LTC2920-2IMS8#TRPBF Linear Technology, LTC2920-2IMS8#TRPBF Datasheet - Page 7

• Manufacturer Part Number: LTC2920-2IMS8#TRPBF
• Description: IC SNGL/DUAL PWR SUP CTRLR 8MSOP
• Manufacturer: Linear Technology

Specifications of LTC2920-2IMS8#TRPBF

• Applications: Single/Dual Power Supply Controller
• Voltage - Supply: 2.3 V ~ 6 V
• Current - Supply: 230µA
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 8-MSOP, Micro8™, 8-uMAX, 8-uSOP,
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Input: -

APPLICATIO S I FOR ATIO
Knowing the value of the resistors R
voltage of V
Since the op amp keeps its inverting terminal equal to the
noninverting terminal, the voltage at the inverting terminal
between R
in the feedback resistor network, V
calculated by:
This is the voltage on one side of R
across R
the LTC2920 changes the power supply output voltage.
Figure 2 shows the simplified model with the LTC2920
added.
Again in this circuit, the op amp will keep the voltage at its
inverting input at V
node, the delta current will always be added or subtracted
from I
signed I
added or subtracted at the feedback pin.) Because of this,
the voltage across R
or
and finally
Note that the delta voltage V
I
MARGIN
V
V
V
V
V
PSOUT
PSOUT
RF
RF
PSOUT
R
FB
SET
= (I
= (I
MARGIN
, and never I
and R
F
. This equation is helpful in understanding how
Figure 2. Simplified Power Supply Model
FBNOM
FBNOM
= V
= V
= V
F
REF
and R
R
LTC2920
REF
REF
REF
F
S
, V
, not R
value to emphasize the fact that current is
± I
• R
• [1+ (R
+ (I
I
+ (I
PSOUT
G
M
REF
U
MARGIN
is V
RG
F
F
FB
) ± (I
FBNOM
. If we add or subtract current at this
G
I
will be:
MARGIN
. (“±I
V
REF
• R
or V
REF
can be calculated by:
I
R
F
RG
U
MARGIN
G
/R
. Knowing the current flowing
F
) • R
+
–
)
MARGIN
• R
REF
G
)]
MARGIN
F
.
F
) ± (I
–
+
• R
” is used rather than a
W
I
R
FB
F
MARGIN
F
PSOUT
F
)
F
, plus the voltage
depends only on
and R
can be also
• R
G
V
2920-1/2 F02
U
, and the
PSOUT
F
)
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
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 Supply Module Design Considerations
There are usually practical limits to V
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
age drop across R
supply module specifications for details. See the “Select-
ing The R
tions on how to choose R
POWER MODULE
V
V
SET
IN
IN
Figure 3. Margining a Power Supply Module
CC
+
–
LTC2920-1/LTC2920-2
Resistor” section of this datasheet for instruc-
SENSE
SENSE
or ground with analog switches or relays.
TRIM
V
V
O
O
+
+
–
–
SYSTEM
I
MARGIN
R
SYSTEM
. See the manufacturer’s power
SET
in module applications.
I
M
V
V
PSOUT
O
LTC2920
MARGIN
–
O
+
. For instance, V
R
S
plus any volt-
2920-1/2 F03
R
SET
292012fa
7
O
+