TC1240 datasheet(8/16 Pages) MICROCHIP

8 / 16 page

TC1240/TC1240A

DS21516C-page 8

 2003 Microchip Technology Inc.

The switching frequency in Equation 5-1b is defined as

one-half the oscillator frequency (i.e., FSW = FOSC/2).

The 1/(FSW)(C1) term in Equation 5-1b is the effective

output resistance of an ideal switched capacitor circuit

(Figure 5-1 and Figure 5-2).

The output voltage ripple is given by Equation 5-2.

EQUATION 5-2:

FIGURE 5-1:

Ideal Switched Capacitor

Model.

FIGURE 5-2:

Equivalent Output

Resistance.

5.3

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 5-1b).

Table 5-1 shows various values of C1 and the

corresponding output resistance values @ +25°C. It

assumes a 0.1

Ω ESRC1 and 0.9Ω RSW. Table 5-2

shows the output voltage ripple for various values of

C2. The VRIPPLE values assume 5mA output load

current and 0.1

Ω ESRC2.

TABLE 5-1:

OUTPUT RESISTANCE

VS. C1 (ESR = 0.1Ω)

TABLE 5-2:

OUTPUT VOLTAGE RIPPLE

VS. C2 (ESR = 0.1Ω)

IOUT 5mA

VRIPPLE

IOUT

2FSW

() C2

()

-------------------------------- 2IOUT

() ESRC2

()

V+

VOUT

FSW x C1

V+

VOUT

REQUIV

REQUIV =

C1 (µF)

TC1240

ROUT(Ω)

TC1240A

ROUT(Ω)

0.47

28.5

20.5

2.2

19.5

3.3

4.7

15.5

10.5

13.6

9.3

12.5

8.3

100

12.2

8.1

C1 (µF)

TC1240/TC1240A

VRIPPLE (mV)

0.47

142

167

2.2

3.3

4.7

6.7

2.5

100

1.6

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