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TC7662B Folha de dados(PDF) 3 Page - Microchip Technology |
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TC7662B Folha de dados(HTML) 3 Page - Microchip Technology |
3 / 11 page 3 TC7662B CHARGE PUMP DC-TO-DC VOLTAGE CONVERTER TC7662B-8 9/11/96 © 2001 Microchip Technology Inc. DS21469A Figure 2. Idealized Negative Voltage Capacitor THEORETICAL POWER EFFICIENCY CONSIDERATIONS In theory, a voltage converter can approach 100% efficiency if certain conditions are met: A. The drive circuitry consumes minimal power. B. The output switches have extremely low ON resistance and virtually no offset. C. The impedances of the pump and reservoir capacitors are negligible at the pump frequency. The TC7662B approaches these conditions for nega- tive voltage conversion if large values of C1 and C2 are used. 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 C1 (V12 – V22) where V1 and V2 are the voltages on C1 during the pump and transfer cycles. If the impedances of C1 and C2 are relatively high at the pump frequency (refer to Figure 2) 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 C2 as large as possible to eliminate output voltage ripple, but also to employ a correspondingly large value for C1 in order to achieve maximum efficiency of operation. Dos and Don’ts 1. Do not exceed maximum supply voltages. 2. Do not connect the LV terminal to GND for supply voltages greater than 3.5 volts. 3. Do not short circuit the output to V+ supply for voltages above 5.5 volts for extended periods; however, transient conditions including start-up are okay. DETAILED DESCRIPTION The TC7662B contains all the necessary circuitry to complete a negative voltage converter, with the exception of two external capacitors which may be inexpensive 1 µF polarized electrolytic types. The mode of operation of the device may be best understood by considering Figure 2, which shows an idealized negative voltage converter. Ca- pacitor C1 is charged to a voltage V+ for the half cycle when switches S1 and S3 are closed. (Note: Switches S2 and S4 are open during this half cycle.) During the second half cycle of operation, switches S2 and S4 are closed, with S1 and S3 open, thereby shifting capacitor C1 negatively by V+ volts. Charge is then transferred from C1 to C2 such that the voltage on C2 is exactly V+, assuming ideal switches and no load on C2. The TC7662B approaches this ideal situation more closely than existing non-mechanical circuits. In the TC7662B, the four switches of Figure 2 are MOS power switches; S1 is a P-channel device and S2, S3 and S4 are N-channel devices. The main difficulty with this ap- proach is that in integrating the switches, the substrates of S3 and S4 must always remain reverse biased with respect to their sources, but not so much as to degrade their “ON” resistances. In addition, at circuit start up, and under output short circuit conditions (VOUT = V+), the output voltage must be sensed and the substrate bias adjusted accordingly. Failure to accomplish this would result in high power losses and probable device latchup. The problem is eliminated in the TC7662B by a logic network which senses the output voltage (VOUT) together with the level translators, and switches the substrates of S3 and S4 to the correct level to maintain necessary reverse bias. The voltage regulator portion of the TC7662B is an integral part of the anti-latchup circuitry; however, its inher- ent voltage drop can degrade operation at low voltages. Therefore, to improve low voltage operation, the “LV” pin should be connected to GND, disabling the regulator. For supply voltages greater than 3.5 volts, the LV terminal must be left open to insure latchup proof operation and prevent device damage. Figure 1. TC7662B Test Circuit 1 2 3 4 8 7 6 5 TC7662B + V+ (+5V) V+ VO C1 10 µF + C2 10 µF IL RL NOTE: For large values of COSC (>1000 pF), the values of C1 and C2 should be increased to 100 µF. IS VIN S3 S1 S2 S4 C2 VOUT = – VIN C1 |
Nº de peça semelhante - TC7662B |
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Descrição semelhante - TC7662B |
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