3

CHARGE PUMP DC-TO-DC CONVERTER

TC7662A

TC7662A-5 9/11/96

© 2001 Microchip Technology Inc.

DS21468

Note one of its characteristics is ESR (equivalent series

resistance). This parasitic resistance winds up in series with

the load. Thus, both voltage and power conversion effi-

ciency are compromised if a low ESR capacitor is not used.

capacitors from typical ESR to low ESR types, the effective

converter output impedance changed from 45

Ω to 40Ω, an

improvement of 12%.

This applies to all types of capacitors, including film

types (polyester, polycarbonate etc.).

Some applications information suggests that the ca-

pacitor is not critical and attributes the limiting factor to the

capacitor's reactance value. Let's examine this:

where DS (duty cycle) = 50%.

For the TC7662A, f = 12,000Hz, and a typical value of

C would be 10

µF. This is a reactive impedance of ≈1.33Ω.

If the ESR is as great as 5

Ω, the reactive value is not as

critical as it would first appear, since the ESR would dominate.

The 5

Ω value is typical of a general-purpose electrolytic

capacitor.

Synchronizing

The TC7662A may be synchronized by connecting pin

7 of the TC7662A through a 100k resistor in series with a

diode to a negative-going pulse source. The negative pulse

voltage can be +5V with a 5 microsecond duration going

negative to 0V.

TEST CIRCUIT

Figure 2.

Synchronization

,

DS

APPLICATIONS INFORMATION

Theory of Operation

The TC7662A is a capacitive charge pump (some-

times called a switched-capacitor circuit), where four

MOSFET switches control the charge and discharge of a

capacitor.

The functional diagram (page 1) shows how the switch-

ing action works. SW1 and SW2 are turned on simulta-

assumes that the ON resistance of the MOSFETs in series

with the capacitor produce a charging time (3 time con-

stants) 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 with all switches OFF (preventing

large currents from occurring due to cross conduction),

SW3 and SW4 are turned ON. The charge in C1 is then

VERTED. In this way, a negative voltage is derived.

An oscillator supplies pulses to a flip-flop that is 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 a pin that controls the frequency of

oscillation. Pin 7 can have a capacitor added that is con-

nected to ground. This will lower the frequency of the

oscillator by adding capacitance to the internal timing ca-

page 5.)

Capacitors

In early charge pump converters, capacitors were not

switches. In order to understand this, let’s look at a model of

a typical electrolytic capacitor (Figure 1).

1

2

πf C

TC7662A

1

2

3

4

8

7

5

CP

+

10

µF

COSC

R L

V

(–5V)

OUT

10

µF

C R

I L

IS

V

(+5V)

+

NC

NC

6

+

TTL

Q

Q

100 k

TO PIN 7

TC7662A

Figure 1.

Capacitor Equivalent Circuit

EPR

ESL

ESR

C