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TC7662AMJA датащи(PDF) 5 Page - Microchip Technology |
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TC7662AMJA датащи(HTML) 5 Page - Microchip Technology |
5 / 14 page 2002 Microchip Technology Inc. DS21468B-page 5 TC7662A 3.0 DETAILED DESCRIPTION The TC7662A is a capacitive charge pump (sometimes called a switched-capacitor circuit), where four MOSFET switches control the charge and discharge of a capacitor. The functional block diagram shows how the switching action works. SW1 and SW2 are turned on simulta- neously, charging CP to the supply voltage, VDD. This assumes that the ON resistance of the MOSFETs in series with the capacitor produce a charging time (3 time constants) less than the ON time provided by the oscillator frequency, as shown: 3 (RDS(ON) CP) <CP/(0.5 fOSC). In the next cycle, SW1 and SW2 are turned OFF and, after a very short interval with all switches OFF (preventing large currents from occurring due to cross conduction), SW3 and SW4 are turned ON. The charge in CP is then transferred to CR, but with the polarity inverted. In this way, a negative voltage is derived. An oscillator supplies pulses to a flip-flop that is fed to a set of level shifters. These level shifters then drive each set of switches at one-half the oscillator frequency. The oscillator has a pin that controls the frequency of oscillation. Pin 7 can have a capacitor added that is connected to ground. This will lower the frequency of the oscillator by adding capacitance to the internal timing capacitor of the TC7662A. (See Typical Characteristics – Oscillator Frequency vs. COSC.) FIGURE 3-1: TC7662A TEST CIRCUIT 3.1 Theoretical Power Efficiency Considerations In theory, a voltage converter can approach 100% efficiency if certain conditions are met: 1. The drive circuitry consumes minimal power. 2. The output switches have extremely low ON resistance and virtually no offset. 3. The impedances of the pump and reservoir capacitors are negligible at the pump frequency. The TC7662A approaches these conditions for negative voltage conversion if large values of CP and CR are used. Note: Energy is lost only in the transfer of charge between capacitors if a change in voltage occurs. The energy lost is defined by: E = 1/2 CP (V1 2 – V 2 2) V1 and V2 are the voltages on CP during the pump and transfer cycles. If the impedances of CP and CR are relatively high at the pump frequency (refer to Figure 3- 1), compared to the value of RL, there will be a substantial difference in voltages V1 and V2. Therefore, it is desirable not only to make CR as large as possible to eliminate output voltage ripple, but also to employ a correspondingly large value for CP in order to achieve maximum efficiency of operation. 3.2 Dos and Don’ts • Do not exceed maximum supply voltages. • Do not short circuit the output to V+ supply for voltages above 5.5V for extended periods; however, transient conditions including start-up are okay. • When using polarized capacitors in the inverting mode, the + terminal of CP must be connected to pin 2 of the TC7662A and the + terminal of CR must be connected to GND (pin 3). • If the voltage supply driving the TC7662A has a large source impedance (25-30 ohms), then a 2.2 µF capacitor from pin 8 to ground may be required to limit the rate of rise of the input voltage to less than 2V/ µsec. TC7662A 1 2 3 4 8 7 5 + 10 µF 10 µF VDD (+5V) NC NC 6 + CP CR VOUT (-5V) COSC RL IS IL |
Аналогичный номер детали - TC7662AMJA |
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Аналогичное описание - TC7662AMJA |
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