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US3011 Datasheet(PDF) 10 Page - UNISEM |
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US3011 Datasheet(HTML) 10 Page - UNISEM |
10 / 11 page 4-10 Rev. 1.3 12/8/00 US3011 Following the same procedure for the Schottcky diode results in a heatsink with θsa = 25 °C/W. Although it is possible to select a slightly smaller heatsink, for sim- plicity the same heatsink as the one for the high side MOSFET is also selected for the synchronous MOSFET. Switcher Current Limit Protection The PWM controller uses the MOSFET Rds-on as the sensing resistor to sense the MOSFET current and com- pares to a programmed voltage which is set externally via a resistor (Rcs) placed between the drain of the MOSFET and the “CS+” terminal of the IC as shown in the application circuit. For example, if the desired cur- rent limit point is set to be 22A and from our previous selection, the maximum MOSFET Rds-on=19m Ω, then the current sense resistor, Rcs is calculated as : Vcs=IcL*Rds=22*0.019=0.418V Rcs=Vcs/Ib=(0.418V)/(200uA)=2.1k Ω Where: Ib=200uA is the internal current setting of the device Switcher Timing Capacitor Selection The switching frequency can be programmed using an external timing capacitor. The value of Ct can be ap- proximated using the equation below: Switcher Output Voltage Adjust As it was discussed earlier,the trace resistance from the output of the switching regulator to the Slot 1 can be used to the circuit advantage and possibly reduce the number of output capacitors, by level shifting the DC regulation point when transitioninig from light load to full load and vice versa. To account for the DC drop, the output of the regulator is typically set about half the DC drop that results from light load to full load. For example, if the total resistance from the output capacitors to the F C Where C =Ti g Capacitor F Switching Frequency SW T T SW ≈ × = − 3 5 10 5 . : min If, F = kHz : C pF SW T 200 3 5 10 200 10 175 5 3 ≈ × × = − . Slot 1 and back to the GND pin of the device is 5m Ω and if the total ∆I, the change from light load to full load is 14A, then the output voltage measured at the top of the resistor divider which is also connected to the output capacitors in this case, must be set at half of the 70 mV or 35mV higher than the DAC voltage setting. To do this, the top resistor of the resistor divider, Rtop is set at 100 Ω, and the bottom resistor, Rb is calculated. For example, if DAC voltage setting is for 2.8V and the desired output under light load is 2.835V, then Rb is calculated using the following formula : Rb= 100* {Vdac /(Vo - 1.004*Vdac)} [Ω] Rb= 100* {2.8 /(2.835 - 1.004*2.800)} = 11.76 kΩ Select 11.8 k Ω , 1% Note: The value of the top resistor must not exceed 100 Ω Ω. The bottom resistor can then be adjusted to raise the output voltage. Soft Start Capacitor Selection The soft start capacitor must be selected such that dur- ing the start up when the output capacitors are charging up, the peak inductor current does not reach the current limit treshold. A minimum of 1uF capacitor insures this for most applications. An internal 10uA current source charges the soft start capacitor which slowly ramps up the inverting input of the PWM comparator Vfb3. This insures the output voltage to ramp at the same rate as the soft start cap thereby limiting the input current. For example, with 1uF and the 10uA internal current source the ramp up rate is ( ∆V/ ∆t)=I/C = 1V/100mS. Assum- ing that the output capacitance is 9000uF, the maxi- mum start up current will be: I=9000uF*(1V/100mS)=0.09A Input Filter It is highly recommended to place an inductor between the system 5V supply and the input capacitors of the switching regulator to isolate the 5V supply from the switching noise that occurs during the turn on and off of the switching components. Typically an inductor in the range of 1 to 3 uH will be sufficient in this type of appli- cation. Switcher External Shutdown The best way to shutdown the part is to pull down on the soft start pin using an external small signal transistor such as 2N3904 or 2N7002 small signal MOSFET. This allows slow ramp up of the output, the same as the power up. |
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