TC1240

4

TC1240-1 7/7/00 DS21333A

Positive Doubling Charge Pump with

Shutdown in SOT Package

© 2001 Microchip Technology Inc.

Figure 3. Test Circuit

3

1

2

6

5

C3

C1

C2

V

IN

V

OUT

R

L

TC1240

C1–

VIN

OUT

C1+

GND

Voltage Doubler

Device

C1

C2

C3

TC1240

SHDN

4

3

1

2

6

5

C3

C1

C2

V

IN

V

OUT

R

L

TC1240

C –

VIN

OUT

C +

GND

Device

C1

C2

C3

TC1240

SHDN

4

3.3

µF

3.3

µF3.3µF

VOLTAGE DOUBLER

The most common application for charge pump

devices is the doubler (Figure 3). This application uses

two external capacitors - C1 and C2 (plus a power supply

bypass capacitor, if necessary). The output is equal to 2 x

1 and Table 2 for capacitor selection.

CASCADING DEVICES

Two or more TC1240s can be cascaded to increase

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

seen that the output resistance rises rapidly for multiple

cascaded devices. For the case of the two-stage ‘tripler’output

stage.

Figure 4. Cascading Multiple Devies to Increase Output Voltage

C1A

C1B

C2A

3

4

2

6

2

5

1

1

5

6

C2B

V

IN

V

OUT

V

TC1240

"1"

TC1240

"n"

SHDN

SHDN

C+

GND

C–

OUT

OUT

C–

GND

C+

3

4

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

5 mA output load current and 0.1

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

TC1240

C1 (

0.47

47

1

28.5

2.2

19.5

3.3

17

4.7

15.5

10

13.6

47

12.5

100

12.2

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

TC1240

C1 (

0.47

142

167

2.2

30

3.3

20

4.7

14

10

6.7

47

2.5

100

1.6

INPUT SUPPLY BYPASSING

AC impedance and minimize noise effects due to the switch-

ing internal to the device. The recommended capacitor

should be a large value (at least equal to C1) connected from

the input to GND.

SHUTDOWN INPUT

TheTC1240 is disabled when SHDN is high, and

enabled when SHDN is low. This input cannot be allowed

to float.