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CS5211GDR14 Datasheet(PDF) 10 Page - ON Semiconductor |
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CS5211GDR14 Datasheet(HTML) 10 Page - ON Semiconductor |
10 / 16 page CS5211 http://onsemi.com 10 qT t (TJ * TA) PD where; θT is the total thermal impedance (θJC + θSA). θJC is the junction–to–case thermal impedance of the MOSFET. θSA is the sink–to–ambient thermal impedance of the heatsink assuming direct mounting of the MOSFET (no thermal “pad” is used). TJ is the specified maximum allowed junction temperature. TA is the worst case ambient operating temperature. For TO–220 and TO–263 packages, standard FR–4 copper clad circuit boards will have approximate thermal resistances ( θSA) as shown below: Pad Size (in2/mm2) Single–Sided 1 oz. Copper 0.5/323 60–65 °C/W 0.75/484 55–60 °C/W 1.0/645 50–55 °C/W 1.5/968 45–50 °C/W 2.0/1290 38–42 °C/W 2.5/1612 33–37 °C/W As with any power design, proper laboratory testing should be performed to insure the design will dissipate the required power under worst case operating conditions. Variables considered during testing should include maximum ambient temperature, minimum airflow, maximum input voltage, maximum loading, and component variations (i.e. worst case MOSFET RDS(on)). Also, the inductors and capacitors share the MOSFET’s heatsinks and will add heat and raise the temperature of the circuit board and MOSFET. For any new design, its advisable to have as much heatsink area as possible – all too often new designs are found to be too hot and require re–design to add heatsinking. Compensation Capacitor Selection The nominal output current capability of the error amp is 30 µA. This current charging the capacitor on the COMP pin is used as soft start for the converter. The COMP pin is going to ramp up to a voltage level that is within 70 mV of what VFFB is going to be when in regulation. This is the voltage that will determine the soft start. Therefore, the COMP capacitor can be established by the following relationship: C + 30 mA soft start VFFB(REG) where: soft start = output ramp–up time VFFB(REG) = VFFB voltage when in regulation 30 µA = COMP output current, typ. The COMP output current range is given in the data sheet and will affect the ramp–up time. The value of the capacitor on the COMP pin will have an effect on the loop response and the transient response of the converter. Transient response can be enhanced by the addition of a parallel combination of a resistor and capacitor between the COMP pin and the comp capacitor. ROSC Selection The switching frequency is programmed by selecting the resistor connected between the ROSC pin and SGND (pin 7). The grounded side of this resistor should be directly connected to the SGND pin, without any other currents flowing between the bottom of the resistor and the pin. Also, avoid running any noisy signals under the resistor, since injected noise could cause frequency jitter. The graph in Figure 6 shows the required resistance to program the frequency. Below 500 kHz, the following formula is accurate: R + 13500 fSW ) 6kW where fSW is the switching frequency in kHz. Figure 6. Frequency vs. ROSC 10 0 Resistance (k Ω) 100 200 300 400 500 800 20 600 30 40 50 60 70 700 80 90 100 110 Differential Remote Sense Operation The ability to implement fully differential remote sense is provided by the CS5211. The positive remote sense is implemented by bringing the output remote sense connection to the positive load connection. A low value resistor is connected from Vout to the feedback point at the regulator to provide feedback in the instance when the remote sense point is not connected. The negative remote sense connection is provided by connecting the SGND of the CS5211 to the negative of the load return. Again, a low value resistor should be connected between SGND and LGND at the regulator to provide feedback in the instance when the remote sense point is not connected. The maximum voltage differential between the three grounds for this part is 200 mV. |
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