LT3081
13
3081fc
For more information
applicaTions inForMaTion
Again, the lower current levels used in the LT3081 neces-
sitate attention to board leakages as error sources (see the
Programming Linear Regulator Output Voltage section).
In a current source configuration, programmable cur-
rent limit and current monitoring functions are often
unused. When not used, tie I
MON
to OUT and leave I
LIM
open. The TEMP pin is still available for use, if unused tie
TEMP to OUT.
Selecting R
SET
and R
OUT
in Current Source Applications
In Figure 3, both resistors R
SET
and R
OUT
program the
value of the output current. The question now arises: the
ratio of these resistors is known, but what value should
each resistor be?
The first resistor to select is R
SET
. The value selected should
generate enough voltage to minimize the error caused by
the offset between the SET and OUT pins. A reasonable
starting level is ~200mV of voltage across R
SET
(R
SET
equal
to 4.02k). Resultant errors due to offset voltage are a few
percent. The lower the voltage across R
SET
becomes, the
higher the error term due to the offset.
From this point, selecting R
OUT
is easy, as it is a straight-
forward calculation from R
SET
. Take note, however, resistor
errors must be accounted for as well. While larger voltage
drops across R
SET
minimize the error due to offset, they
also increase the required operating headroom.
Obtaining the best temperature coefficient does not require
the use of expensive resistors with low ppm temperature
coefficients. Instead, since the output current of the LT3081
is determined by the ratio of R
SET
to R
OUT
, those resis-
tors should have matching temperature characteristics.
Less expensive resistors made from the same material
provide matching temperature coefficients. See resistor
manufacturers’ data sheets for more details.
Higher output currents necessitate the use of higher watt-
age resistors for R
OUT
. There may be a difference between
the resistors used for R
OUT
and R
SET
. A better method to
maintain consistency in resistors is to use multiple resis-
tors in parallel to create R
OUT
, allowing the same wattage
and type of resistor as R
SET
.
Programming Current Limit Externally
A resistor placed between I
LIM
and OUT on the LT3081
externally sets current limit to a level lower than the internal
current limit. Connect this resistor directly at the OUT pins
for best accuracy. The value of this resistor calculates as:
R
ILIM
= I
LIMIT
/360mA/kΩ + 450Ω
The resistor for a 1.3A current limit is: R
ILIM
= 1.3A/360mA/
kΩ + 450Ω = 4.06k. Tolerance over temperature is ±15%,
so current limit is normally set 20% above maximum load
current. The 450Ω offset resistance built in to the pro-
grammable current limit allows for lowering the maximum
output current to only bias currents (see curve of Minimum
Load Current in Typical Performance Characteristics) us-
ing external switches.
The LT3081’s internal current limit overrides the pro-
grammed current limit if the input-to-output voltage dif-
ferential in the power transistor is excessive. The internal
current limit is ≈2A with a foldback characteristic dependent
on input-to-output differential voltage, not output voltage
per se
(see Typical Performance Characteristics).
Stability and Input Capacitance
The LT3081 does not require an input capacitor to main-
tain stability. Input capacitors are recommended in linear
regulator configurations to provide a low impedance input
source to the LT3081. If using an input capacitor, low
ESR, ceramic input bypass capacitors are acceptable for
applications without long input leads. However, applica-
tions connecting a power supply to an LT3081 circuit’s
IN and GND pins with long input wires combined with
low ESR, ceramic input capacitors are prone to voltage
spikes, reliability concerns and application-specific board
oscillations. The input wire inductance found in many
battery-powered applications, combined with the low ESR
ceramic input capacitor, forms a high Q LC resonant tank
circuit. In some instances this resonant frequency beats
against the output current dependent LDO bandwidth and
interferes with proper operation. Simple circuit modifica-
tions/solutions are then required. This behavior is not
indicative of LT3081 instability, but is a common ceramic
input bypass capacitor application issue.