PCA9620 datasheet(34/71 Pages) NXP

34 / 71 page

PCA9620

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Product data sheet

Rev. 1 — 9 December 2010

34 of 71

NXP Semiconductors

PCA9620

Universal LCD driver for low multiplex rates

(9)

The ratio between these two numbers determines the charge pump power efficiency:

(10)

Loading the charge pump with higher currents decreases the output voltage. This

decrease is determined by the charge pump driving capability, respectively by the output

resistance of the charge pump (see Table 35 on page 55).

The power efficiency calculation is only valid when the charge pump is running at its

maximum peak frequency and regulates the generated VLCD voltage with full speed. In

this case, the ripple on the VLCD voltage equals the internal charge pump frequency.

Approximately, this could also be calculated with the parameter of the output resistance of

the charge pump (see Table 35 on page 55), the load current, and the voltage needed to

be provided by using Equation 7 on page 33. This value of Iload is close to the value of the

load current needed for the application.

If the application runs with VDD2 = 3.0 V, the load currents are up to 400 μA

(DC measured), and the VLCD generated voltages are up to 5.0 V, then - concerning

power efficiency - it would be the best to have a charge pump frequency set to half

frequency.

Charge pump set to 2

× V

DD2; VDD2 =3V.

(1)

p, full charge pump frequency.

(2)

p, half charge pump frequency.

(3) VLCD, full frequency.

(4) VLCD, half frequency.

Fig 26. Power efficiency of the charge pump

DD2

o Pi

⁄

Iload (μA)

1000

800

400

600

200

001aan027

ηp

(%)

VLCD

(V)

(3)

(4)

(1)

(2)

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