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TC7660EOA 数据表(PDF) 8 Page - Microchip Technology |
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TC7660EOA 数据表(HTML) 8 Page - Microchip Technology |
8 / 20 page TC7660 DS21465C-page 8 2002-2011 Microchip Technology Inc. 5.4 Changing the TC7660 Oscillator Frequency The operating frequency of the TC7660 can be changed in order to optimize the system performance. The frequency can be increased by over-driving the OSC input (Figure 5-4). Any CMOS logic gate can be utilized in conjunction with a 1 k series resistor. The resistor is required to prevent device latch-up. While TTL level signals can be utilized, an additional 10 k pull-up resistor to V+ is required. Transitions occur on the rising edge of the clock input. The resultant output voltage ripple frequency is one half the clock input. Higher clock frequencies allow for the use of smaller pump and reservoir capacitors for a given output volt- age ripple and droop. Additionally, this allows the TC7660 to be synchronized to an external clock, eliminating undesirable beat frequencies. At light loads, lowering the oscillator frequency can increase the efficiency of the TC7660 (Figure 5-5). By lowering the oscillator frequency, the switching losses are reduced. Refer to Figure 2-3 to determine the typi- cal operating frequency based on the value of the external capacitor. At lower operating frequencies, it may be necessary to increase the values of the pump and reservoir capacitors in order to maintain the desired output voltage ripple and output impedance. FIGURE 5-4: External Clocking. FIGURE 5-5: Lowering Oscillator Frequency. 5.5 Positive Voltage Multiplication Positive voltage multiplication can be obtained by employing two external diodes (Figure 5-6). Refer to the theory of operation of the TC7660 (Section 4.1 “Theory of Operation”). During the half cycle when switch S2 is closed, capacitor C1 of Figure 5-6 is charged up to a voltage of V+ - VF1, where VF1 is the forward voltage drop of diode D1. During the next half cycle, switch S1 is closed, shifting the reference of capacitor C1 from GND to V+. The energy in capacitor C1 is transferred to capacitor C2 through diode D2, pro- ducing an output voltage of approximately: EQUATION FIGURE 5-6: Positive Voltage Multiplier. 5.6 Combined Negative Voltage Conversion and Positive Supply Multiplication Simultaneous voltage inversion and positive voltage multiplication can be obtained (Figure 5-7). Capacitors C1 and C3 perform the voltage inversion, while capaci- tors C2 and C4, plus the two diodes, perform the posi- tive voltage multiplication. Capacitors C1 and C2 are the pump capacitors, while capacitors C3 and C4 are the reservoir capacitors for their respective functions. Both functions utilize the same switches of the TC7660. As a result, if either output is loaded, both outputs will drop towards GND. CMOS GATE 1k VOUT “1” + V+ + 1 2 3 4 8 7 6 5 TC7660 10 µF 10 µF V+ VOUT + + 1 2 3 4 8 7 6 5 TC7660 C1 C2 V+ COSC VOUT 2 V + VF1 VF2 + – = where: VF1 is the forward voltage drop of diode D1 and VF2 is the forward voltage drop of diode D2. + C2 D1 D2 + C1 VOUT = 1 2 3 4 8 7 6 5 TC7660 V+ (2 V+) - (2 VF) |
类似零件编号 - TC7660EOA |
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类似说明 - TC7660EOA |
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