TCM680 datasheet(4/7 Pages) TELCOM

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4-16

TELCOM SEMICONDUCTOR, INC.

TCM680

+5V TO

±10V VOLTAGE CONVERTER

EFFICIENCY CONSIDERATIONS

Theoretically a charge pump can approach 100% effi-

ciency under the following conditions:

• The charge Pump switches have virtually no offset

and extremely low on resistance

• Minimal power is consumed by the drive circuitry

• The impedances of the reservoir and pump capaci-

tors are negligible

For the TCM680, efficiency is as shown below:

Efficiency V

+ = V

DD /(2VIN)

VDD = 2VIN – V

DROP

DROP = (I

OUT)(R

OUT)

Efficiency V

– = V

SS /(– 2VIN)

VSS = 2VIN – V

–

DROP

–

DROP = (I

–

OUT)(R

–

OUT)

Power Loss =(V

DROP)(I

OUT) + (V

–

DROP)(I

–

OUT)

There will be a substantial voltage difference between

OUT – VIN) and VIN for the positive pump and between

OUT and V –

OUT if the impedances of the pump capacitors C1

and C2 are high with respect to the output loads.

Larger values of reservoir capacitors C3 and C4 will

reduce output ripple. Larger values of both pump and

reservoir capacitors improve the efficiency. See "Capacitor

Selection" in Applications Section.

APPLICATIONS

Positive and negative Converter

The most common application of the TCM680 is as a

dual charge pump voltage converter which provides positive

and negative outputs of two times a positive input voltage.

The simple circuit of Figure 6 performs this same function

using the TCM680 and external capacitors, C1, C2, C3 and C4.

Figure 6. Positive and Negative Converter

VOUT

GND

TCM680

µF

VOUT

–

Capacitor Selection

The TCM680 requires only 4 external capacitors for

operation. These can be inexpensive polarized aluminum

electrolytic types. For the circuit in Figure 6 the output

characteristics are largely determined by the external

capacitors. An expression for ROUT can be derived as shown

below:

OUT = 4(RSW1 + RSW2 + ESRC1 + RSW3 + RSW4 + ESRC2)

+4(RSW1 + RSW2 + ESRC1 + RSW3 + RSW4 + ESRC2)

+1/(fPUMP x C1) + 1/(fPUMP x C2) + ESRC4

–

OUT = 4(RSW1 + RSW2 + ESRC1 + RSW3 + RSW4 + ESRC2)

+4(RSW1 + RSW2 + ESRC1 + RSW3 + RSW4+ ESRC2)

+1/(fPUMP x C1) + 1/(fPUMP x C2) + ESRC3

Assuming all switch resistances are approximately

equal...

OUT = 32RSW + 8ESRC1 + 8ESRC2 + ESRC4

+1/(fPUMP x C1) + 1/(fPUMP x C2)

–

OUT = 32RSW + 8ESRC1 + 8ESRC2 + ESRC3

+1/(fPUMP x C1) + 1/(fPUMP x C2)

ROUT is typically 140Ω at +25°C with VIN = +5V and C1

and C2 as 4.7

µF low ESR capacitors. The fixed term

(32RSW) is about 130Ω. It can be seen easily that increasing

or decreasing values of C1 and C2 will affect efficiency by

changing ROUT. However, be careful about ESR. This term

can quickly become dominant with large electrolytic capaci-

tors. Table 1 shows ROUT for various values of C1 and C2

(assume 0.5

Ω ESR). C1 and C4 must be rated at 6VDC or

greater while C2 and C3 must be rated at 12VDC or greater.

Output voltage ripple is affected by C3 and C4. Typically

the larger the value of C3 and C4 the less the ripple for a

given load current. The formula for VRIPPLE(p-p) is given

below:

V+RIPPLE(p-p) = {1/[2(fPUMP /3) x C4] + 2(ESRC4)}(I

OUT)

V–RIPPLE(p-p) = {1/[2(fPUMP /3) x C3] + 2(ESRC3)}(I

–

OUT)

For a 10

µF (0.5Ω ESR) capacitor for C3, C4,

fPUMP = 21kHz and IOUT = 10mA the peak-to-peak ripple

voltage at the output will be less than 100mV. In most

applications (IOUT < = 10mA) 10-20µF output capacitors and

1-5

µF pump capacitors will suffice. Table 2 shows VRIPPLE

for different values of C3 and C4 (assume 1

Ω ESR).

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