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LT3573 Datasheet(PDF) 9 Page - Linear Technology |
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LT3573 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 26 page  LT3573 9 3573fd For more information www.linear.com/LT3573 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 LT3573 operation. Minimum Current Limit The LT3573 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: LPRI ≥ VOUT • tMIN IMIN •NPS = VOUT •NPS • 1.4µH V tMIN = minimum off-time, 350ns IMIN = minimum current limit, 250mA 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: RFB = RREF •NPS VOUT + VF ( )a+VTC VBG 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 RFB not as an 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 LT3573 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. 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 |
Similar Part No. - LT3573_15 |
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Similar Description - LT3573_15 |
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