4

TCM828/829-4

5/22/00

Switched Capacitor

Voltage Converters

TCM828

TCM829

© 2001 Microchip Technology Inc.

DS21488A

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. (See Equation 1(b)).

Table 1 shows various values of C1 and the correspond-

ing output resistance values @ +25

°C. It assumes a 0.1Ω

output load current and 0.1

Table 1. Output Resistance vs. C1 (ESR = 0.1

C1(

0.1

850

302

1

100

45

3.3

42

25

10

25

19

47

18

17

100

17

17

Table 2. Output Voltage Ripple vs. C2 (ESR = 0.1

C2(

1

835

286

3.3

254

88

10

85

31

47

20

8

100

10

5

Input Supply Bypassing

AC impedance and minimize noise effects due to the switch-

ing internal to the device. The recommended capacitor

depends on the configuration of the TCM828/829.

If the device is loaded from OUT to GND it is recom-

mended that a large value capacitor (at least equal to C1) be

connected from the input to GND. If the device is loaded

from IN to OUT a small (0.1

µF) capacitor from IN to OUT is

sufficient.

Voltage Inverter

The most common application for charge pump devices

is the inverter (Figure 3). This application uses two external

capacitors – C1 and C2 (plus a power supply bypass

capacitor, if necessary). The output is equal to V–

voltage drops due to loading. Refer to Table 1 and Table 2

for capacitor selection.

Cascading Devices

Two or more TCM828/829’s can be cascaded to

increase output voltage (Figure 4). If the output is lightly

can be seen that the output resistance rises rapidly for

multiple cascaded devices. For large negative voltage

requirements see the TC682 or TCM680 data sheets.

Paralleling Devices

can be connected in parallel (Figure 5). The output resis-

tance will be reduced by a factor of N where N is the number

of TCM828/829’s. Each device will require it’s own pump

capacitor (C1), but all devices may share one reservoir

capacitor (C2). However, to preserve ripple performance the

value of C2 should be scaled according to the number of

paralleled TCM828/829’s.

Figure 3. Test Circuit

C1

C1

C2

5

5

4

3

4

1

2

2

1

3

C2

V

IN

V

OUT

V

+

TCM828

TCM829

"1"

TCM828

TCM829

"n"

. . .

. . .

Figure 4. Cascading TCM828s or TCM829s to Increase Output Voltage

3

2

4

5

1

C3

3.3

µF*

C1

3.3

µF*

C2

3.3

µF*

V

IN

V

OUT

R

L

TCM828

TCM829

C1–

IN

OUT

C1+

GND

Voltage Inverter

*10

µF (TCM828)