LTC2925CUF Linear Technology, LTC2925CUF Datasheet - Page 18

• Manufacturer Part Number: LTC2925CUF
• Description: IC POWER SUPPLY CONTROLLER 24QFN
• Manufacturer: Linear Technology

Specifications of LTC2925CUF

• Applications: Power Supply Controller
• Voltage - Supply: 2.9 V ~ 5.5 V
• Current - Supply: 1.5mA
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 24-QFN
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Voltage - Input: -

LTC2925
APPLICATIONS INFORMATION
Final Sanity Checks
The collection of equations below is useful for identifying
unrealizable solutions.
As stated in step 2, the slave supply must fi nish ramping
before the master signal has reached its fi nal voltage. This
can be verifi ed by the following equation:
Here, V
ter signal, either the supply voltage ramped up through the
optional external FET or V
It is possible to choose resistor values that require the
LTC2925 to supply more current than the Electrical Char-
acteristics table guarantees. To avoid this condition, check
that I
exceed ±3mA.
To confi rm that I
mum guaranteed current, verify that:
Finally, check that the RAMPBUF pin will not be forced to
sink more than 3mA when it is at 0V or be forced to source
more than 3mA when it is at V
18
V
R
R
V
V
TRACK
R
TRACK
TA
V
TA1
TRACKx
TRACK
MASTER
TB1
TRACK
R
+
TB
R
⎛
⎜
⎝
+
1+
TB1
does not exceed 1mA and I
= 0.8V. V
< 1
V
R
TRACK
R
R
+
mA
TRACKx
TB2
TA
TB
R
TA2
⎞
⎟ <
⎠
V
MASTER
MASTER
+
+
V
< 1mA, the TRACKx pin(s) maxi-
V
MASTER
R
R
TRACK
CC
TB2
TB3
is the fi nal voltage of the mas-
when no FET is present.
MASTER
+
<
R
3mA and
TA3
V
MASTER
.
+
RAMPBUF
R
TB3
<
does not
3mA
Caution with Boost Regulators and Linear Regulators
Note that the LTC2925’s tracking cell is not able to control
the outputs of all types of power supplies. If it is necessary
to control a supply, where the output is not controllable
through its feedback node, the series FET can be used to
control its output. For example, boost regulators commonly
contain an inductor and diode between the input supply
and the output supply providing a DC current path when
the output voltage falls below the input voltage. Therefore,
the LTC2925’s tracking cell will not effectively drive the
supply’s output below the input.
Special caution should be taken when considering the use
of linear regulators. Three-terminal linear regulators have
a reference voltage that is referred to the output supply
rather than to ground. In this case, driving current into
the regulator’s feedback node will cause its output to rise
rather than fall. Even linear regulators that have their ref-
erence voltage referred to ground, including low-dropout
regulators (LDOs), may be problematic. Linear regulators
commonly contain circuitry that prevents driving their
outputs below their reference voltage. This may not be
obvious from the datasheets, so lab testing is recom-
mended whenever the LTC2925’s tracking cell is used to
control linear regulators.
2925fc