TCM680 datasheet(8/18 Pages) MICROCHIP

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TCM680

DS21486C-page 8

5.0

APPLICATIONS INFORMATION

5.1

Voltage Multiplication and

Inversion

The TCM680 performs voltage multiplication and inver-

sion simultaneously, providing positive and negative

outputs (Figure 5-1). The magnitude of both outputs is,

approximately, twice the input voltage. Unlike other

switched capacitor converters, the TCM680 requires

only four external capacitors to provide both functions

simultaneously.

FIGURE 5-1:

Positive and Negative

Converter.

5.2

Capacitor Selection

The TCM680 requires only 4 external capacitors for

operation, which can be inexpensive, polarized alumi-

num electrolytic types. For the circuit in Figure 5-1, the

output characteristics are largely determined by the

external capacitors. An expression for ROUT can be

derived as shown below:

EQUATION

Assuming all switch resistances are approximately

equal:

EQUATION

ROUT is typically 140Ω at +25°C with VIN = +5V and C1

and C2 as 4.7 µF low ESR capacitors. The fixed term

(32RSW) is about 130Ω. It can easily be seen 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 5-1 shows

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

ESR). C1 and C4 must be rated at 6 VDC or greater

while C2 and C3 must be rated at 12 VDC or greater.

Output voltage ripple is affected by C3 and C4.

Typically, the larger the value of C3 and C4, the less the

ripple for a given load current. The formula for

VRIPPLE(p-p) is given below:

EQUATION

For a 10 µF (0.5

Ω ESR) capacitor for C3, C4,

fPUMP = 21 kHz and IOUT = 10 mA, the peak-to-peak

ripple voltage at the output will be less than 100 mV. In

most applications (IOUT ≤ 10 mA), 10-20 µF output

capacitors and 1-5 µF pump capacitors will suffice.

Table 5-2 shows VRIPPLE for different values of C3 and

C4 (assume 1 Ω ESR).

TABLE 5-1:

OUTPUT RESISTANCE

VS. C1, C2

TABLE 5-2:

VRIPPLE PEAK-TO-PEAK

VS. C3, C4 (IOUT 10 mA)

GND

TCM680

VIN

VOUT

VIN

22 µF

ROUT

+ =4(R

SW1 +RSW2 +ESRC1 +RSW3 +RSW4 +ESRC2)

+4(RSW1 +RSW2 +ESRC1 +RSW3 +RSW4 +ESRC2)

+1/(fPUMP x C1) + 1/(fPUMP x C2) + ESRC4

ROUT

– =4(R

SW1 +RSW2 +ESRC1 +RSW3 +RSW4 +ESRC2)

+4(RSW1 +RSW2 +ESRC1 +RSW3 +RSW4 +ESRC2)

+1/(fPUMP x C1) + 1/(fPUMP x C2) + ESRC3

ROUT

+ = 32R

SW + 8ESRC1 + 8ESRC2 + ESRC4

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

ROUT

– = 32R

SW + 8ESRC1 + 8ESRC2 + ESRC3

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

C1, C2 (µF)

ROUT

+, R

OUT

- (Ω)

0.1

1089

0.47

339

1232

3.3

165

4.7

157

146

141

100

137

C3, C4 (µF)

VRIPPLE(p-p)

+,V

RIPPLE(p-p)

- (mV)

0.47

1540

1734

3.3

236

4.7

172

100

VRIPPLE(p-p)

+ ={1/[2(f

PUMP /3) x C4] + 2(ESRC4)} (IOUT

VRIPPLE(p-p)

– ={1/[2(f

PUMP /3) x C3] + 2(ESRC3)} (IOUT

–)

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