PCA9620 datasheet(33/71 Pages) NXP

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PCA9620

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Product data sheet

Rev. 1 — 9 December 2010

33 of 71

NXP Semiconductors

PCA9620

Universal LCD driver for low multiplex rates

7.4.5 Charge pump driving capability

Figure 25 illustrates the main factor determining how much current the charge pump can

deliver.

The output resistance of the charge pump is specified in Table 35 on page 55. With these

values it can be calculated how much current the charge pump can drive under certain

conditions.

Example: Assuming the user would like to have the normal operation point at 25

°C with

VLCD = 7.0 V and VDD2 = 5.0 V and the charge pump is set to 2 × VDD2. Then the

theoretical value of VLCD is 10.0 V and the desired one is 7.0 V. The difference between

the theoretical maximum value and desired one is 3.0 V. The charge pump resistance is

nominally 0.85 k

Ω. Equation 7 shows the possible current that the charge pump could

deliver:

(7)

For this example we get:

In cases where no extreme driving capability is needed, a command is available for

decreasing the charge pump frequency (see Table 22 on page 11) and thus reducing the

total current consumption. If the charge pump frequency is halved, then the driving

capability is halved as well, whereas the output resistance doubles.

7.4.6 Charge pump frequency settings and power efficiency

The PCA9620 offers the possibility to use different frequency settings for the charge

pump. Bit CPF controls the frequency at which the charge pump is running (see Table 22

on page 11). This frequency has a direct influence on the current consumption of the IC

but also on the charge pump driving capability. Using a lower charge pump frequency

decreases the current consumption and the driving capability.

The power efficiency of the charge pump determines in certain applications which

frequency settings to choose for the CPF bit. In the example shown in Figure 26, the

current consumption was measured with the charge pump set to 2

× V

DD2 and with

VDD2 = 3.0 V and VPR[7:0] set to maximum to obtain the maximum possible VLCD with this

setup, which is close to 6.0 V. The current load on pin VLCD determines the output power

delivered by the IC:

(8)

The current consumption on pin VDD2 determines the input power taken by the IC:

Fig 25. Charge pump model (used to characterize the driving strength)

Theoretical VLCD value

VLCD = 2 × VDD2 or

VLCD = 3 × VDD2

Output Resistance

Ro(cp)

Regulated desired VLCD

This supplies the segments

and backplanes

013aaa259

load

ΔV

LCD

ocp

()

⁄

load

3.0 V 0.85 k

⁄

3.5 mA

load

LCD

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