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TC7662A Arkusz danych(PDF) 8 Page - Microchip Technology |
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TC7662A Arkusz danych(HTML) 8 Page - Microchip Technology |
8 / 16 page TC7662A DS21468B-page 8 2001-2012 Microchip Technology Inc. 4.5 Changing the TC7662A Oscillator Frequency It is possible to increase the conversion efficiency of the TC7662A at low load levels by lowering the oscillator frequency. This reduces the switching losses, and is shown in Figure 4-5. However, lowering the oscillator frequency will cause an undesirable increase in the impedance of the pump (CP) and reservoir (CR) capacitors; this is overcome by increasing the values of CP and CR by the same factor that the frequency has been reduced. For example, the addition of a 100pF capacitor between pin 7 (OSC) and VDD will lower the oscillator frequency to 2kHz from its nominal frequency of 12kHz (multiple of 6), and thereby necessitate a corresponding increase in the value of CP and CR (from 10 F to 68F). FIGURE 4-5: LOWERING OSCILLATOR FREQUENCY 4.6 Positive Voltage Doubling The TC7662A may be employed to achieve positive voltage doubling using the circuit shown in Figure 4-6. In this application, the pump inverter switches of the TC7662A are used to charge CP to a voltage level of VDD – VF (where VDD is the supply voltage and VF is the forward voltage on CP plus the supply voltage (VDD) applied through diode D2 to capacitor CR). The voltage thus created on CR becomes (2 VDD) – (2 VF), or twice the supply voltage minus the combined forward voltage drops of diodes D1 and D2. The source impedance of the output (VOUT) will depend on the output current, but for VDD = 5V and an output current of 10 mA, it will be approximately 60 . FIGURE 4-6: POSITIVE VOLTAGE MULTIPLIER 4.7 Combined Negative Voltage Conversion and Positive Supply Multiplication Figure 4-7 combines the functions shown in Figure 4-1 and Figure 4-6 to provide negative voltage conversion and positive voltage doubling simultaneously. This approach would be, for example, suitable for generat- ing +9V and -5V from an existing +5V supply. In this instance, capacitors C1 and C3 perform the pump and reservoir functions, respectively, for the generation of the negative voltage, while capacitors C2 and C4 are pump and reservoir, respectively, for the doubled positive voltage. There is a penalty in this configuration which combines both functions, however, in that the source impedances of the generated supplies will be somewhat higher due to the finite impedance of the common charge pump driver at pin 2 of the device. FIGURE 4-7: COMBINED NEGATIVE CONVERTER AND POSITIVE DOUBLER 4.8 Voltage Splitting The same bidirectional characteristics can be used to split a higher supply in half, as shown in Figure 4-8. The combined load will be evenly shared between the two sides. Because the switches share the load in parallel, the output impedance is much lower than in the standard circuits, and higher currents can be drawn from the device. By using this circuit, and then the circuit of Figure 4-4, +15V can be converted (via +7.5V and -7.5V) to a nominal -15V, though with rather high series resistance (~250 ). FIGURE 4-8: SPLITTING A SUPPLY IN HALF 1 2 3 4 8 7 6 5 + VOUT COSC + TC7662A 10 μF 10 μF VDD 1 2 3 4 8 7 6 5 VOUT = (2 VDD) – (2 VF) + CR D1 D2 + CP TC7662A VDD 1 2 3 4 8 7 6 5 + VDD VOUT = (2 VDD) – (2 VF) C1 D1 + + C3 C4 VOUT = -(VDD – VF) C2 TC7662A D2 + + RL1 RL2 VOUT = VDD – V – 2 50 μF 50 μF VDD V – 50 μF + 1 2 8 7 TC7662A 3 4 6 5 + – – – |
Podobny numer części - TC7662A_13 |
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Podobny opis - TC7662A_13 |
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