TC682_13 datasheet(4/14 Pages) MICROCHIP

4 / 14 page

TC682

DS21453D-page 4

 2002-2012 Microchip Technology Inc.

3.0

DETAILED DESCRIPTION

FIGURE 3-1:

TC682 Test Circuit

3.1

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 C

+ is at +5V, the voltage

potential across capacitor C2 is now -10V.

FIGURE 3-2:

Charge Pump – Phase 1

3.2

Phase 2

VSS transfer – phase two of the clock connects the neg-

ative 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 3-3:

Charge Pump – Phase 2

3.3

Maximum Operating Limits

The TC682 has on-chip Zener diodes that clamp VIN

to approximately 5.8V, and VOUT 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.

3.4

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

capacitors 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

VOUT and -2VIN if the impedances of the pump capaci-

tors 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 Section 4.2

“Capacitor Selection” “Capacitor Selection”.

(+5V)

GND

–

GND

VIN

All Caps = 3.3

μF

TC682

VOUT

C1+

VIN

C2+

C1–

C2–

COUT

V–OUT

–

VIN = +5V

VOUT

-5V

SW4

SW1

SW2

SW3

–

+5V

VOUT

-10V

SW4

SW2

SW1

SW3

–

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