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ICL7660S Datasheet(PDF) 9 Page - Intersil Corporation |
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ICL7660S Datasheet(HTML) 9 Page - Intersil Corporation |
9 / 13 page 9 FN3179.7 January 23, 2013 Typical Applications Simple Negative Voltage Converter The majority of applications will undoubtedly utilize the ICL7660S and ICL7660A for generation of negative supply voltages. Figure 15 shows typical connections to provide a negative supply where a positive supply of +1.5V to +12V is available. Keep in mind that pin 6 (LV) is tied to the supply negative (GND) for supply voltage below 3.5V. The output characteristics of the circuit in Figure 15 can be approximated by an ideal voltage source in series with a resistance as shown in Figure 15B. The voltage source has a value of -(V+). The output impedance (RO) is a function of the ON resistance of the internal MOS switches (shown in Figure 14), the switching frequency, the value of C1 and C2, and the ESR (equivalent series resistance) of C1 and C2. A good first order approximation for RO is shown in Equation 2: Combining the four RSWX terms as RSW, we see in Equation 3 that: RSW, the total switch resistance, is a function of supply voltage and temperature (see the output source resistance graphs, Figures 2, 3, and 11), typically 23 Ω at +25°C and 5V. Careful selection of C1 and C2 will reduce the remaining terms, minimizing the output impedance. High value capacitors will reduce the 1/(fPUMP x C1) component, and low ESR capacitors will lower the ESR term. Increasing the oscillator frequency will reduce the 1/(fPUMP x C1) term, but may have the side effect of a net increase in output impedance when C1 > 10µF and is not long enough to fully charge the capacitors every cycle. Equation 4 shows a typical application where fOSC = 10kHz and C = C1 = C2 = 10µF: 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 C1 term, rendering an increase in switching frequency or filter capacitance ineffective. Typical electrolytic capacitors may have ESRs as high as 10 Ω. Output Ripple ESR also affects the ripple voltage seen at the output. The peak-to-peak output ripple voltage is given by Equation 5: A low ESR capacitor will result in a higher performance output. Paralleling Devices Any number of ICL7660S and ICL7660A voltage converters may be paralleled to reduce output resistance. The reservoir capacitor, C2, serves all devices, while each device requires its own pump capacitor, C1. The resultant output resistance is approximated in Equation 6: Cascading Devices The ICL7660S and ICL7660A may be cascaded as shown to produce larger negative multiplication of the initial supply voltage. However, due to the finite efficiency of each device, the practical limit is 10 devices for light loads. The output voltage is defined as shown in Equation 7: where n is an integer representing the number of devices cascaded. The resulting output resistance would be approximately the weighted sum of the individual ICL7660S and ICL7660A ROUT values. Changing the ICL7660S and ICL7660A Oscillator Frequency It may be desirable in some applications, due to noise or other considerations, to alter the oscillator frequency. This can be achieved simply by one of several methods. By connecting the Boost Pin (Pin 1) to V+, the oscillator charge and discharge current is increased and, hence, the oscillator frequency is increased by approximately 3.5 times. The result is a decrease in the output impedance and ripple. 1 2 3 4 8 7 6 5 + - 10µF 10µF ICL7660S VOUT = -V+ V+ + - RO VOUT V+ + - 15A. 15B. FIGURE 15. SIMPLE NEGATIVE CONVERTER AND ITS OUTPUT EQUIVALENT ICL7660A R0 2RSW1 RSW3 ESRC1 ++ () 2R SW2 RSW4 ESRC1 ++ () + () ≅ (EQ. 2) 1 fPUMP C1 × -------------------------------- ESRC2 + fPUMP fOSC 2 -------------- = RSWX MOSFET Switch Resistance = () R0 2xRSW 1 fPUMP C1 × -------------------------------- 4xESRC1 ESRC2 ++ + ≅ (EQ. 3) R0 2x23 1 510 3 × 10 × 10 6 – × --------------------------------------------------- 4xESRC1 ESRC2 ++ + ≅ (EQ. 4) R0 46 20 5 ++ ESRC × ≅ VRIPPLE 1 2fPUMP × C2 × ----------------------------------------- 2ESRC2 IOUT × + ⎝⎠ ⎛⎞ ≅ (EQ. 5) ROUT ROUT of ICL7660S () n number of devices () --------------------------------------------------------- = (EQ. 6) VOUT nVIN () – = (EQ. 7) ICL7660S, ICL7660A |
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