LT1584/LT1585/LT1587
APPLICATIONS INFORMATION
load current of 10mA. The current out of the adjust pin adds
to the current from R1 and is typically 55µA. Its output
voltage contribution is small and only needs consideration
when very precise output voltage setting is required.
LT1584
V
IN
IN
C1
10µF
I
ADJ
55µA
V
OUT
= V
REF
(1 + R2/R1) + I
ADJ
(R2)
R2
LT1584
LT1585 • F03
+
OUT
ADJ
V
REF
R1
+
V
OUT
C2
22µF
Figure 3. Basic Adjustable Regulator
Load Regulation
It is not possible to provide true remote load sensing
because the LT1584/LT1585/LT1587 are three-terminal
devices. Load regulation is limited by the resistance of the
wire connecting the regulators to the load. Load regulation
per the data sheet specification is measured at the bottom
of the package.
For fixed voltage devices, negative side sensing is a true
Kelvin connection with the ground pin of the device re-
turned to the negative side of the load. This is illustrated in
Figure 4.
LT1584-3.3
IN
OUT
GND
R
L
R
P
PARASITIC
LINE RESISTANCE
V
IN
LT1585 • F04
Figure 4. Connection for Best Load Regulation
For adjustable voltage devices, negative side sensing is a
true Kelvin connection with the bottom of the output divider
returned to the negative side of the load. The best load
regulation is obtained when the top of resistor divider R1
connects directly to the regulator output and not to the
load. Figure 5 illustrates this point. If R1 connects to the
load, the effective resistance between the regulator and the
load is:
R
P
×
(1 + R2/R1), R
P
= Parasitic Line Resistance
10
U
W
U U
The connection shown in Figure 5 does not multiply R
P
by
the divider ratio. As an example, R
P
is about four milliohms
per foot with 16-gauge wire. This translates to 4mV per foot
at 1A load current. At higher load currents, this drop
represents a significant percentage of the overall regula-
tion. It is important to keep the positive lead between the
regulator and the load as short as possible and to use large
wire or PC board traces.
R
P
PARASITIC
LINE RESISTANCE
OUT
ADJ
R1*
R
L
R2*
V
IN
IN
*CONNECT R1 TO CASE
CONNECT R2 TO LOAD
LT1584 • F05
Figure 5. Connection for Best Load Regulation
Thermal Considerations
The LT1584/LT1585/LT1587 family protects the device
under overload conditions with internal power and thermal
limiting circuitry. However, for normal continuous load
conditions, do not exceed maximum junction temperature
ratings. It is important to consider all sources of thermal
resistance from junction-to-ambient. These sources in-
clude the junction-to-case resistance, the case-to-heat
sink interface resistance, and the heat sink resistance.
Thermal resistance specifications have been developed to
more accurately reflect device temperature and ensure safe
operating temperatures. The electrical characteristics sec-
tion provides a separate thermal resistance and maximum
junction temperature for both the control circuitry and the
power transistor. Older regulators, with a single junction-
to-case thermal resistance specification, use an average of
the two values provided here and allow excessive junction
temperatures under certain conditions of ambient tem-
perature and heat sink resistance. Calculate the maximum
junction temperature for both sections to ensure that both
thermal limits are met.
Junction-to-case thermal resistance is specified from the
IC junction to the bottom of the case directly below the die.
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