LTC2920-1/LTC2920-2
7
292012fa
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.
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.
APPLICATIO S I FOR ATIO
WUU
U
R
SET
2920-1/2 F02
R
G
V
REF
V
PSOUT
+
–
R
F
I
MARGIN
I
FB
I
RG
I
M
R
S
LTC2920
+
–
Figure 2. Simplified Power Supply Model
LTC2920
I
M
R
S
POWER MODULE
SENSE
–
V
IN
–
V
O
–
TRIM
V
IN
+
V
O
+
V
O
–
V
PSOUT
R
SET
R
SYSTEM
2920-1/2 F03
I
MARGIN
SENSE
+
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.
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
.
