### DN372 - Power Supply Tracker Can Also Margin Supplies

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.
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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.
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