 2001-2012 Microchip Technology Inc.

DS21484D-page 5

TC962

3.0

APPLICATIONS INFORMATION

3.1

Theory of Operation

The TC962 is a capacitive pump (sometimes called a

switched capacitor circuit), where four MOSFET

switches control the charge and discharge of a

capacitor.

The functional block diagram shows how the switching

action works. SW1 and SW2 are turned on simultane-

assumes that the on resistance of the MOSFETs in

series with the capacitor results in a charging time

(3 time constants) that is less than the on time provided

by the oscillator frequency as shown:

In the next cycle, SW1 and SW2 are turned off and after

a very short interval of all switches being off (this

prevents large currents from occurring due to cross

conduction), SW3 and SW4 are turned on. The charge

inverted. In this way, a negative voltage is now derived.

An oscillator supplies pulses to a flip-flop that is then

fed to a set of level shifters. These level shifters then

drive each set of switches at one-half the oscillator

frequency.

The oscillator has two pins that control the frequency of

oscillation. Pin 7 can have a capacitor added that is

returned to ground. This will lower the frequency of the

oscillator by adding capacitance to the timing capacitor

internal to the TC962. Grounding pin 6 will turn on a

current source and double the frequency. This will

double the charge current going into the internal

capacitor, as well as any capacitor added to pin 7.

A Zener diode has been added to the TC962 for use as

a reference in building external regulators. This Zener

runs from pin 1 to ground.

3.2

Latch-Up

All CMOS structures contain a parasitic SCR. Care

must be taken to prevent any input from going above or

below the supply rail, or latch-up will occur. The result

Unless the power supply input has a current limit, this

latch-up phenomena will result in damage to the

device. (See AN763, Latch-up Protection for MOSFET

Drivers.)

FIGURE 3-1:

Test Circuit

FIGURE 3-2:

Typical Applications

TC962

1

2

3

4

8

690

7

5

+

10

μF

(–5V)

10

μF

V

(+5V)

+

NC

+

1

2

3

TC962

4

8

7

6

5

10

μF

V+

+

+

+

10

μF

+

10

μF

Combined Negative Converter and Positive Multiplier

+

10

μF

+

1

2

3

4

8

7

6

5

10

μF

+

V

2

+

Split V+ In Half

TC962

Positive Voltage Multiplier

Lowering Output Resistance by Paralleling Devices

TC962

1

2

3

4

8

7

6

5

10

μF

10

μF

+

+

+

V+

10

μF

+

10

μF

+

TC962

TC962

1

2

3

4

8

7

6

5

1

2

3

4

8

7

6

5

+

V+

10

μF

V+