LTC2920-1_15 Datasheet, PDF (7/16 Pages) Linear Technology – Single/Dual Power Supply Margining Controller

LTC2920-1/LTC2920-2
APPLICATIO S I FOR ATIO
Knowing the value of the resistors R_F and R_G, and the
voltage of V_REF, V_PSOUT can be calculated by:
V_PSOUT = V_REF • [1+ (R_F/R_G)]
Since the op amp keeps its inverting terminal equal to the
noninverting terminal, the voltage at the inverting terminal
between R_F and R_G is V_REF. Knowing the current flowing
in the feedback resistor network, V_PSOUT can be also
calculated by:
V_PSOUT = V_REF + (I_FB • R_F)
This is the voltage on one side of R_F, plus the voltage
across R_F. This equation is helpful in understanding how
the LTC2920 changes the power supply output voltage.
Figure 2 shows the simplified model with the LTC2920
added.
R_SET
I_M
LTC2920
R_S
I_MARGIN
R_G
I_RG
V_{REF –+}
R_F
I_FB
–
+
V_PSOUT
2920-1/2 F02
Figure 2. Simplified Power Supply Model
Again in this circuit, the op amp will keep the voltage at its
inverting input at V_REF. If we add or subtract current at this
node, the delta current will always be added or subtracted
from I_FB, and never I_RG. (“±I_MARGIN” is used rather than a
signed I_MARGIN value to emphasize the fact that current is
added or subtracted at the feedback pin.) Because of this,
the voltage across R_F will be:
V_RF = (I_FBNOM ± I_MARGIN) • R_F
or
V_RF = (I_FBNOM • R_F) ± (I_MARGIN • R_F)
and finally
V_PSOUT = V_REF + (I_FBNOM • R_F) ± (I_MARGIN • R_F)
Note that the delta voltage V_MARGIN depends only on
I_MARGIN and R_F, not R_G or V_REF.
POWER SUPPLY MODULE VOLTAGE MARGINING
Another method of accomplishing voltage margining is
useful for power supply “brick” modules with voltage
adjust pins. Typically, the power supply manufacturer will
design the power supply to be adjusted up or down, using
external resistors connected to the trim pin. The values of
these resistors are usually calculated by the design engi-
neer using two different equations supplied by the manu-
facturer. There is usually one equation for trimming the
voltage up, and another equation for trimming the voltage
down. In most cases, the power supply module is treated
like a “black box” and very little information is given on
how the trimming is accomplished from an internal circuit
standpoint.
Traditionally such power supply modules are margined by
calculating the two resistors, and alternately connecting
each to V_CC or ground with analog switches or relays.
Figure 3 shows how the LTC2920 can be used in these
applications as well. Using the LTC2920 for these applica-
tions can save a significant amount of PCB real estate
and cost.
POWER MODULE
SENSE⁺
V_IN+
V_O+
TRIM
R_SYSTEM
I_MARGIN
V_IN–
V_O–
SENSE^–
V_PSOUT
LTC2920
I_M
R_S
V_O–
R_SET
2920-1/2 F03
Figure 3. Margining a Power Supply Module
Power Supply Module Design Considerations
There are usually practical limits to V_O+. For instance, V_O+
usually has upper and lower voltage limits specified by the
power module manufacturer. A common value is 10%
above and 20% below the rated output voltage of the power
supply module. This limit includes V_MARGIN plus any volt-
age drop across R_SYSTEM. See the manufacturer’s power
supply module specifications for details. See the “Select-
ing The R_SET Resistor” section of this datasheet for instruc-
tions on how to choose R_SET in module applications.
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