Application Note 2 September 2008 PDF: margin-volt_prog_an.pdf Application Guidelines for Non-Isolated Converters AN04-005: Margining and Voltage Programming Introduction The Austin LynxTM, Lynx II, TLynx and Naos Raptor series of non-isolated modules all have the ability to adjust their output voltage by interfacing through the Trim pin. In addition to the output voltage set function, two other types of output voltage adjustment are frequently used. The first is margining, where the output voltage is adjusted a fixed percentage (typically ±5 or 10%) off the nominal during system level test. This adjustment is generally used for checking the operational margin of circuits and thereby assuring the robustness of the application. The second is voltage programming where an external voltage is provided to either provide more accurate adjustment of the output voltage of the module or to implement a margining-like function. This application note describes circuits and provides design guidelines for implementing margining or voltage programming. To understand how margining or voltage programming are done, it is beneficial to understand the circuit used to scale down the output voltage in the Austin Lynx and Lynx II series of modules. Figure 1 shows a portion of the output voltage divider and feedback circuitry that is used in all Lynx and Lynx II series modules (except the fixedoutput voltage 5/3.3V and 12V input Lynx modules – these come in different output voltage versions and do not have an wide-output voltage adjust capability). The output voltage is scaled down by a divider consisting of resistors R1 and R2. The mid point of the divider is connected to the inverting input of the op-amp which is used to process the error voltage between the internal reference voltage Vref (0.7V) and the divider midpoint voltage. The voltage control loop of the module regulates the output voltage so as to maintain the voltage at the mid point of the divider equal to the reference voltage. The trim pin is connected through a 1KΩ resistor to the mid point of the divider. By connecting an external resistor between the Trim pin and GND, the divider ratio is changed thus allowing adjustment of the output voltage. In both margining and voltage programming, the output voltage is adjusted by in essence effecting a change in the scaling between the output and voltage divider mid point voltages. In the TLynx and Naos Raptor series of modules, a simpler circuit is used to divide down the voltage as shown in Fig. 2. However, the same functionality is obtained is obtained through the TRIM pin. TLynx or Naos Raptor Module PWM Controller R1 - AMP Trim + Austin Lynx or Lynx II Series Module Vout VREF PWM Controller R1 AMP Trim + R2 VREF R1 2K 10K 2K 2K VREF 0.6V 0.591V 0.591V 0.6V Fig. 2. Circuit diagram of the output voltage divider and feedback circuitry used in the TLynx and Naos Raptor series of modules. GND R1 R2 R3 VREF 30.1K 402K 1K 0.7V 15K 200K 1K 0.7V 1.5K Open 100 0.8V Fig. 1. Circuit diagram of the output voltage divider and feedback circuitry used in the Austin Lynx and Lynx II series of modules (except fixed-output 5/3.3V in Austin Lynx and 12V in Austin Lynx modules). LINEAGE POWER GND 3.3Vin TLynx 12Vin TLynx 3A/6A/10A/20A Naos Raptor 40A/50A/60A Naos Raptor R3 - 5Vin Austin Lynx or Lynx II 12Vin Austin Lynx or Lynx II Austin MegaLynx Vout Implementing Margining Margining is implemented by essentially changing the divider by small amounts to obtain the desired margining voltages. One method to accomplish this is shown in Fig. 3. This scheme connects a pre-determined resistor with a small FET in series between the trim pin and ground (to margin up or slightly increase the output voltage), and a similar FET+resistor combination in series between TRIM 1 Application Guidelines for Non-Isolated Converters Application Note September 2, 2008 AN04-005: Margining and Voltage Programming and Vout to margin down (or slightly decrease the output voltage). Under normal operation, both FETs are kept off. To margin up, the lower FET (Q1) would be turned on, while to margin down the upper FET (Q2) would be turned on. Table 1 shows a summary of the various states and their effect on the output voltage. Another scheme to implement margining is shown in Fig. 4, where two FET+resistor combinations are shown connected in parallel between the Trim and Ground pins. For normal operation, one of the FETs (say Q1) is kept on while the other one is kept off. To margin up, both FETs would be on, while to margin down, both FETs would be off. The summary of the various states and effect on output voltage are shown in Table 2. Similar results are obtained using either scheme. The POL Programming Tool (free download from www.lineagepower.com) can calculate the values of resistors to be used with the margining schemes shown in Fig. 3 for different POL modules. Vo VO Austin Lynx or Lynx II Series Trim R1 R2 Rtrim Q1 Q2 GND Fig. 4. Alternate circuit for implementing margining capability in the Lynx and Lynx II series modules. Vo Rmargin-down Austin Lynx or Lynx II Series Q2 Trim Rmargin-up Rtrim Q1 GND Fig. 3. Circuit for implementing margining capability in the Lynx, Lynx II, TLynx and Naos Raptor series of modules. Table 1. Action Q1 Q2 Normal OFF OFF Margin-up ON OFF Margin-down OFF ON Table 2. Action Q1 Q2 Normal ON OFF Margin-up ON ON Margin-down OFF OFF LINEAGE POWER Margining through Voltage Programming Another method of adjusting the output voltage of the module is through programming using an external voltage source. Recall that the output voltage is scaled down by a voltage divider network, and this voltage is then fed back to compare with a fixed reference voltage inside the module. The Trim point represents the mid point of this divider. By injecting an additional voltage through a resistor into the Trim pin, the scaling between the output voltage and the voltage at the Trim pin is changed, effectively changing the output voltage. This method of adjustment is referred to as voltage programming of the module output voltage. The voltage programming equation for the 12V input Austin Lynx and Lynx II modules is (Vref − Vt )⎤ ⎡ Vref Vout = Vref + 15000 × ⎢ + 200 , 000 1000 ⎥⎦ ⎣ Vout = (16.075 × Vref ) − (15 × Vt ) where, Vt is the voltage at the Trim pin. This equation shows that the output voltage is highly sensitive to tolerances in Vref and Vt, since these terms are multiplied by 16.075 and 15, respectively. Similar considerations apply to the 5/3.3V in Austin Lynx and Lynx II modules as well. To reduce the sensitivity to tolerances in the reference and programming voltages, a modified circuit as shown in Fig. 4 needs to be used. The external output voltage Vs is fed to the module's trim pin through a resistor network to cause the change in the output voltage. When the external voltage VS is increased, a larger current flows through Rext and R3 into the inverting terminal of the opamp, thereby reducing the current that flows from Vout through resistor R1. This effectively reduces the 2 Application Guidelines for Non-Isolated Converters Application Note September 2, 2008 AN04-005: Margining and Voltage Programming scaling of the output voltage and results in a decrease of Vout. The reverse occurs when VS is reduced resulting in an increase in Vout. Table 3 shows values of various circuit parameters for the case of the 5/3.3V input Lynx and Lynx II series modules, while Table 4 shows a similar set of data for the 12V input series modules. If margining is implemented along with voltage programming, both functions can be implemented together by adjusting the source voltage VS to perform the margining. Figure 5 shows a slightly modified version of Figure 4 that can be used. Table 5 provides the values of VS needed to margin either +/-5% or +/-10% for the 5/3.3V in Lynx and Lynx II Series modules, while Table 6 provides a similar set of data for 12V in Lynx and Lynx II Series of modules. For the Austin MegaLynx, TLynx and NSR series of modules, the POL Programming tool can be used to select external voltage source and series resistor values for programming the module output voltage over a range. Summary Margining and output voltage programming are useful features of the Lynx series of modules. Along with simple external circuits, they allow versatile output voltage adjustment of the module to fit various application needs. Margining is used for checking the robustness of the circuits, as well as ensuring the proper functionality of the module under a noisy environment. Voltage programming can be used to provide a precise adjustment of the module output voltage in demanding applications or provide dynamic voltage adjustment when needed. Austin Lynx or Lynx II Series Module Vout PWM Controller R1 R3 - AMP Austin Lynx or Lynx II Series Module + Vout PWM Controller Rg VS Trim R2 Rtrim Vref 0.7V R1 GND R3 - AMP Rext1 Rg1 VS Trim + R2 Rtrim1 Vref 0.7V Table 5. External Source Value for Margining 5/3.3V Lynx Series Modules GND Fig. 4. Recommended circuit to be used in voltage programming of the Austin Lynx Series of modules. Table 3. Programming Voltage Values for 5/3.3V Lynx Modules Output Voltage Rext Rtrim Rg Vs,nom % Error 1.2V 8k 33.9k 73.2k 0.505 1.74 % 1.5V 3.5k 19.6k 55k 0.485 1.75 % 1.8V 2k 13k 39.2k 0.453 1.74 % 2.5V 1k 5.94k 14.5k 0.345 1.75 % 3.3V 0.6k 2.56k 1.685k 0.217 1.85 % Table 4. Programming Voltage Values for 12V Lynx Modules Output Voltage Rext Rtrim Rg Vs,nom Output Voltage Rtrim Rg Vs,nom Vs,up (+10%) Vs,down (-10%) Vs,up (+5%) Vs,down (-5%) 1.2 41.96k 124k 0.624 0.05 1.2 0.335 0.91 1.5V 23.07k 82k 0.573 0.05 1.095 0.310 0.835 1.8V 15.004k 55k 0.522 0.05 0.995 0.287 0.757 2.5V 6.947k 21.6k 0.403 0.05 0.758 0.226 0.58 3.3V 3.161k 5.6k 0.2675 0.05 0.485 0.159 0.376 Table 6. External Source Value for Margining 12V Lynx Series Modules Output Voltage Rtrim Rg Vs,nom Vs,up (+10%) Vs,down (-10%) Vs,up (+5%) Vs,down (-5%) 1.2 22.464k 73.2k 0.67 0.05 1.29 0.365 0.98 1.5V 13.047k 54.9k 0.65 0.05 1.25 0.350 0.950 1.8V 9.024k 42.2k 0.63 0.05 1.20 0.345 0.918 2.5V 5.009k 26.1k 0.58 0.05 1.12 0.315 0.855 3.3V 3.122k 15.8k 0.53 0.05 1.01 0.290 0.770 5.0V 1.472k 6.04k 0.417 0.05 0.79 0.231 0.603 % Error 1.2 9.53k 22.464k 73.2k 0.385 2.4% 1.5V 6.34k 6.71k 10.0k 0.335 2.4 % 2.4 % 1.8V 6.19k 2.834k 1.58k 0.198 2.5V 2.21k 2.8k 6.19k 0.326 2.4 % 3.3V 1.47k 1.652k 3.19k 0.280 2.4 % 5.0V 0.32k 1.15k 3.19k 0.325 2.4 % LINEAGE POWER Fig. 5. Recommended circuit to be used in voltage programming along with margining of the Austin Lynx Series of modules. 3 Application Guidelines for Non-Isolated Converters AN04-005: Margining and Voltage Programming Application Note September 2, 2008 Asia-Pacific Headquarters Tel: +65 6416 4283 World Wide Headquarters Lineage Power Corporation 3000 Skyline Drive, Mesquite, TX 75149, USA +1-800-526-7819 (Outside U.S.A.: +1-972-284-2626) www.lineagepower.com e-mail: [email protected] Europe, Middle-East and Africa Headquarters Tel: +49 89 6089 286 India Headquarters Tel: +91 80 28411633 Lineage Power reserves the right to make changes to the product(s) or information contained herein without notice. No liability is assumed as a result of their use or application. No rights under any patent accompany the sale of any such product(s) or information. © 2008 Lineage Power Corporation, (Mesquite, Texas) All International Rights Reserved. LINEAGE POWER 4