TC7662AMJA datasheet(7/14 Pages) MICROCHIP

7 / 14 page

 2002 Microchip Technology Inc.

DS21468B-page 7

TC7662A

4.2

Output Ripple

ESR also affects the ripple voltage seen at the output.

The total ripple is determined by 2 voltages, A and B,

as shown in Figure 4-2. Segment A is the voltage drop

across the ESR of CR at the instant it goes from being

charged by CP (current flowing into CR) to being dis-

charged through the load (current flowing out of CR).

The magnitude of this current change is 2 x IOUT, hence

the total drop is 2 x IOUT x ESRCR volts. Segment B is

the voltage change across CR during time t2, the half of

the cycle when CR supplies current to the load. The

drop at B is IOUT x t2/CR volts. The peak-to-peak ripple

voltage is the sum of these voltage drops:

FIGURE 4-2:

OUTPUT RIPPLE

4.3

Paralleling Devices

Any number of TC7662A voltage converters may be

paralleled to reduce output resistance (Figure 4-3).

The reservoir capacitor, CR, serves all devices, while

each device requires its own pump capacitor, CP. The

resultant output resistance would be approximately:

4.4

Cascading Devices

The TC7662A may be cascaded as shown (Figure 4-4)

to produce larger negative multiplication of the initial

supply voltage. However, due to the finite efficiency of

each device, the practical limit is 10 devices for light

loads. The output voltage is defined by:

VOUT = – n (VIN)

where n is an integer representing the number of

devices cascaded. The resulting output resistance

would be approximately the weighted sum of the

individual TC7662A ROUT values.

FIGURE 4-3:

PARALLELING DEVICES LOWERS OUTPUT IMPEDANCE

FIGURE 4-4:

INCREASED OUTPUT VOLTAGE BY CASCADING DEVICES

2 x fPUMP x CR

VRIPPLE ≅ (

+ 2 x ESRCR x IOUT)

-(VDD)

ROUT =

ROUT (of TC7662A)

n (number of devices)

TC7662A

VDD

TC7662A

"n"

"1"

VDD

µF

"n"

"1"

µF

OUT*

TC7662A

*VOUT = -nVDD

µF

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