TC828A datasheet(4/8 Pages) TELCOM

4 / 8 page

Switched Capacitor

Voltage Converters

TC828A

TC828A-1 5/1/00

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

ESRC1 and 2Ω RSW. Table 2 shows the output voltage ripple

for various values of C2. The VRIPPLE values assume 10mA

output load current and 0.1

Ω ESRC2.

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

Ω)

C1(

µF)

ROUT (

Ω)

0.1

850

100

3.3

18.3

100

17.3

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

Ω) I

OUT 10mA

C2(

µF)

VRIPPLE (mV)

835

3.3

254

19.7

100

10.3

Input Supply Bypassing

The VIN input should be capacitively bypassed to reduce

AC impedance and minimize noise effects due to the switch-

ing internal to the device. The recommended capacitor

depends on the configuration of the TC828A.

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–

IN plus any

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

for capacitor selection.

Cascading Devices

Two or more TC828As can be cascaded to increase

output voltage (Figure 4). If the output is lightly loaded, it will

be close to (– 2 x VIN) but will droop at least by ROUT of the

first device multiplied by the IQ of the second. It 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

To reduce the value of ROUT, multiple TC828As can be

connected in parallel (Figure 5). The output resistance will

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

TC828As. 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

TC828As.

Figure 3. Test Circuit

OUT

OUT = –nVIN

TC828

“1”

TC828

“N”

. . .

Figure 4. Cascading TC828As to Increase Output Voltage

µF*

OUT

TC828

C1–

OUT

C1+

GND

Voltage Inverter

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