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LTC2920-1IS5 Datasheet(PDF) 9 Page - Linear Technology |
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LTC2920-1IS5 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 16 page  LTC2920-1/LTC2920-2 9 292012fa IMARGIN current. Select a new IMARGIN current that is within one of the two LTC2920’s IMARGIN ranges, then calculate the scaling factor: IFACTOR = IMARGIN(OLD)/IMARGIN(NEW) The new feedback resistors would then be: RF(NEW) = RF(OLD) • IFACTOR RG(NEW) = RG(OLD) • IFACTOR And RSET can then be calculated as descibed above. WARNING In some cases, adjusting the feedback resistors on a switching supply might require recompensating the power supply. Please refer to the applications information sup- plied with the power supply for further information. APPLICATIO S I FOR ATIO between the trim pin and the power supply positive voltage output or the trim pin and the negative power supply output (ground). The polarity of the voltage trim and trim resistor configuration are chosen by the manufacturer. The equations describing the resistor values versus the desired output voltage changes are typically not linear. Fortunately, the relationship between trim pin current and output voltage change is typically linear. The current trim equation is usually the same (in magnitude) for changing the output voltage up or down. Once the equation for trim current is determined, it is much easier to use than trim resistors. To illustrate this, Figure 8 shows a typical resistor trim down curve for a power module. Figure 9 shows a typical current trim down curve for the same power module. LTC2920 IM RS POWER MODULE SENSE– VIN – VO – TRIM VIN + VO + VO – VPSOUT RSET 2920-1/2 F07 IMARGIN SENSE+ Figure 7. Using a Power Module Trim Pin for Voltage Margining Selecting the RSET Resistor Using Voltage Trim Pins with ‘Brick’ Type Power Supply Modules ‘Brick’ power supply modules often have a trim pin which can be used for voltage margining. Figure 7 shows a typical connection using the LTC2920 for voltage margin- ing a power supply module. The amount of current necessary to adjust the output voltage of the power supply module is not normally given directly by the manufacturer. However, by using informa- tion that is supplied by the manufacturer, a measurement can be made to determine a simple equation that is useful for power supply module voltage margining. Typically, the manufacturer will supply two different equa- tions for selecting trim resistors: one for trimming the output voltage up and a different one for trimming the output voltage down. Trim resistors are nominally placed TRIM VOLTAGE (V) 0 0.1 0.2 0.3 0.4 0.5 2920-1/2 F08 1M 100k 10k 1k 100 10 1 Figure 8. Typical Trim Voltage vs Trim Resistor Curve TRIM VOLTAGE (V) 0 0.1 0.2 0.3 0.4 0.5 2920-1/2 F09 300 250 200 150 100 50 0 Figure 9. Typical Trim Voltage vs Trim Current Curve |
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