AMS682CS datasheet(4/7 Pages) ADMOS

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AMS682

APPLICATIONS (Continued)

Capacitor Selection

The output resistance of the AMS682 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 = 16 RSW+ 4ESRC1+ 4ESRC2+ ESRC3

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

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

low ESR capacitors. The fixed term (16RSW) is about 80-

Ω. Increasing or decreasing values of C1 and C2 will affect

efficiency by changing ROUT.

Table 1 shows ROUT for various values of C1 and C2 (assume

0.5

Ω 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 :

VRIPPLE = [1/[2(fPUMP X C3)]+2(ESRC3)] (IOUT)

For a 10

µF (0.5Ω ESR), f

PUMP = 10kHz and IOUT=10mA the

peak -to-peak ripple voltage at the output will be less than

60mV. In most applications (IOUT ≤ 10mA) a 10-20µF

capacitor and 1-5

µF pump capacitors will be sufficient. Table

2 shows VRIPPLE for different values of C3 (assume 1Ω ESR).

Paralleling devices

Paralleling multiple AMS682 reduces the output resistance of

the converter. The effective output resistance is the output

resistance of one device divided by the number of devices.

Figure 5 illustrates how each device 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 AMS682 provides -6V at V

OUT , which is regulated to -5V

by the negative LDO. The AMS682 input can vary from 3V to

5.5V without affecting regulation significantly. A voltage

detector 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 AMS682.

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 drop 1.5V.

Table 1. ROUT vs. C1, C2

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

C1, C2 (

µF)

ROUT (Ω)

C3(

µF)

VRIPPLE (mV)

0.05

4085

0.50

1020

0.10

2084

1.00

520

0.47

510

3.30

172

1.00

285

5.00

120

3.30

145

10.00

5.00

125

22.00

10.00

105

100.00

22.00

100.00

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