TC682EPA datasheet(5/14 Pages) MICROCHIP

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 2002-2012 Microchip Technology Inc.

DS21453D-page 5

TC682

4.0

TYPICAL APPLICATIONS

4.1

Negative Doubling Converter

The most common application of the TC682 is as a

charge pump voltage converter which provides a

negative output of two times a positive input voltage

(Figure 4-1).

FIGURE 4-1:

Inverting Voltage Doubler

4.2

Capacitor Selection

The output resistance of the TC682 is determined, in

part, by the ESR of the capacitors used. An expression

for ROUT is derived as shown below:

Assuming all switch resistances are approximately

equal:

ROUT is typically 140 at +25°C with VIN = +5V and 3.3

F low ESR capacitors. The fixed term (16R

SW) is

about 80-90

. It can be seen easily that increasing or

decreasing values of C1 and C2 will affect efficiency by

changing ROUT. However, be careful about ESR. This

term can quickly become dominant with large electro-

lytic capacitors. Table 4-1 shows ROUT for various

values of C1 and C2 (assume 0.5

 ESR). C1 must be

rated at 6VDC or greater while C2 and C3 must be

rated at 12VDC or greater.

Output voltage ripple is affected by C3. Typically the

larger the value of C3 the less the ripple for a given load

current. The formula for P-P VRIPPLE is given below:

VRIPPLE = {1/[2(fPUMP x C3)] + 2(ESRC3)} (IOUT)

For a 10

F (0.5 ESR) capacitor for C3, f

PUMP = 10

kHz and IOUT = 10 mA the peak-to-peak ripple voltage

at the output will be less then 60 mV. In most

applications (IOUT < = 10 mA) a 10-20 F capacitor and

1-5

F pump capacitors will suffice. Table 4-2 shows

VRIPPLE for different values of C3 (assume 1 ESR).

TABLE 4-1:

OUTPUT RESISTANCE

VS. C1, C2

TABLE 4-2:

VRIPPLE PEAK-TO-PEAK

VS. C3 (IOUT 10mA)

GND

TC682

μF

VIN

C1+

C2+

C1–

C2–

V–OUT

VIN

ROUT =2(RSW1 +RSW2 +ESRC1 +RSW3 +RSW4 +ESRC2)

+2(RSW1 +RSW2 +ESRC1 +RSW3 +RSW4 +ESRC2)

+1/(fPUMP x C1) +1/(fPUMP x C2)

+ESRC3

ROUT = 16RSW + 4ESRC1 + 4ESRC2 + ESRC3

+1/(fPUMP x C1) +1/(fPUMP x C2)

C1, C2 (

F)

ROUT()

0.05

4085

0.10

2084

0.47

510

1.00

285

3.30

145

5.00

125

10.00

105

22.00

100.00

C3 (

F)

VRIPPLE (mV)

0.50

1020

1.00

520

3.30

172

5.00

120

10.00

22.00

100.00

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