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TC7660EV Datasheet(PDF) 6 Page - Microchip Technology |
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TC7660EV Datasheet(HTML) 6 Page - Microchip Technology |
6 / 12 page 6 TC7660 CHARGE PUMP DC-TO-DC VOLTAGE CONVERTER © 2001 Microchip Technology Inc. DS21465A TC7660-7 9/30/96 The TC7660 approaches these conditions for negative voltage multiplication 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) 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 not only desirable 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 • Do not exceed maximum supply voltages. • Do not connect LV terminal to GND for supply voltages greater than 3.5V. • 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 C1 must be connected to pin 2 of the TC7660 and the + terminal of C2 must be connected to GND Pin 3. Simple Negative Voltage Converter Figure 3 shows typical connections to provide a nega- tive supply where a positive supply is available. A similar scheme may be employed for supply voltages anywhere in the operating range of +1.5V to +10V, keeping in mind that pin 6 (LV) is tied to the supply negative (GND) only for supply voltages below 3.5V. Figure 3. Simple Negative Converter Paralleling Devices Any number of TC7660 voltage converters may be paralleled to reduce output resistance (Figure 4). The reser- voir capacitor, C2, serves all devices, while each device requires its own pump capacitor, C1. The resultant output resistance would be approximately: ROUT (of TC7660) n (number of devices) ROUT = 2 2 πf C1 The output characteristics of the circuit in Figure 3 are those of a nearly ideal voltage source in series with 70 Ω. Thus, for a load current of –10mA and a supply voltage of +5V, the output voltage would be – 4.3V. The dynamic output impedance of the TC7660 is due, primarily, to capacitive reactance of the charge transfer capacitor (C1). Since this capacitor is connected to the output for only 1/2 of the cycle, the equation is: XC = = 3.18 Ω, where f = 10kHz and C1 = 10µF. 1 2 3 4 8 7 6 5 TC7660 10 µF + V + 10 µF + VOUT* 1. VOUT = –n V + for 1.5V V + 10V NOTES: * C1 C2 |
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