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TC7662AEPA Arkusz danych(PDF) 6 Page - Microchip Technology |
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TC7662AEPA Arkusz danych(HTML) 6 Page - Microchip Technology |
6 / 14 page TC7662A DS21468B-page 6 2002 Microchip Technology Inc. 4.0 TYPICAL APPLICATIONS 4.1 Simple Negative Voltage Converter The majority of applications will undoubtedly utilize the TC7662A for generation of negative supply voltages. Figure 4-1 shows typical connections to provide a negative supply where a positive supply of +3V to +18V is available. FIGURE 4-1: SIMPLE NEGATIVE CONVERTER AND ITS OUTPUT EQUIVALENT The output characteristics of the circuit in Figure 4-1 are those of a nearly ideal voltage source in series with a resistance as shown in Figure 4-1b. The voltage source has a value of -(VDD). The output impedance (RO) is a function of the ON resistance of the internal MOS switches (shown in the Functional Block Diagram), the switching frequency, the value of CP and CR, and the ESR (equivalent series resistance) of CP and CR. A good first order approximation for RO is: Combining the four RSWX terms as RSW, we see that: RSW, the total switch resistance, is a function of supply voltage and temperature (See Section 5.0, Typical Characteristics “Output Source Resistance” graphs), typically 23 Ω at +25°C and 5V. Careful selection of CP and CR will reduce the remaining terms, minimizing the output impedance. High value capacitors will reduce the 1/(fPUMP x CP) component, and low ESR capacitors will lower the ESR term. Increasing the oscillator frequency will reduce the 1/(fPUMP x CP) term, but may have the side effect of a net increase in output impedance when CP > 10µF and there is not enough time to fully charge the capacitors every cycle. In a typ- ical application when fOSC = 12kHz and C = CP = CR = 10 µF: Since the ESRs of the capacitors are reflected in the output impedance multiplied by a factor of 5, a high value could potentially swamp out a low 1/(fPUMP x CP) term, rendering an increase in switching frequency or filter capacitance ineffective. Typical electrolytic capacitors may have ESRs as high as 10 Ω. 1 2 3 4 8 7 6 5 TC7662A 10 µF + VDD + 10 µF VOUT = -V+ – VOUT RO VDD VDD VDD VDD + – AB RO ≅ 2(RSW1 + RSW2 + ESRCP) + 2(RSW3 + RSW4 + ESRCP) + + ESRCR 1 fPUMP x CP (fPUMP = , RSWX = MOSFET switch resistance) fOSC 2 RO ≅ 2 x RSW + + 4 x ESRCP + ESRCRΩ 1 fPUMP x CP RO ≅ 2 x 23 + + 4 x ESRCP + ESRCR 1 (5 x 123 x 10 x 10-6) RO ≅ (46 + 20 + 5 x ESRC)Ω |
Podobny numer części - TC7662AEPA |
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Podobny opis - TC7662AEPA |
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