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XRP6142 Datasheet(PDF) 11 Page - Exar Corporation |
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XRP6142 Datasheet(HTML) 11 Page - Exar Corporation |
11 / 17 page X XR RP P6 61 14 42 2 S Sy yn ncch hrro on no ou us s S Stte ep p--D Do ow wn n C Co on nttrro olllle err w wiitth h D DD DR R M Me em mo orry y T Te errm miin na attiio on n © 2010 Exar Corporation Page 11 of 17 Rev. 1.0.0 () 2 , 02 . 0 IN IN OUT IN OUT MAX OUT IN V V fs V V V I C × × − × × = SYNCHRONOUS FET (LOW-SIDE FET) Select the synchronous FET for voltage rating BVDSS, on resistance rating RDS(ON) and gate drive rating VGS. As a rule of thumb, voltage rating should be at least twice the converter input voltage. FETs with voltage rating of up to 30V should provide satisfactory performance. Drive voltage of 4.5V is sufficient for applications with minimum input voltage of 4.5V. For applications with a lower input voltage a FET with 2.5V gate drive should be selected. Switching losses of the Synchronous FET are negligible in comparison to its conduction losses. RDS(ON) is calculated based on conduction losses from: () 2 ) ( 1 OUT Conduction ON DS I D P R × − ≤ It is common practice to allocate 50% of the total FET losses to the synchronous FET. As an example, consider a 10W buck converter with a target efficiency of 90%. Therefore, the target total power loss is 1.1W. Assume that the only significant non-FET loss is the inductor loss estimated at 0.1W. Thus the maximum conduction loss of the synchronous FET should not exceed 0.5W. By using this value in the above equation RDS(ON) can be calculated and a suitable FET selected. SWITCHING FET (HIGH-SIDE FET) Select the switching FET for voltage rating BVDSS, on-resistance rating RDS(ON), gate drive rating VGS, rise time tr and fall time tf. BVDSS and VGS selection guidelines are the same as Synchronous FET. The switching FET incurs switching (i.e., transitional) as well as conduction losses. RDS(ON) is calculated based on conduction losses from: 2 ) ( OUT Conduction ON DS I D P R × ≤ It is common practice to allocate 50% of the total high-side FET losses to conduction. Proceeding with the example from previous section the total target loss is 0.5W, and thus target conduction loss equals 0.25W. By using this value in the above equation RDS(ON) can be calculated. Rise and fall time can be approximated from: s out IN Switching f r f I V P t t × × = + Since the allotted switching loss budget is 0.25W, tr and tf can be calculated from the above equation. For a detailed explanation of FET losses and FET selection procedure refer to EXAR application note ANP-20. R-C SNUBBER (OPTIONAL) An R-C snubber placed across the synchronous FET eliminates the ringing and reduces the amplitude of overshoot at SW node. Use surface-mount components and place them close to the FET drain-source. Calculate the value of snubber capacitor Csnb from: Coss Csnb × = 3 Coss is the output capacitance of the synchronous FET corresponding to VIN. Calculate the value of the snubber resistor Rsnb from: OUT OUT I V Rsnb × = 2 DDR MEMORY POWER APPLICATIONS XRP6142 can be used to generate the required VDDQ (VDD) or VTT Reference voltages for DDR I, II and III memories and provides a 40mA buffered VTT Reference voltage. When used in conjunction with Exar’s SP2996 DDR Memory Termination, the XRP6142 provides a complete DDR power management solution. A cost- effective DDR2 solution is shown on page 15. XRP6142 provides the VDDQ and VTTREF voltages. SP2996 provides the VTT voltage. Please note that the current output of VDDQ can be increased up to 10A by using a larger QT/QB MOSFET and scaling the L1 and C3 accordingly. |
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