CS5522-AP datasheet(45/56 Pages) CIRRUS

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CS5521/22/23/24/28

DS317F2

2.8.1 Charge Pump Drive Circuits

The CPD (Charge Pump Drive) pin of the converter

can be used with external components (shown in

Figure 21) to develop an appropriate negative bias

voltage for the NBV pin. When CPD is used to gen-

erate the NBV, the NBV voltage is regulated with

an internal regulator loop referenced to VA+.

Therefore, any change on VA+ results in a propor-

tional change on NBV. With VA+ = 5 V, NBV’s

regulation is set proportional to VA+ at approxi-

mately -2.1 V.

Figure 24 illustrates a charge pump circuit when

the converters are powered from a +3.0 V digital

supply. Alternatively, the negative bias supply can

be generated from a negative supply voltage or a

resistive divider as illustrated in Figure 25.

For ground based signals with the instrumentation

amplifier engaged (when in the 25 mV, 55 mV, or

100 mV ranges), the voltage on the NBV pin

should at no time be less negative than -1.8 V or

more negative than -2.5 V. To prevent excessive

voltage stress to the chip when the instrumentation

amplifier isn’t engaged (when in the 1 V, 2.5 V, or

5 V ranges) the NBV voltage should not be more

negative than -2.5 V.

The components in Figure 21 are the preferred

components for the CPD filter. However, smaller

capacitors can be used with acceptable results. The

µF ensures very low ripple on NBV. Intrinsic

safety requirements prohibit the use of electrolytic

capacitors. In this case, four 0.47

µF ceramic ca-

pacitors in parallel can be used.

Note: The charge pump is designed to nominally provide

400

µA of current for the instrumentation amplifier

when

0.033

µF pumping capacitor is used

(XIN = 32.768 kHz). When a larger pumping capaci-

tor is used, the charge pump can source more current

to power external loads. Refer to Applications Note

152 “Using the CS5521/23, CS5522/24/28, and

CS5525/26 Charge Pump Drive for External Loads”

for more details on using the charge pump with exter-

nal loads.

2.9 Digital Gain Scaling

The CS5521/22/23/24 and CS5528 all feature a

gain register capable of being scaled from 0.6 to 4-

2-22 in decimal. The specified ranges of the con-

verter are defined with a voltage reference of 2.5 V

and the gain register set at approximately 1.0. The

gain register can be manipulated to scale the input

for ranges other than those specified. For example,

when using a 2.5 V voltage reference, and the

25 mV input range setting, the gain register can be

changed from 1.000 to 2.000 (shift the entire regis-

ter contents to the left one position) to achieve an

input span of 12.5 mV. Under this condition the

full span of the converter codes will appear across

a 12.5 mV span. The amount of noise in the con-

Figure 24. Charge Pump Drive Circuit for VD+ = 3 V

-5V

NBV

30.1K

Ω

34.8K

Ω

2N5087

or similar

-5V

2.1K

Ω

2.0K

Ω

NBV

µF

BAT85

Figure 25. Alternate NBV Circuits

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