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LT3574IMSPBF Datasheet(PDF) 9 Page - Linear Technology |
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LT3574IMSPBF Datasheet(HTML) 9 Page - Linear Technology |
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9 / 24 page  LT3574 3574f applications inForMation ERROR AMPLIFIER—DYNAMIC THEORY Due to the sampling nature of the feedback loop, there are several timing signals and other constraints that are required for proper LT3574 operation. Minimum Current Limit The LT3574 obtains output voltage information from the SW pin when the secondary winding conducts current. The sampling circuitry needs a minimum amount of time to sample the output voltage. To guarantee enough time, a minimum inductance value must be maintained. The primary side magnetizing inductance must be chosen above the following value: L V t I N V N µH V PRI OUT MIN MIN PS OUT PS ≥ = • • • • 2 tMIN = minimum off-time, 350ns IMIN = minimum current limit, 175mA The minimum current limit is higher than that on the Elec- trical Characteristics table due to the overshoot caused by the comparator delay. Leakage Inductance Blanking When the output switch first turns off, the flyback pulse appears.However,ittakesafinitetimeuntilthetransformer primary-side voltage waveform approximately represents the output voltage. This is partly due to the rise time on the SW node, but more importantly due to the trans- former leakage inductance. The latter causes a very fast voltage spike on the primary side of the transformer that is not directly related to output voltage (some time is also required for internal settling of the feedback amplifier circuitry). The leakage inductance spike is largest when the power switch current is highest. In order to maintain immunity to these phenomena, a fixed delay is introduced between the switch turn-off command andthebeginningofthesampling.Theblankingisinternally set to 150ns. In certain cases, the leakage inductance may not be settled by the end of the blanking period, but will not significantly affect output regulation. Selecting RFB and RREF Resistor Values The expression for VOUT, developed in the Operation sec- tion, can be rearranged to yield the following expression for RFB: R R N V V V V FB REF PS OUT F TC BG = + ( ) + • a where, VOUT = Output voltage VF = Switching diode forward voltage a = Ratio of Q1, IC to IE, typically 0.986 NPS = Effective primary-to-secondary turns ratio VTC = 0.55V The equation assumes the temperature coefficients of the diode and VTC are equal, which is a good first-order approximation. Strictly speaking, the above equation defines RFBnotasan absolute value, but as a ratio of RREF. So, the next ques- tion is, “What is the proper value for RREF?” The answer is that RREF should be approximately 6.04k. The LT3574 is trimmed and specified using this value of RREF. If the impedance of RREF varies considerably from 6.04k, ad- ditional 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. The RFB resistor given by this equation should also be verified experimentally, and adjusted if necessary for best output accuracy. Tables 1-4 are useful for selecting the resistor values for RREF and RFB with no equations. The tables provide RFB, RREF and RTC values for common output voltages and common winding ratios. Table 1. Common Resistor Values for 1:1 Transformers VOUT (V) NPS RFB (kΩ) RREF (kΩ) RTC (kΩ) 3.3 1.00 18.7 6.04 19.1 5 1.00 27.4 6.04 28 12 1.00 64.9 6.04 66.5 15 1.00 80.6 6.04 80.6 20 1.00 107 6.04 105 |
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