How to Generate Auxiliary Voltage Using an External Charge Pump

This article will explain how to use an external charge pump to generate two additional voltages from a single-channel boost converter such as the TPS61087 from TI. The example will provide the reader with an introduction to charge pumps and a cost-effective solution for using them to generate the required voltage rails in the system.

Using an external charge pump is a flexible and easy way to generate auxiliary voltage rails from a boost converter. These voltage rails can theoretically be any voltage, positive or negative, and can power any application that requires two or more voltages. For example, a single-channel boost converter with a single 5V input supply line can provide all three main voltages (+27V, -7V, and 15V) necessary for a TFT-LCD application. The +/- 5V supply voltage required by an op amp can also generate a 3.3V supply.

Figure 1 Typical application voltage of 5V to 15V for TFT LCD (fsw = 1.2MHz) using an external charge pump (VGH, VGL)

Figure 1 shows an external positive charge pump structure that provides up to 3 times the boost converter output voltage VS, or 45 V, before regulating down to voltage VGH (27 V) for this application. In this case, the negative charge pump regulation stage regulates the output voltage VGL from the -15 V generated by the boost converter to -7 V.

Ideal situation

Positive Charge Pump

Figure 2 shows a typical application of the positive charge pump driver circuit, which will generate 2 times the voltage of VS in the voltage doubling mode. You can get an insight into the operation of the charge pump driver from this figure. The following study is based on the voltage tripling mode.

Figure 2 External positive charge pump - ideal case

The following description describes the charge pump behavior during steady-state operation in a simple and understandable manner. First, we assume that all components are ideal and the duty cycle of the boost converter is 50%. The resistance of R1 in Figure 2 is 0 ohms, and this is where the current flowing into capacitors C1 and C2 is measured.

During the on-time, since VSW=0V, the flying capacitor C1 can be charged to VS through the diode D1. Similarly, the energy storage capacitor C3 is also charged to 2 times VS through D3. Diodes D2 and D4 are both blocked. Since the output VCPP is no longer available, the output capacitor C4 has to power the circuit with the required 20mA load current.

During the off period, the switch node voltage VSW goes high, increasing the energy storage in the flying capacitors C1 and C2, and boosting C3 and C4 to 2xVS and 3xVS, respectively (when VSW=VS). Diode D2 becomes forward biased and allows current to flow into C3, charging it up to 2xVS (after the voltage drops across its terminals during the on period). Similarly, D4 turns on, and C3 recharges the output capacitor to 3xVS, while at the same time powering the output circuit with the necessary 20mA load current.

Finally, during the off-time, the inductor provides 80mA to the flying capacitor and 40mA to the output capacitor of the boost converter, which discharges into C1 during the on-time. Thus, the current provided by the boost converter is, on average, three times the positive charge pump output current, or 60mA.

Negative Charge Pump

The operation of the external negative charge pump is also divided into two stages (charge pump stage and voltage regulation stage). The charge pump provides a negative output voltage –VS (see Figure 1), and the voltage regulation stage then regulates the output voltage VGL to the desired level. You can get a deeper understanding of the operation of the charge pump driver from Figure 3.

Figure 3 External negative charge pump - ideal case

The following description describes the negative charge pump behavior during steady-state operation, again assuming that all components are ideal, the duty cycle of the boost converter is 50%, and the resistance value of R1 is 0 ohms.

At the beginning of the off period, the switch node voltage VS is high, and the flying capacitor C6 is charged to VSW = VS through D6. Among them, the output capacitor C7 can provide 20 mA of output load current.

During the on-time, since VSW = 0V, the positive terminal of the flying capacitor C6 is previously pulled to ground and the energy storage voltage shifts downward by –VS. This causes diode D7 to become forward biased, allowing current to flow and power the output circuitry.

Similar to the positive charge pump, in this example, the current provided on VCPN is 20mA, and the average current provided by the boost converter is equal to twice the output current of the negative charge pump, or 40mA.

Voltage stabilization level

The voltage regulator stage has selectable output voltages, and users can flexibly select the corresponding output voltage according to their specific applications.

We have already seen how the positive and negative charge pumps build their voltages. The next stage (see Figure 4) is similar to the positive and negative charge pumps and can regulate the output voltages VGH and VGL by dissipating the excess energy into the bipolar transistors.

The Zener diode clamps the voltage at the desired output value and the bipolar diode is also used to reduce the current consumption. Finally, the output voltage at VGH and VGL will be equal to VZ -Vbe. Figures 5a and 5b show the output voltage regulation measured before and after the regulation stage. It can be seen that as long as the voltage generated at VCPP and VCPN is always higher than the regulated output voltage, the system will be regulated by adding the transistor voltage drop. For example, by increasing the charge pump from tripler to quadrupler mode and selecting the device according to current and voltage, more power can be generated with appropriately rated components. The maximum possible output current also depends on the sum of the entire current consumption of the system, which should not exceed the current limit of the boost converter.

Instead of using the structure of Figure 4 for regulation, a shunt regulator such as the TL432 can also be used.

Figure 4 Positive charge pump regulator stage

Figure 5 Stable output voltage

The advantage of external charge pumps is that they are cost-effective and provide users with great flexibility. Using a separate boost converter (such as TI's TPS61085 or TPS61087) and the simulation tool TinaTI? to assist in the design, it is easy to obtain a high-power positive/negative charge pump.