TCM828ECT datasheet(14/26 Pages) MICROCHIP

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TCM828/TCM829

DS21488B-page 14

5.0.3

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. (See Equation 5-1).

Table 5-1 shows various values of C1 and the

corresponding output resistance values @ +25°C. It

assumes a 0.1

Ω ESR

C1 and 2Ω RSW. Table 5-2 shows

the output voltage ripple for various values of C2. The

VRIPPLE values assume 10 mA output load current and

0.1

Ω ESR

C2.

5.0.4

INPUT SUPPLY BYPASSING

The VIN input should be capacitively bypassed to

reduce AC impedance and minimize noise effects due

the

switching

internal

the

device.

The

recommended capacitor depends on the configuration

of the TCM828/TCM829 devices.

If the device is loaded from OUT to GND, it is

recommended 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 is sufficient.

5.0.5

VOLTAGE INVERTER

The most common application for charge pump

devices is the inverter (Figure 5-3). 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 5-1 and Table 5-1 for capacitor

selection.

FIGURE 5-3:

Test Circuit.

5.0.6

CASCADING DEVICES

Two or more TCM828/829 devices can be cascaded to

increase output voltage (Table 5-4). 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.

FIGURE 5-4:

Cascading TCM828 or

TCM829 Devices to Increase Output Voltage.

TABLE 5-1:

OUTPUT RESISTANCE VS. C1

(ESR = 0.1

Ω)

C1 (µF)

TCM828 ROUT (Ω)

TCM829 ROUT (Ω)

0.1

850

302

100

3.3

100

TABLE 5-2:

OUTPUT VOLTAGE RIPPLE

VS. C2 (ESR = 0.1

Ω) IOUT

10MA

C2 (µF)

TCM828 VRIPPLE

(mV)

TCM829 ROUT (Ω)

835

286

3.3

254

100

3.3 µF*

VOUT

C1-

OUT

C1+

GND

Voltage Inverter

*10 µF (TCM828)

V+IN

VOUT

VOUT =-nVIN

"1"

"n"

. . .

TCM828/

TCM829

TCM828/

TCM829

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