SS6845G datasheet(11/13 Pages) SSC

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SS6845G

7/21/2005 Rev.3.01

]

D(1

ESR)

(2R

[2V

)

ESR

(2V

)

ESR

(2V

)

OUT

AVE

OFF

AVE

OFF

AVE

OUT

AVE

OFF

AVE

OFF

−

∆

−

………(13)

Thus the output current can be written as

]

D(1

)

ESR

(2R

[2V

)

OUT

OFF

OUT

−

∆

(14)

When the duty cycle is 0.5, the output current can be written as:

]

4ESR)

(8R

[2V

]

0.5)

0.5(1

ESR)

(2R

[2V

OUT

−

(15)

And equation (15) can be re-written as:

OUT

4ESR)

(8R

−

(16)

VOUT

2VIN

COUT

1/fC1

8RDS-ON

IOUT

LOAD

4ESR

Fig. 22 The equivalent circuit of charge pump

According to equation (16), when the duty cycle

is 0.5, the equivalent circuit of the charge pump is

shown in Fig. 22. The term 8RDS-ON is the total

effect of switching resistance, 1/fC1 is the effect

of flying capacitor and 4ESR is its equivalent

resistance.

From the equivalent circuit shown in Fig. 22, it is

seen that the terms 1/fC1, 4ESR and 8RDS-ON

should be as small as possible to get large output

current. However, since the R DS-ON is internal to

the SS6845G

, all that can be done is to lower the

values of

1/fC1 and ESR. However even if the

values

of 1/fC1 and ESR can be kept as small as

possible, the term 8RDS-ON still dominates the

limit of the maximum output current.

Layout Considerations

With the high

switching frequency and transient

currents of the SS6845G, careful consideration of

PCB

layout is important. To achieve the best

performance, it is necessary to minimize the distance

between every component and also to minimize

the length of

every connection and maximize the

trace width. Make sure each device connects to an

immediate ground plane. Fig. 23 to Fig. 25 show

recommended layout.

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