IR3721 datasheet(9/16 Pages) IRF

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IR3721

DATA SHEET

Page 9 of 16

www.irf.com

07/14/08

COMPONENT SELECTION GUIDELINES

Use a 0.1 μF, 6.3V, X7R ceramic bypass capacitor

from VDD to GND and from VK to GND.

Filter the DI output with an RC filter to give a stable

analog representation of the current or power. Some

of the DI source resistance of this filter is internal to

the IR3721 and specified in the electrical

specifications table. Add twenty thousand to fifty

thousand additional ohms externally to minimize

resistance variation. As the DI source resistance

increases beyond these guidelines, the voltage

measurement error caused by non-ideal voltmeter

conductance will increase.

Select a filter capacitor that limits 512 kHz sampling

frequency ripple to an acceptable value. Sampling

frequency ripple will appear as an error, but can be

reduced 20 dB for each decade that the filter corner

frequency is below 512 kHz. Select a capacitor value

that achieves the desired balance between low

sampling frequency ripple and adequate bandwidth.

Resistor current sensing

For resistor current sensing select a precision resistor

for RT inside the RT resistance range limits specified

in the Electrical Specifications table, such as 25.5kΩ

and 1% tolerance.

Next, select a shunt resistor that will provide the most

current sensing voltage while also considering the

allowable power dissipation limitations. The DI output

will saturate to the VK voltage when full scale current

IFS flows through this shunt. Recommended

maximum current sensing voltage range is 5 to 150

mV. Maximum sensing voltages less than 5 mV will

cause comparator input offset voltage errors to

dominate, and voltages larger than 150 mV will cause

comparator leakage current, ICS, errors to dominate.

Select RCS2 to be the next higher standard value

resistor from (RSHUNT·IFS·RT) / VRT in order to

accommodate full scale current IFS.

Bypass VCS to VO with capacitor CCS2. The value of

this capacitor limits the bandwidth, but is required

because it is the integrator of the delta sigma

modulator. Consider selecting the value of CCS2 to

place a filter corner frequency at 5 kHz, which will

reduce sampling ripple by 40 dB.

DCR current sensing

Select an RT network resistance between 20kΩ and

45.3kΩ. Consider the RT network of Figure 5 for DCR

current sensing.

26.1 kΩ, 1%

15.0 kΩ, 1%

2.00 kΩ, 1%

Murata Thermistor

NCP15WB473F03RC

47 kΩ, 1%

Figure 5 RT network

The resistance of the network above at 25°C, RT(25),

is 37.58kΩ. Over temperature RT(T) is multiplied by

copper resistance, DCR(25)·(1+(T-25)·0.0039),

divided by (DCR(25)·( RT(25)) to normalize the

results, and plotted as nominal error in Figure 6.

-5%

-4%

-3%

-2%

-1%

020

100

Temperature [°C]

Figure 6 Nominal error vs. Temperature

Note that the error due to temperature compensation

at 25°C is zero, assuming ideal RT components. At

other temperatures the results are over or under

reported by the factor in percent indicated.

Proceed to calculate RSUM, defined as the sum of

RCS1 plus RCS2, as follows.

RSUM=IFS·DCR(25) ·RT(25) / VRT

Again, IFS is full scale current and VRT is the reference

voltage establishing the current in RT.

Estimate the capacitance CCS1 with the following

equation.

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