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LT3748 Datasheet(PDF) 10 Page - Linear Technology |
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LT3748 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 30 page  LT3748 10 3748fa APPLICATIONS INFORMATION Selecting Actual RREF, RFB and RTC Resistor Values The preceding equations define how the LT3748 would regulate the output voltage if the system had no time de- lays and no error sources. However, there are a number of repeatable delays and parasitics in each application which will affect the output voltage and force a re-evaluation of the RFBandRTCcomponentvalues.Thefollowingapproach is the best method for selecting the correct values. The expression for VOUT, developed in the Operation sec- tion, can be rearranged to yield the following expression for RFB: R FB = RREF •NPS VOUT + VF () + VTC ⎡⎣ ⎤⎦ VBG where: VOUT = Output voltage VF = Output diode forward voltage NPS = Effective primary-to-secondary turns ratio VTC = 0.55V The equation assumes the temperature coefficients of the output diode and VTC are equal and substitutes RFB/NPS for the value of RTC. This is a good first order approximation but will be revisited later. First, the value of RREF should be approximately 6.04k since the LT3748 is trimmed and specified using this value. If the impedance of RREF varies considerably from 6.04k, additional errors will result. However, a variation in RREF of several percent is acceptable. This yields a bit of freedom in selecting standard 1% resistor values to yield nominal RFB/RREF ratios. With starting values for RFB and RTC, an initial iteration of the application should be built with final selections of all external components (transformer, diode, MOSFET, etc.). The resulting VOUT should be measured and used to re-evaluate the value of RFB due to non-idealities in the sampling system: RFB(NEW) = VOUT(DESIRED) VOUT(MEASURED) •RFB(OLD) With a new value of RFB selected, the temperature co- efficient of the output diode in the application can be tested to verify the nominal RTC value. The RTC resistor should be removed from the circuit under test (this will cause VOUT to increase for this step) and VOUT should be measured over temperature at a desired target output load. It is very important for this evaluation that uniform temperature be applied to both the output diode and the LT3748—if freeze spray or a heat gun is used there can be a significant mismatch in temperature between the two devices that causes significant error. Attempting to extrapolate the data from a diode datasheet or assuming the nominal RTC value may yield a better result if there is no method to apply uniform heat or cooling such as an oven. With at least two data points (although more data points from hot to cold are recommended), the change in V/°C can be determined by: ΔVOUT ΔTEMP = VOUT1 –VOUT2 TEMP1– TEMP2 Using the measured VOUT temperature coefficient, an exact RTC value can be selected using the following equation: RTC = RFB NPS • 1.85mV/°C ΔVOUT ΔTEMP If the value of RTC has changed significantly, which can happen with the use of some output diodes that have a very low forward drop, the RFB value may need to be changed to restore VOUT to the desired value. As in the previous iteration, after measuring VOUT, a new RFB can once again be selected using: RFB(NEW) = VOUT(DESIRED) VOUT(MEASURED) •RFB(OLD) Once the values of RFB and RTC are selected, the regulation accuracy from board to board for a given application will be very consistent, typically under ±5% when including device variation of all the components in the system (assuming resistor tolerances and transformer windings matching of 1% or better). However, if the transformer, the output diode or MOSFET switch are changed or the layout is dramatically altered, there may be some change in VOUT. |
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