TC682 datasheet(4/6 Pages) TELCOM

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TELCOM SEMICONDUCTOR, INC.

TC682

INVERTING VOLTAGE DOUBLER

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:

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

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

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

+ESRC3

Assuming all switch resistances are approximately

equal...

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

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

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

3.3

µF low ESR capacitors. The fixed term (16RSW) is about

80-90

Ω. 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 electrolytic capacitors. Table 1

shows ROUT for various values of C1 and C2 (assume 0.5Ω

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

µF (0.5Ω ESR) capacitor for C3, fPUMP = 10kHz

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

output will be less then 60mV. In most applications (IOUT <

= 10mA) a 10-20

µF capacitor and 1-5µF pump capacitors

will suffice. Table 2 shows VRIPPLE for different values of C3

(assume 1

Ω ESR).

Table 1. ROUT vs. C1, C2

C1, C2 (

µF)

ROUT (

Ω)

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

Table 2. VRIPPLE Peak- to-Peak vs. C3 (IOUT = 10mA)

C3 (

µF)

VRIPPLE (mV)

0.50

1020

1.00

520

3.30

172

5.00

120

10.00

22.00

100.00

Paralleling Devices

Paralleling multiple TC682s reduces the output resis-

tance of the converter. The effective output resistance is the

output resistance of a single device divided by the number

of devices. As illustrated in Figure 5, each requires separate

pump capacitors C1 and C2, but all can share a single

reservoir capacitor.

–5V Regulated Supply From A Single

3V Battery

Figure 6 shows a – 5V power supply using one 3V

battery. The TC682 provides – 6V at V

–

OUT, which is regu-

lated to – 5V by the negative LDO. The input to the TC682

can vary from 3V to 5.5V without affecting regulation appre-

ciably. A TC54 device is connected to the battery to detect

undervoltage. This unit is set to detect at 2.7V. With higher

input voltage, more current can be drawn from the outputs

of the TC682. With 5V at VIN, 10mA can be drawn from the

regulated output. Assuming 150

Ω source resistance for the

converter, with IL

–= 10mA, the charge pump will droop 1.5V.

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