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IR3721MTRPBF Datasheet(PDF) 9 Page - International Rectifier |
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IR3721MTRPBF Datasheet(HTML) 9 Page - International Rectifier |
9 / 16 page IR3721 DATA SHEET Page 9 of 16 www.irf.com 09/15/08 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% 0% 1% 2% 3% 4% 5% 020 40 60 80 100 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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