Electronic Design

Power-Supply Tracker Also Margins Supplies

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. In addition to its usual task of tracking multiple power supplies, the LTC2923 power-supply tracking controller can be used to margin supplies.

The LTC2923 employs 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 0.8 V 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. Typically, a resistive divider connects the master signal to the TRACK pin. Selecting the appropriate resistor values, RTA and RTB, configures the relationship of the slave power supply relative to the master signal. But in this application, an LTC2923 tracking cell is used to margin a supply high and low under the control of a three-state I/O pin.

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 illustration. If the feedback voltage, VFB, of the power supply is 0.8 V, simply 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 2.5 V. To margin 10% high to 2.75 V requires a 54.4-k V 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 is to 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.

In Figure 3's circuit, an LTC2923 ramps up a 3.3-V supply through a series FET, tracks a 2.5-V supply to that 3.3-V supply, and margins the 2.5-V supply up and down by 10%. The first tracking cell connected to pins TRACK1 and FB1 causes the 2.5-V supply to track this 3.3-V 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 2.5-V supply up and down by 10%.

Operation of the circuit in Figure 3 is fairly straightforward. To margin high, the I/O pin is pulled above 1.6 V. This pulls the TRACK2 pin above 0.8 V so that no current is sourced into the power supply's feedback node. The supply then defaults to its margined high output of 2.75 V. For a nominal 2.5-V output, the I/O is high impedance. Now, no current flows through RTM2 but 14.7 m A flows through RTM1 and is mirrored at the power supply's feedback node. This forces the output voltage down by 250 mV to 2.50 V. For a margined low output, the I/O pin is pulled to ground. Now, 14.7 m A flows through RTM2 in addition to the 14.7 m A flowing through RTM1. This current is mirrored at the power-supply feedback node and drives the output down by an extra 250 mV 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.

If the desired high- and low-voltage margins, D VHIGH and D VLOW, aren't equal, simply adjust RTM2. In this case, choose RFM1 as above to configure the high margin, and set RTM1 = RFM1. Scale the voltage step D VLOW relative to the voltage step D VHIGH by choosing RTM2 via: RTM1/RTM2 = D VLOW/ D VHIGH. For example, to change the margins in the above example to 10% high and 20% low, leave RFM1 and RTM1 unchanged at 54.4 k V , but reduce RTM2 to 27.2 k V .

If the feedback voltage, VFB, of the power supply isn't 0.8 V, then the values of RTM1 and RTM2 are scaled by 0.8 V/VFB. If the feedback voltage in the above example were 1.23 V, then RTM1 and RTM2 would be scaled so that RTM1 = RTM2 = RFM1 3 0.8 V/1.23 V = 35.4 k V .

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