors reduce the output resistance. With this function, designer can estimate the maximum output voltage under specific output current and input voltage.

TI has lots of negative output charger pump can meet specific application requirements. The LM27761 has not only integrated a negative charger pump but also integrates an LDO for better power supply performance and noise removal. See Figure 2-2. Figure 2-3 shows the typical application diagram. The advantages of the LM27761 are better performance, simple design, low cost and compact size. Equation 2 shows the output voltage.

Figure 2-2 LM27761 Block Diagram

Figure 2-3 Typical Application Circuit of LM27761

The maximum output current for a charger pump in the market generally is less than 300mA. The maximum output current for LM27761 is 250mA and this can meet many applications. The second constraint for the charger pump is the amplitude of output voltage must not be larger than the input voltage and must keep a little margin. For example, a designer cannot obtain -5V output voltage from a 5V input voltage. To obtain a -5V output voltage under 250mA output current, the input voltage must be larger than 5.5V for LM27761 according to the data sheet.