LT1584/LT1585/LT1587
U
W U U
APPLICATIONS INFORMATION
loadcurrentof10mA.Thecurrentoutoftheadjustpinadds
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.
The connection shown in Figure 5 does not multiply RP by
the divider ratio. As an example, RP is about four milliohms
perfootwith16-gaugewire.Thistranslatesto4mVperfoot
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.
LT1584
V
IN
IN
OUT
V
OUT
C2
22µF
+
+
C1
10µF
ADJ
V
REF
R1
R2
I
ADJ
55µA
(1 + R2/R1) + I
ADJ
R
P
PARASITIC
LINE RESISTANCE
V
= V
REF
(R2)
OUT
LT1584
ADJ
LT1585 • F03
V
IN
OUT
IN
Figure 3. Basic Adjustable Regulator
R1*
R2*
Load Regulation
R
L
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.
*CONNECT R1 TO CASE
CONNECT R2 TO LOAD
LT1584 • F05
Figure 5. Connection for Best Load Regulation
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.
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
moreaccuratelyreflectdevicetemperatureandensuresafe
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.
R
P
PARASITIC
LINE RESISTANCE
LT1584-3.3
IN OUT
V
IN
GND
R
L
LT1585 • F04
Figure 4. Connection for Best Load Regulation
For adjustable voltage devices, negative side sensing is a
trueKelvinconnectionwiththebottomoftheoutputdivider
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:
RP × (1 + R2/R1), RP = Parasitic Line Resistance
Junction-to-case thermal resistance is specified from the
IC junction to the bottom of the case directly below the die.
10
Linear [ Linear ]
