Datasheet

LT3090
16
3090fa
For more information www.linear.com/LT3090
applicaTions inForMaTion
The LT3090’s positive or negative current monitor circuitry
is designed to remain accurate even under short circuit
or dropout conditions.
Externally Programmable Current Limit
The ILIM pin internally regulates to 225mV above IN.
Connecting a resistor from ILIM to IN sets the current
flowing out of the ILIM pin, which in turn programs the
LT3090’s current limit. The programming scale factor is
10kΩ • A. For example, a 20k resistor between ILIM and
IN programs current limit to 500mA. For good accuracy,
Kelvin connect this resistor to the LT3090’s IN pin.
In cases where the OUT-to-IN differential is greater than
7V, the LT3090’s foldback circuitry decreases the internal
current limit. Therefore, internal current limit may over
-
ride the
externally programmed current limit level to keep
the
LT3090 within its Safe-Operating-Area (SOA). See
the Internal Current Limit vs Input-to-Output differential
graph in the Typical Performance Characteristics section.
ILIM can be tied to IN if external programmable current limit
is not needed. However, because the ILIM pin is internally
regulated to 225mV above IN, if ILIM pin is shorted to
IN, then this loop will current limit, thereby
causing the
LT3090’s
quiescent current to increase by about 300µA.
Hence, when unused, it is recommended to tie ILIM to IN
through a 10k resistor.
Load Regulation
The LT3090 does not have a separate Kelvin connection
for sensing output voltage. Therefore, it is not possible
to provide true remote load sensing. The connectivity
resistance between the regulator and the load limits load
regulation. The data sheet specification for load regulation
is Kelvin sensed at the OUT pin of the package. GND side
Kelvin sensing is a true Kelvin connection, with the top of
the voltage setting resistor returned to the positive side of
the load (see Figure 8). Connected as shown, system load
regulation is the sum of the LT3090 load regulation and the
parasitic line resistance multiplied by the output current.
It is therefore important to keep the negative connection
between the regulator and the load as short as possible
and to use wide wires or PC board traces.
resistor to GND – this generates a negative voltage (pro
-
portional to output current) on IMONN. Furthermore, as
illustrated
in Figure 6, the negative current monitor pin
can also be used for cable drop compensation. Cable drop
compensation corrects for load
dependent voltage drop
caused by a resistive connection between the LT3090’s
OUT pin and its load.
Figure 6. Simple Cable Drop Compensation
Figure 7. Positive Output Current Monitor
For a positive current monitor application, as illustrated
in Figure 7, tie IMONP through a resistor to GND—this
generates a positive voltage (proportional to output cur
-
rent) on IMONP. And tie IMONN to a supply at least 2V
above the maximum operating IMONP voltage.
When unused, IMONN and IMONP pins can be left floating;
however, this slightly reduces (~5%) the device’s internal
current limit. Hence, if the current monitor functionality
is not used, as shown in Figure 1, it is recommended to
tie IMONN to GND and IMONP to IN.
LT3090
R
CBL
= R
CBL1
+ R
CBL2
SET
GND
ILIM
SHDN
3090 F06
–
+
50µA
IMONN
OUT
0.1µF
4.7µF
R
CDC
= R
CBL
• 1K
100k
IN
V
IN
≤–6V
IMONP
4.7µF
R
CBL1
R
CBL2
10k
LOAD
LT3090
GND
≥3V
ILIM
2k
1mV PER mA
SHDN
3090 F07
I
OUT
2000
IMONN
OUT
SET
4.7µF
IN
V
IN
–3V TO –10V
IMONP
4.7µF
V
OUT
: –2.5V
MAX I
OUT
: 600mA
10k
0.1µF 49.9k 0.1µF
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