TC682 datasheet(3/6 Pages) TELCOM

3 / 6 page

4-23

TELCOM SEMICONDUCTOR, INC.

Phase 2

VSS transfer – phase two of the clock connects the

negative terminal of C2 to the negative side of reservoir

capacitor C3 and the positive terminal of C2 to ground,

transferring the generated – 10V to C3. Simultaneously, the

positive side of capacitor C1 is switched to +5V and the

negative side is connected to ground. C2 is then switched to

VCC and GND and Phase 1 begins again.

Figure 3. Charge Pump – Phase 2

DETAILED DESCRIPTION

Phase 1

VSS charge storage – before this phase of the clock

cycle, capacitor C1 is already charged to +5V. C1

+ is then

switched to ground and the charge in C1

– is transferred to C2–

. Since C2

+ is at +5V, the voltage potential across capacitor

C2 is now –10V.

Figure 2. Charge Pump – Phase 1

MAXIMUM OPERATING LIMITS

The TC682 has on-chip zener diodes that clamp VIN to

approximately 5.8V, and V

–

OUT to – 11.6V. Never exceed the

maximum supply voltage or excessive current will be shunted

by these diodes, potentially damaging the chip. The TC682

will operate over the entire operating temperature range with

an input voltage of 2V to 5.5V.

EFFICIENCY CONSIDERATIONS

Theoretically a charge pump voltage multiplier can

approach 100% efficiency under the following conditions:

• The charge pump switches have virtually no offset

and are 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 TC682, efficiency is as shown below:

Voltage Efficiency = VOUT / (– 2VIN)

VOUT = – 2VIN + VDROP

VDROP = (IOUT) (ROUT)

Power Loss

= IOUT (VDROP)

There will be a substantial voltage difference between

–

OUT and 2 VIN if the impedances of the pump capacitors

C1 and C2 are high with respect to their respective output

loads.

Larger values of reservoir capacitor C3 will reduce

output ripple. Larger values of both pump and reservoir

capacitors improve the efficiency. See "Capacitor Selec-

tion" in Applications section.

APPLICATIONS

Negative Doubling Converter

The most common application of the TC682 is as a

charge pump voltage converter which provides a negative

output of two times a positive input voltage (Figure 4).

Figure 4. Inverting Voltage Doubler

IN = +5V

OUT

–5V

SW4

SW1

SW2

SW3

–

+5V

OUT

–10V

SW4

SW2

SW1

SW3

–

VOUT

GND

TC682

µF

VOUT

–

INVERTING VOLTAGE DOUBLER

TC682

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