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LTC2920-2 Datasheet(PDF) 11 Page - Linear Technology |
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LTC2920-2 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  LTC2920-1/LTC2920-2 11 292012fa APPLICATIO S I FOR ATIO the IMACCURACY specification. If VT does not fall within this range, the LTC2920 cannot be used for this application. Figure 10. Power Module ITRIM Model SENSE– VIN – VO – TRIM VIN + VO + RVT VPS VO – ITRIM RTRIM 2920-1/2 F10 VT SENSE+ VREF + – RSET = 20k 2920-1/2 F12 RG = 944k VREF = 1.2V VPSOUT = 3.3V + – RF = 1.65k IMARGIN = ± 50μA IFB = 1.27mA IM RS LTC2920 + – Figure 12. Power Supply Voltage Margin Model POWER SUPPLY VOLTAGE MARGINING (%) 0 1 2 3 4 56 2920-1/2 F11 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 1% FEEDBACK RESISTOR INACCURACY 1% RSET RESISTOR INACCURACY 5% LTC2920 IMARGIN INACCURACY Figure 11. Sources of Power Supply Margined Voltage Errors for this example). The second error is the power supply initial set point accuracy. In this example the RF resistor has a 1% accuracy error causing a 0.6% initial set point error in the power supply. Because the margined power supply voltage is the change in the voltage, VMARGIN, from the power supply initial set point voltage, this error shows up in the margined power supply voltage. When these two errors are combined, the error is: Error = ⏐1 – (3.4043/3.3825)⏐ • 100 = 0.65% The error caused by a 1% inaccuracy in RG will be similar since the dominate error source is the power supply initial set point voltage. Errors caused by RF and RG can be a major contributor to voltage margin errors. Using 0.1% resistors for both RF and RG is often the best choice for improving both voltage margin accuracy and power supply initial accuracy. Accuracy of Power Supply Voltages when Margining The accuracy of margined power supply voltages depends on several factors. Figure 11 shows the magnitude of the errors discussed in detail below as a function of power supply margining percentage. In a typical feedback model (Figure 12), the delta voltage is a function of the margin current, IMARGIN, and the feedback resistor, RF. VMARGIN = IMARGIN • RF Errors in VMARGIN are directly proportional to errors in IMARGIN and errors in RF. A 5% error in IMARGIN will cause a 5% error in VMARGIN. In this example, a 3.3V power supply is margined by 2.5%, or 0.0825V to 3.3825V. With a 5% VMARGIN error, the actual margin voltage is 0.0866V and the actual power supply voltage is 3.3866V. The error in the expected voltage is then: Error = ⏐1 – (3.3866/3.3825)⏐ • 100 = 0.12% Similarly, a 1% inaccuracy in the RSET resistor would cause only 0.024% error in the expected power supply margined voltage. In effect, IMARGIN errors caused by the RSET resistor or the LTC2920 are attenuated by the voltage margining percentage. The accuracy of the RF resistor introduces two errors in the margined supply voltage. The first is the error in VMARGIN (IMARGIN • RF). This error is similar in magnitude to the errors described above and is generally quite small (0.024% |
Similar Part No. - LTC2920-2_15 |
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Similar Description - LTC2920-2_15 |
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