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To A/D

AO

1 kW

UCD7230

SLUS741 – NOVEMBER 2006

APPLICATION INFORMATION (continued)

While the amp faithfully passes the sensed current signal, it should be noted that the amplifier is bandwidth

limited for normal switching frequencies. Therefore, AO represents a moving average of the sensed current.

Should noise filtering be desired, a capacitor, not to exceed 220 pF, can be placed from AO to AGND. There is

a 1-k

Ω resistor between the amplifier output and A0 for this cap to work with. Alternately, a capacitor can be

connected between POS and NEG to filter against the R

time constant is the same as L/R, then the voltage across C is the same as the IR drop across the equivalent R

of the inductor. If the time constants don’t match, the average voltage across C is still the same as the average

voltage across R, but the indication of ripple current amplitude will be off. Tolerance of the value of R in the

inductor has a direct effect on measurement accuracy, as does the temperature coefficient of R. Copper has a

temperature coefficient of approximately 3800 ppm/°C. For a 100 °C rise in winding temperature, the dc

resistance of the inductor increases by 38%. The worst case scenario would be a cracked core or

under-designed inductor in which cases the core could tend towards saturation. In that scenario, inductor current

could change slope drastically and is not correctly modeled by the capacitor voltage.

The amp output can go up to 3.3 V, so reasonable designs limit full scale to 3.0 V. Should attenuation be

necessary, use a resistive divider between AO and the control chip A/D input as shown in Figure 4.

Figure 4. Level Shifting and Filtering the Voltage Representation of the Average Output Current

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