advertisement Power Supply Tracker Can Also Margin Supplies – Design Note 372 Dan Eddleman Power supply margining is a technique commonly used to test circuit boards in production. By adjusting power supply output voltages, electrical components are tested at the upper and lower supply voltage limits specified for a design. The LTC®2923 power supply tracking controller can be used to margin supplies in addition to its usual task of tracking multiple power supplies. The LTC2923 uses the simple tracking cell shown in Figure 1 to control the ramp-up and ramp-down behavior of multiple supplies. This cell servos the TRACK input to 0.8V and mirrors the current supplied by that pin at the FB output pin. The FB pin connects to the feedback node of the slave power supply. Normally, a resistive divider connects the master signal to the TRACK pin. By selecting the appropriate resistor values, RTA and RTB, the relationship of the slave power supply is configured relative to the master signal. The supply margining application uses an LTC2923 tracking cell to margin a supply high and low under the control of a three-state I/O pin. illustration. If the feedback voltage, VFB, of the power supply is 0.8V, solve for the value of RFM1 that must be added in parallel with RF1 of the existing design to produce the desired high margin output. In Figure 2, the feedback resistors RF2 and RF1 produce an output voltage of 2.5V. To margin 10% high to 2.75V requires a 54.4k resistor, RFM1, in parallel with RF1. Now connect a resistor, RTM1, whose value is equal to RFM1 between the TRACK pin and ground. If the output will be margined low by the same voltage that it was margined high, then connect another resistor, RTM2, equal to RFM1, between the TRACK pin and the three-state I/O pin. THREE-STATE I/O RTM2 54.4k LTC2923 DC/DC TRACK2 FB2 VFB = 0.8V VOUT DN372 F02 RTM1 54.4k RFM1 54.4k RF1 8k RF2 17k Figure 2. The LTC2923 Margins the Output of a 2.5V Supply 10% High or Low Under the Control of a Three-State I/O VCC + + – MASTER 0.8V – RTB TRACK DC/DC FB FB OUT SLAVE RTA RFA RFB DN372 F01 Figure 1. Simplified Tracking Cell In the circuit shown in Figure 2, the supply is margined to its high, low and nominal output voltages by driving the I/O pin to its high, low and high impedance states respectively. This example shows calculated resistor values rather than standard resistor values for ease of 09/05/372 In the circuit shown in Figure 3, an LTC2923 ramps up a 3.3V supply through a series FET, tracks a 2.5V supply to that 3.3V supply, and margins the 2.5V supply up and down by 10%. The first tracking cell connected to pins TRACK1 and FB1 causes the 2.5V supply to track this 3.3V supply during power up and power down as shown in Figure 4. The tracking cell connected to TRACK2 and FB2 is used to margin the 2.5V supply up and down by 10%. The operation of the circuit in Figure 3 is simple. To margin high, the I/O pin is pulled above 1.6V. This pulls the TRACK2 pin above 0.8V so that no current is sourced into the feedback node of the power supply. The supply then defaults to its margined high output of 2.75V. For a nominal output, the I/O is high impedance. Now, no , LTC and LT are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Q1 VIN 3.3V 3.3V 3.3V CGATE 0.1µF RON2 138k RON1 100k RT2 17k VCC GATE LTC2923 RAMPBUF FB1 50ms/DIV DC/DC FB2 TRACK2 RTM1 54.4k 10% 0.5V/DIV RAMP RT1 8k RTM2 54.4k 10% ON TRACK1 3-STATE I/O 2.5V FB 0.8V OUT 2.5V GND RFM1 54.4k RF1 8k RF2 17k DN372 F03 Figure 3. The 2.5V Supply Tracks the 3.3V Supply and Can be Margined High or Low by 10% Under Control of a Three-State I/O current flows through RTM2 but 14.7µA flows through RTM1 and is mirrored at the feedback node of the power supply. This forces the output voltage down by 250mV to 2.50V. For a margined low output, the I/O pin is pulled to ground. Now, 14.7µA flows through RTM2 in addition to the 14.7µA flowing through RTM1. This current is mirrored at the power supply feedback node, and drives the output down by an extra 250mV from nominal. Note that the ability to configure a current driven into the feedback node with RTM1 often allows the nominal output voltage to be closer to the ideal value than is possible with a single pair of standard value resistors, RF1 and RF2, in the power supply feedback network. DN372 F04 Figure 4. Output of Circuit in Figure 3. The 2.5V Supply Tracks the 3.3V Supply and is Margined High and Low by 10% If the desired high and low voltage margins, ∆VHIGH and ∆VLOW, are not equal simply adjust RTM2. In this case, choose RFM1 as above to configure the high margin, and set RTM1 = RFM1. Scale the voltage step ∆VLOW relative to the voltage step ∆VHIGH by choosing RTM2 by RTM1/RTM2 = ∆VLOW/∆VHIGH. For example, to change the margins in the above example to 10% high and 20% low, leave RFM1 and RTM1 unchanged at 54.4k, but reduce RTM2 is to 27.2k. If the feedback voltage, VFB, of the power supply is not 0.8V then the values of RTM1 and RTM2 are scaled by 0.8V/VFB. If the feedback voltage in the above example were 1.23V, then RTM1 and RTM2 would be scaled so that RTM1 = RTM2 = RFM1 • 0.8V/1.23V = 35.4k. Conclusion The LTC2923’s primary application is tracking power supplies, but its versatile architecture is suited to other functions as well. The application described here allows a three-state I/O to control supply margining using a few resistors and an LTC2923 tracking cell. Data Sheet Download http://www.linear.com Linear Technology Corporation For applications help, call (408) 432-1900, Ext. 2452 dn372f LT/TP 0905 409K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2005