 2002 Microchip Technology Inc.

DS21367B-page 5

TC1221/TC1222

4.0

APPLICATIONS INFORMATION

4.1

Output Voltage Considerations

The TC1221/TC1222 perform voltage conversion but

do not provide regulation. The output voltage will droop

in a linear manner with respect to load current. The

value of this equivalent output resistance is approxi-

mately 25

approximately -5V at light loads, and droops according

to the equation below:

4.2

Charge Pump Efficiency

The overall power efficiency of the charge pump is

affected by four factors:

1.

Losses from power consumed by the internal

oscillator, switch drive, etc. (which vary with

input

voltage,

temperature

and

oscillator

frequency).

2.

MOSFET switches on-board the charge pump.

3.

Charge pump capacitor losses due to effective

series resistance (ESR).

4.

Losses that occur during charge transfer (from

the

commutation

capacitor

to

the

output

capacitor) when a voltage difference between

the two capacitors exists.

Most of the conversion losses are due to factors (2) and

(3) above. These losses are given by Equation 4-1(b).

EQUATION 4-1:

effective output resistance of an ideal switched

capacitor circuit (Figure 4-1 and Figure 4-2). The value

TC1221/TC1222.

The remaining losses in the circuit are due to factor (4)

above, and are shown in Equation 4-2. The output

voltage ripple is given by Equation 4-3.

EQUATION 4-2:

EQUATION 4-3:

FIGURE 4-1:

IDEAL SWITCHED

CAPACITOR MODEL

FIGURE 4-2:

EQUIVALENT OUTPUT

RESISTANCE

4.3

Capacitor Selection

In order to maintain the lowest output resistance and

output ripple voltage, it is recommended that low ESR

capacitors be used. Additionally, larger values of C1

will lower the output resistance and larger values of

C2 will reduce output ripple. (Equation 4-1(b) and

Equation 4-3).

OUT

OUT

V+

C2

C1

f

V+

C2

f x C1

1