Datasheet ICL7660S, ICL7660A (Renesas) - 9

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Typical Applications
Equation 4 shows a typical application where fOSC = 10kHz and C = C1 = C2 = 10µF:
Simple Negative Voltage Converter
The majority of applications will undoubtedly utilize the 1 R  + -------------------------- + 4xESR + ICL7660S and ICL7660A for generation of negative supply 0 2x23 C1 ESRC2 5 103   10  10–6 (EQ. 4) voltages. Figure 15 shows typical connections to provide a R  + +  ESR negative supply where a positive supply of +1.5V to +12V is 0 46 20 5 C available. Keep in mind that pin 6 (LV) is tied to the supply Since the ESRs of the capacitors are reflected in the output negative (GND) for supply voltage below 3.5V. impedance multiplied by a factor of 5, a high value could potentially swamp out a low 1/fPUMP x C1 term, rendering an
V+
increase in switching frequency or filter capacitance ineffective. Typical electrolytic capacitors may have ESRs as high as 10.
1 8 10µF 2 7 Output Ripple ICL7660S + ICL7660A
ESR also affects the ripple voltage seen at the output. The
3 6 - RO VOUT
peak-to-peak output ripple voltage is given by Equation 5:
4 5 -
 1 V --------------------- + 2ESR    (EQ. 5)
- VOUT = -V+ V+
RIPPLE    C2 IOUT
+
2 fPUMP C2
10µF +
A low ESR capacitor will result in a higher performance output.
15A. 15B. Paralleling Devices FIGURE 15. SIMPLE NEGATIVE CONVERTER AND ITS
Any number of ICL7660S and ICL7660A voltage converters
OUTPUT EQUIVALENT
may be paralleled to reduce output resistance. The reservoir The output characteristics of the circuit in Figure 15 can be capacitor, C2, serves all devices, while each device requires its approximated by an ideal voltage source in series with a own pump capacitor, C1. The resultant output resistance is resistance as shown in Figure 15B. The voltage source has a approximated in Equation 6: value of -(V+). The output impedance (RO) is a function of the R ON resistance of the internal MOS switches (shown in Figure R OUTof ICL7660S = ----------------------------- (EQ. 6) OUT nnumber of devices 14), the switching frequency, the value of C1 and C2, and the ESR (equivalent series resistance) of C1 and C2. A good first
Cascading Devices
order approximation for RO is shown in Equation 2: The ICL7660S and ICL7660A may be cascaded as shown to R    +   0 2 RSW1 + RSW3 + ESRC1 2 RSW2 + RSW4 + ESRC1 produce larger negative multiplication of the initial supply voltage. However, due to the finite efficiency of each device, 1 ---------------- + ESR the practical limit is 10 devices for light loads. The output C2 (EQ. 2) f  PUMP C1 voltage is defined as shown in Equation 7: f f OSC = -------   (EQ. 7) PUMP R = V = –   2 SWX MOSFET Switch Resistance OUT n VIN where n is an integer representing the number of devices Combining the four RSWX terms as RSW, we see in Equation 3 cascaded. The resulting output resistance would be that: approximately the weighted sum of the individual ICL7660S 1 R  + ---------------- + 4xESR + and ICL7660A R (EQ. 3) OUT values. 0 2xRSW f  C1 ESRC2 PUMP C1
Changing the ICL7660S and ICL7660A Oscillator
RSW, the total switch resistance, is a function of supply voltage
Frequency
and temperature (see the output source resistance graphs, It may be desirable in some applications, due to noise or other Figures 2, 3, and 11), typically 23 at +25°C and 5V. Careful considerations, to alter the oscillator frequency. This can be selection of C1 and C2 will reduce the remaining terms, achieved simply by one of several methods. minimizing the output impedance. High value capacitors will reduce the 1/(f By connecting the Boost Pin (Pin 1) to V+, the oscillator charge PUMP x C1) component, and low ESR capacitors will lower the ESR term. Increasing the oscillator frequency will and discharge current is increased and, hence, the oscillator reduce the 1/(f frequency is increased by approximately 3.5 times. The result PUMP x C1) term, but may have the side effect of a net increase in output impedance when C is a decrease in the output impedance and ripple. This is of 1 > 10µF and is not long enough to fully charge the capacitors every cycle. major importance for surface mount applications where capacitor size and cost are critical. Smaller capacitors, such as FN3179 Rev 7.01 Page 9 of 14 Feb 10, 2020