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Typical Applications

Simple Negative Voltage Converter

1

2

3

4

8

7

6

5

+

TL7660

+

-

-

10 µF

V+

10 µF

V

= –V+

OUT

TL7660

CMOS VOLTAGE CONVERTER

SCAS794 – JUNE 2006

APPLICATION INFORMATION (continued)

The majority of applications will undoubtedly utilize the TL7660 for generation of negative supply voltages.

Figure 3 shows typical connections to provide a negative supply negative (GND) for supply voltages below

3.5 V.

Figure 3. Simple Negative-Voltage Converter

The output characteristics of the circuit in Figure 3 can be approximated by an ideal voltage source in series with

a resistance. The voltage source has a value of –VCC. The output impedance (RO) is a function of the ON

resistance of the internal MOS switches (shown in Figure 2), 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:

RO ≈ 2(RSW1 + RSW3 + ESRC1) + 2(RSW2 + RSW4 + ESRC1)

RO ≈ 2(RSW1 + RSW3 + ESRC1) + 1/fPUMPC1 + ESRC2

Where fPUMP = fOSC/2 , RSWX = MOSFET switch resistance.

Combining the four RSWX terms as RSW, we see that:

RO ≈ 2 (RSW) + 1/fPUMPC1 + 4 (ESRC1) + ESRC2

RSW, the total switch resistance, is a function of supply voltage and temperature (See the Output Source

Resistance graphs). Careful selection of C1 and C2 reduces the remaining terms, minimizing the output

impedance. High value capacitors reduce the 1/fPUMPC1 component, and low ESR capacitors lower the ESR

term. Increasing the oscillator frequency reduces the 1/fPUMPC1 term but may have the side effect of a net

increase in output impedance when C1 > 10 µF and there is no longer enough time to fully charge the capacitors

every cycle. In a typical application where fOSC = 10 kHz and C = C1 = C2 = 10 µF:

RO ≈ 2(23) + 1/(5 × 10

3)(10–5) + 4(ESR

C1) + ESRC2

RO ≈ 46 + 20 + 5 (ESRC)

Because 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/fPUMPC1 term, rendering an increase in switching frequency or filter

capacitance ineffective. Typical electrolytic capacitors may have ESRs as high as 10

Ω.

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