Datasheet LT3748 (Analog Devices) - 10

LT3748
applications inForMation Selecting Actual RREF , RFB and RTC Resistor Values
With a new value of RFB selected, the temperature co- The preceding equations define how the LT3748 would efficient of the output diode in the application can be regulate the output voltage if the system had no time delays tested to verify the nominal RTC value. The RTC resistor and no error sources. However, there are a number of should be removed from the circuit under test (this will repeatable delays and parasitics in each application which cause VOUT to increase for this step) and VOUT should will affect the output voltage and force a re-evaluation of be measured over temperature at a desired target output the R load. It is very important for this evaluation that uniform FB and RTC component values. The following approach is the best method for selecting the correct values. temperature be applied to both the output diode and the LT3748—if freeze spray or a heat gun is used there can The expression for VOUT, developed in the Operation sec- be a significant mismatch in temperature between the tion, can be rearranged to yield the following expression two devices that causes significant error. Attempting to for RFB: extrapolate the data from a diode datasheet or assuming R ( the nominal R  )+ V  TC value may yield a better result if there is R REF • NPS VOUT + VF TC  no method to apply uniform heat or cooling such as an FB = V BG oven. With at least two data points (although more data points from hot to cold are recommended), the change where: in V/°C can be determined by: VOUT = Output voltage ∆VOUT = VOUT1 – VOUT2 V F = Output diode forward voltage ∆TEMP TEMP1– TEMP2 N Using the measured V PS = Effective primary-to-secondary turns ratio OUT temperature coefficient, an exact R V TC value can be selected using the following equation: TC = 0.55V 1.85mV/°C The equation assumes the temperature coefficients of the RTC = RFB • ∆V output diode and V N OUT TC are equal and substitutes RFB/NPS for PS the value of R ∆TEMP TC. This is a good first order approximation but will be revisited later. If the value of RTC has changed significantly, which can First, the value of R happen with the use of some output diodes that have REF should be approximately 6.04k since the LT3748 is trimmed and specified using this a very low forward drop, the RFB value may need to be value. If the impedance of R changed to restore V REF varies considerably from OUT to the desired value. As in the 6.04k, additional errors will result. However, a variation in previous iteration, after measuring VOUT , a new RFB can R once again be selected using: REF of several percent is acceptable. This yields a bit of freedom in selecting standard 1% resistor values to yield VOUT(DESIRED) nominal R R • R FB/RREF ratios. FB(NEW) = FB(OLD) VOUT(MEASURED) With starting values for RFB and RTC, an initial iteration of the application should be built with final selections of Once the values of RFB and RTC are selected, the regulation all external components (transformer, diode, MOSFET, accuracy from board to board for a given application will be etc.). The resulting V very consistent, typically under ±5% when including device OUT should be measured and used to re-evaluate the value of R variation of all the components in the system (assuming FB due to non-idealities in the sampling system: resistor tolerances and transformer windings matching of 1% or better). However, if the transformer, the output V R OUT(DESIRED) diode or MOSFET switch are changed or the layout is FB(NEW) = • R V FB(OLD) dramatically altered, there may be some change in V OUT(MEASURED) OUT . 3748fb 10 For more information www.linear.com/LT3748 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration order information Electrical Characteristics Typical Performance Characteristics Pin Functions block diagram Operation Applications Information Package Description Typical Application Related Parts