5. Output Voltage Trimming Design Guide & Applications Manual For VI-200 and VI-J00 Family DC-DC Converters and Configurable Power Supplies OVERVIEW Specifications such as efficiency, ripple and input voltage range are a function of output voltage settings. As the output voltage is trimmed down, efficiency goes down; ripple as a percent of Vout goes up and the input voltage range widens since input voltage dropout (loss of regulation) moves down. As the units are trimmed up, the reverse of the above effects occurs. Example 1. For trimming –10% to +10% with a standard off-the-shelf 10 kΩ potentiometer (R7), values for resistors R6 and R8 need to be calculated. Resistor R6 limits the trim down range. For a given percentage, its value is independent of output voltage. Refer to Table 5–1, for limiting resistor values. All converters have a fixed current limit. The overvoltage protection setpoint is also fixed; trimming the output voltage does not alter its setting. As the output voltage is trimmed down, the current limit setpoint remains constant. Therefore, in terms of output power, if the unit is trimmed down, available output power drops accordingly. +OUT R1 47 Ω Typ. Error Amp – + Do not attempt to trim the module output voltage more than +10%, as overvoltage shut down may occur. Do not exceed maximum rated output power when the module is trimmed up. CAUTION: When trimming up VI- / MI-J00 converter modules, additional care should be taken as an improper component selection could result in module failure. Improper connection of the sense leads on VI- / MI-J00 converter modules can also result in an excessive overvoltage condition and module failure. R8 R2 TRIM R5 10 kΩ[a] 2.5 V[a] The output voltage of most Vicor converters can be trimmed +10%, –50%. Certain modules have restricted trim ranges. Consult the latest datasheet for details. +SENSE R3 C1 R6 R7 Load –SENSE R4 27 Ω Typ. –OUT [a]For Vout <3.3 V, R5 = 3.88 kΩ and internal reference = 0.97 V. Figure 5–1 — External resistive network for variable trimming TRIMMING DOWN –10% A 10% drop of the 2.5 V reference at the TRIM pin is needed to effect a 10% drop in the output voltage. (Figure 5–2) +OUT The following procedures describe methods for output voltage adjustment (–10 to +10% of nominal) of the VI-/ MI-200, VI-/ MI-J00, ComPAC / MI-ComPAC, FlatPAC and MegaMod / MI-MegaMod Families. Modules with nominal 3.3 V outputs and above have the 2.5 V precision reference and 10 k internal resistor. For trim resistor calculations on modules with 2.0 V outputs use 0.97 V in place of the 2.5 V reference and substitute 3.88 kΩ for the internal 10 kΩ resistor. Resistors are 0.25 W. When trimming down any module, always maintain a minimum preload of at least 1% of rated output power and in some cases up to 10% may be required. For more specific information on trimming down a specific module, please consult Vicor’s Applications Engineering Department at (800) 927-9474. 2.5 V [a] reference (internal) R5 10 kΩ [a] (internal) V1 +SENSE TRIM R6 I R6 –SENSE R8 R7 10 kΩ POT –OUT [a] For Vout <3.3 V, R5 = 3.88 kΩ and internal reference = 0.97 V. Figure 5–2 — Circuit diagram “Trim Down” V1 = 2.5 V – 10% = 2.25 V Therefore: IR5 = (2.5 V – 2.25 V) = 25 µA 10 kΩ Since IR5 = IR6 = 25 µA: RESISTIVE ADJUSTMENT PROCEDURE R6 = 2.25 V = 90 kΩ 25 µA To achieve a variable trim range, an external resistor network must be added. (Figure 5–1) This value will limit the trim down range to –10% of nominal output voltage. VI-200 and VI-J00 Family Design Guide Page 8 of 98 Rev 3.5 Apps. Eng. 800 927.9474 vicorpower.com 800 735.6200 5. Output Voltage Trimming Design Guide & Applications Manual For VI-200 and VI-J00 Family DC-DC Converters and Configurable Power Supplies TRIMMING UP +10% FIXED TRIM To trim 10% above the nominal output voltage, the following calculations are needed to determine the value of R8. This calculation is dependent on the output voltage of the module. A 12 V output will be used as an example. (Figure 5–3) Converters can be trimmed up or down with the addition of one external resistor, either Ru for programming up or Rd for programming down. (Figure 5–4) Example 2. Fixed Trim Up (12 V to 12.6 V). To determine Ru, the following calculation must be made: It is necessary for the voltage at the TRIM pin to be 10% greater than the 2.5 V reference. This offset will cause the error amplifier to adjust the output voltage up 10% to 13.2 V. 2.5 V + 5% = 2.625 V VR5 = VTRIM – Vref VR5 = 2.625 – 2.5 = 0.125 V Knowing this voltage, the current through R5 can be found: + OUT + SENSE R5 10 kΩ[a] (internal) 2.5 V [a] reference (internal) V1 TRIM R8 R6 90 kΩ V2 – SENSE 25 µA IR5 = I R8 R7 10 kΩ POT 500 µA VRu = 12.6 V – 2.625 V = 9.975 V Ru = 9.975 = 798 kΩ 12.5 µA – OUT [a] VR5 = 0.125 = 12.5 µA R5 10 kΩ For Vout <3.3 V, R5 = 3.88 kΩ and internal reference = 0.97 V. Figure 5–3 — Circuit diagram “Trim Up” + OUT R5 10 kΩ [a] (internal) V1 = 2.5 V + 10% = 2.75 V 2.5 V [a] reference (internal) IR5 = (2.75 V – 2.5 V) = 25 µA 10 kΩ Therefore, V2 = 2.75 V + 2.25 V = 5 V. The current through R7 (10 kΩ pot) is: V2 R7 = 5 = 500 µA 10 k Using Kirchoff’s current law: IR8 = IR7 + IR6 = 525 µA Thus, knowing the current and voltage, R8 can be determined: VR8 = (Vout + 10%) – V2 = 13.2 V – 5 V = 8.2 V R8 = Ru TRIM – SENSE Rd (8.2 V) = 15.6 kΩ 525 µA [a] For Vout <3.3 V, R5 = 3.88 kΩ and internal reference = 0.97 V. Figure 5–4 — Fixed trimming Connect Ru from the TRIM pin to the +SENSE. Be sure to connect the resistor to the +SENSE, not the +OUT, or drops in the positive output lead as a function of load will cause apparent load regulation problems. Example 3. –25% Fixed Trim Down (24 V to 18 V). The trim down methodology is identical to that used in Example 2, except that it is utilized to trim the output of a 24 V module down 25% to 18 V. The voltage on the TRIM pin must be reduced 25% from its nominal setting of 2.5 V. This is accomplished by adding a resistor from the TRIM pin to –SENSE. 2.5 V – 25% = 1.875 V This resistor configuration allows a 12 V output module to be trimmed up to 13.2 V and down to 10.8 V. Follow this procedure to determine resistor values for other output voltages. VI-200 and VI-J00 Family Design Guide Page 9 of 98 Trim Resistor for UP Programming or Trim Resistor for DOWN Programming – OUT Since IR5 = IR6 , the voltage drop across R6 = (90 kΩ) (25 µA) = 2.25 V. IR7 = + SENSE Rev 3.5 Apps. Eng. 800 927.9474 VR5 = Vbandgap – VTRIM = 2.5 V – 1.875 V = 0.625 V vicorpower.com 800 735.6200 5. Output Voltage Trimming Design Guide & Applications Manual For VI-200 and VI-J00 Family DC-DC Converters and Configurable Power Supplies Knowing this voltage, the current through R5 can be found: IR5 = VR5 R5 = 0.625 = 62.5 µA 10 kΩ The voltage across the resistor, Rd, and the current flowing through it are known: Rd = (2.5 V – 0.625 V) = 30 kΩ 62.5 µA Connect Rd (Figure 5– 4) from the TRIM pin to the –SENSE of the module. Be sure to connect the resistor to the –SENSE, not the –OUT, or drops in the negative output lead as a function of load will cause apparent load regulation problems. Values for Trim Down by Percentage Percent –5 % –10 % –15 % –20 % –25 % –30 % –35 % – 40 % – 45 % –50 % Resistance 190 kΩ 90 kΩ 56.7 kΩ 40 kΩ 30 kΩ 23.3 kΩ 18.6 kΩ 15 kΩ 12.2 kΩ 10 kΩ Table 5–1 — Values for trim down by percentage (Refer to product data sheet for allowable trim ranges at vicorpower.com) Fixed Trim Down Vnom 5 V 15 V 24 V 48 V V (Desired) 4.5 V 3.3 V 2.5 V 13.8 V 20 V 40 V 36 V Trim Resistor [a] 90.9 kΩ 19.6 kΩ 10.0 kΩ 115 kΩ 49.9 kΩ 49.9 kΩ 30.1 kΩ DYNAMIC ADJUSTMENT PROCEDURE Output voltage can also be dynamically programmed by driving the TRIM pin from a voltage or current source; programmable power supplies and power amplifier applications can be addressed in this way. For dynamic programming, drive the TRIM pin from a source referenced to the negative sense lead, and keep the drive voltage in the range of 1.25 – 2.75 V. Applying 1.25 – 2.5 V on the TRIM pin corresponds to 50 – 100% of nominal output voltage. For example, an application requires a +10, 0% (nominal), and a –15% output voltage adjustment for a 48 V output converter. Referring to the table below, the voltage that should be applied to the trim pin would be as follows: VTRIM VOUT Change from nominal 2.125 2.5 2.75 40.8 48 52.8 –15% 0 +10% The actual voltage range is further restricted by the allowable trim range of the converter. Voltages in excess of 2.75 V (+10% over nominal) may cause overvoltage protection to be activated. For applications where the module will be programmed on a continuous basis the slew rate should be limited to 30 Hz sinusoidal. TRIMMING ON THE WEB (VICORPOWER.COM) Trim values are calculated automatically. Design Calculators are available on Vicor’s website in the PowerBenchTM section at www.vicorpower.com/powerbench. Resistor values can be easily determined for fixed trim up, fixed trim down and for variable trimming applications. In addition to trimming information, the website also includes design tips, applications circuits, EMC suggestions, thermal design guidelines and PDF data sheets for all available Vicor products. Table 5–2a — Values for fixed trim down by voltage Fixed Trim Up Vnom 5 V 12 V 15 V 24 V 48 V V (Desired) 5.2 V 5.5 V 12.5 V 13.2 V 15.5 V 16.5 V 25 V 50 V Trim Resistor [a] 261 kΩ 110 kΩ 953 kΩ 422 kΩ 1.62 MΩ 562 kΩ 2.24 MΩ 4.74 MΩ Table 5–2b — Values for fixed trim up by voltage [a] Values listed in the tables are the closest standard 1% resistor values. VI-200 and VI-J00 Family Design Guide Page 10 of 98 Rev 3.5 Apps. Eng. 800 927.9474 vicorpower.com 800 735.6200