Figure 2. Circuit using the charge pump IC
The charge pump circuit is usually called a switched capacitor converter (Switched Capacitorconverter), which includes a diode or a switched network of switches and capacitors. Figure 2 shows the circuit used by the charge pump IC. The IC has two controllable switches and oscillators inside. It is connected to two capacitors C1 and C2. This circuit can also be composed of discrete components as shown in Figure 3(a). As long as one oscillator, such as NE555 and one logic is reversed, such as 4009UB and two diodes D1 and D2 And two capacitors C1, C2 can form a simple charge pump circuit.
Figure 3 (a) simple charge pump circuit
The principle of operation is as follows:
(1) If the control pulse is low, its reverse output is high, and its equivalent circuit is shown in Figure 3(b). At this time, D1 is forward biased, D2 is reverse bias, C1 The voltage across the voltage Vc1 can be charged up to Vc1 - (Vcc - Vd), where Vd is the forward bias of the diode, and the current direction at this time is as shown by I in the figure.
Figure 3 (b) Equivalent circuit when the control pulse (CLK) is low
(2) When the control pulse is high, its reverse output is low. At this time, the positive end of C1 cross-voltage Vc1 is equivalent to ground. As shown in Figure 3(c), the D1 is reverse. Bias, while D2 is forward biased, accepting the previous C1 voltage across, C2 can be charged up to - (Vcc - 2Vd) voltage, and its voltage is negative corresponding to ground.
The C1 flywheel capacitor moves back and forth, the charge is input to the output, and the C2 storage capacitor stabilizes the charge and stabilizes the output voltage. In this circuit, the charge cycle of the pulse can be controlled to achieve the desired output.
Figure 3 (c) Equivalent circuit when the control pulse (CLK) is high
However, the circuit of the charge pump can be applied to the boost as well as the step-down. The charge pump IC with the IC number MAX619 from MAXIM is taken as an example, as shown in Figure 4. The operating principle is as follows:
(1) When the switches SW1, SW3 and SW7, SW5 are turned on, and the other switches are turned "OFF", their C1, C2 are each charged to a voltage of about V1.
(2) In the previous state, when the switches SW2, SW4, and SW6 are turned on, and the other switches are turned on (OFF), the voltage of about V in the front state and the voltages on the capacitors C1 and C2 are connected in series to the C4. The capacitor is charged to obtain an output voltage Vo which can be charged up to 3 times the voltage of the VI.
When the MAX619 is used with a specification of 1.8V-3.6V, the output can be 5V/20mA, and when the input is greater than 3V, the output can be 5V/50mA. The power consumption is as follows: Powerloss=Iout&TImes;[(2 or 3)VI- Vo] its power consumption depends on the amplified voltage (2 or 3) VI and Vo differential pressure and output current Iout, the circuit is a capacitive component, the efficiency of this circuit will be lower than the low drop converter (LDO) It is much higher, and the circuit architecture does not require an inductor, and its capacitor can be used with a ceramic capacitor. Therefore, the electromagnetic interference is small, the volume and the price are also lower than the inductance, and it plays an extremely important role in the design of the portable power source.
2, the characteristics of the charge pump circuit·Easy to use: In addition to adding one power supply to the input and output terminals, add one pump capacitor (Cpump).
• The circuit is more efficient than the LDO.
Low EMI or output ripple.
• The wattage of the output power supply and the VI/Vout voltage ratio are limited.
· The price is medium.
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