© 2005 Microchip Technology Inc.

DS21537C-page 5

TC1682/TC1683/TC1684

4.0

APPLICATIONS INFORMATION

4.1

Output Voltage Considerations

The

TC1682/TC1683/TC1684

perform

inverting

voltage conversions but do not provide any type of

regulation. The output voltage will droop in a linear

manner with respect to the output load current. The

value

of

the

equivalent

output

resistance

is

approximately 120

In this particular case, the output is approximately -10V

at very light loads and will droop according to the

equation below:

4.2

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

and C2 will lower the output resistance and larger

Note:

For proper charge pump operation, C1

must have a voltage rating greater than or

a voltage rating greater than or equal to

Table 4-1 shows various values of C1/C2 and the

@ +25°C.

Table 4-2 shows the output voltage ripple for various

and a 0.1

TABLE 4-1:

OUTPUT RESISTANCE

VS. C1/C2 (ESR = 0.1

Ω)

TABLE 4-2:

OUTPUT VOLTAGE RIPPLE

4.3

Input Supply Bypassing

reduce AC impedance and minimize noise effects due

to

the

switching

internal

to

the

device.

It

is

recommended that a large value capacitor (at least

circuit performance.

4.4

Shutdown Input

The TC1682/TC1683/TC1684 is enabled when SHDN

is low, and disabled when SHDN is high. This input

cannot be allowed to float. (If SHDN is not required, see

the TC2682/TC2683/TC2684 data sheet.) The SHDN

4.5

Inverting Voltage Doubler

The most common application for the TC1682/TC1683/

TC1684 devices is the inverting voltage doubler

(Figure 4-1). This application uses three external

Note:

A

power

supply

bypass

capacitor

is

recommended.

to loading. Refer to Table 4-1 and Table 4-2 for

capacitor selection guidelines.

FIGURE 4-1:

INVERTING VOLTAGE

DOUBLER TEST CIRCUIT

4.6

Layout Considerations

As with any switching power supply circuit, good layout

practice is recommended. Mount components as close

together as possible to minimize stray inductance and

capacitance. Also use a large ground plane to minimize

noise leakage into other circuitry.

C1, C2

(

μF)

TC1682

TC1683

TC1684

0.33

633

184

120

1

262

120

102

3.3

120

95

84

(

μF)

TC1682

(mV)

TC1683

(mV)

TC1684

(mV)

0.33

192

60

27

163

21

16

3.3

17

8

7

C1

C2

TC1682

TC1683

TC1684

C1–

C2–

2

1

7

6

8

4

5

3

C1+

C2+

GND

+

+

+

+

Device

C1

C2

TC1682

3.3

μF

3.3

μF

3.3

μF

3.3

μF

TC1683

1

μF

1

μF

1

μF

1

μF

TC1684

0.33

μF

0.33

μF

0.33

μF

0.33

μF

SHDN