CS5460A datasheet(33/54 Pages) CIRRUS

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CS5460A

The

voltage/current-channel

inputs

have

surge-current limits of 100 mA. This applies to brief

voltage/current spikes (<250 ms). The limit is

10 mA for DC input overload situations. To prevent

permanent damage to the CS5460A, the designer

must include adequate protection circuitry in the

power meter design, to insure that these pin cur-

rent limits are never exceeded, when CS5460A is

operating in the intended power-line metering envi-

ronment.

Focusing specifically on Figure 7, which shows

how voltage/current transformers can be used to

sense the line-voltage/line-current, suppose for ex-

ample that the requirements for a certain 120 VAC

power system require that the power meter must

be able to withstand up to a 8 kV voltage spike on

the power line during normal operating conditions.

To provide a suitable sensor voltage input level to

the voltage channel input pins of the CS5460A, the

turns ratio of the voltage-sense transformer should

be chosen such that the ratio is, for example, on

the order of 1000:1. A voltage-sense transformer

with a 1000:1 turns ratio will provide a 120 mV

(rms) signal to the CS5460A’s differential voltage

channel inputs, when the power line voltage is at

the nominal level of 120 VAC. Therefore, a brief

8 kV surge would be reduced to a 8 V surge across

RV+.

What happens when 8 volts (common-mode) is

present across one of the analog input pins of the

CS5460A? The Vin+/Vin- and Iin+/Iin- pins of the

CS5460A are equipped with internal protection di-

odes. If a voltage is presented to any of these pins

that is larger than approximately 7 V (with respect

to VA- pin) these protection diodes will turn on in-

side the CS5460A. But in order to prevent exces-

sive current levels from flowing through the device,

the value of RV+ must be large enough that when a

8V surge is present across the secondaries of the

voltage-sense transformer, the brief surge current

through RV+ should not be any greater than

100 mA. Therefore, a minimum value for RV+

would be (8 V - 7 V) / 100 mA = 10

Ω. This value

may be increased as needed, to easily obtain the

desired cutoff frequency of the anti-aliasing filter on

the voltage channel (described later), and also to

provide some margin. But the designer should try

to avoid using values for the protection resistors

that are excessively high. A typical value for RV+

would be 470

Ω.

The VIN- pin should also have a protection resistor

(called RV- in Figure 7). To maintain symmetry, the

value of RV- should be made equal to RV+.

For the current channel inputs (Iin+ and Iin-), if the

maximum current rating (Imax) for this power line is

30 A (RMS), then a suitable turns ratio for the cur-

rent-sense transformer might be 200:1. Since the

maximum load for a 120 VAC line rated at 30 A

would be 4

Ω (for unity power factor), a brief 8 kV

surge across “L” and “N” could generate as much

as 2000 A (RMS) of current through the primaries

of the current-sense transformer. This can in turn

generate as much as 10 V across the secondaries

of the current-sense transformer. This voltage is

high enough to turn on one or more of the internal

protection diodes located off of the Iin+/Iin- pins.

Therefore, the value of the protection resistor that

will limit the current flow to less than 100 mA would

be (10 V - 7 V) / 100 mA = 30

Ω. In order to pro-

vide some margin and to use the same resistor val-

ues that are used on the Vin+/Vin- pins, a 470

Ω

resistor can be used as a lower limit for the RI+ and

RI- resistors shown in Figure 7.

Referring to the circuit implementations shown in

Figures 6, 8, and 9, note that when resistor-divider

configurations are used to provide the voltage

channel sense voltage, the VIN+ pin does not need

an additional, separate, dedicated protection resis-

tor. This is because the resistive voltage-divider al-

ready provides the series resistance that is needed

for this protection resistance (from R1 and R2).

(And note in Figure 8 that this is true for both the

VIN+ pin and the VIN- pin.) In Figure 7, a voltage

transformer is used as the voltage sensor. When

any type of transformer is used as the sensor de-

vice for voltage (or current) channel, a dedicated

protection resistor RV+ should be installed in series

with the VIN+ pin, and similarly, a resistor (RV-)

should be installed in series to the VIN- input pin.

Additional

considerations/techniques

regarding

the protection of the analog input pins against sud-

den high-frequency, high-level voltage/current

surges are discussed in Section 3.14.

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