PRODUCT SPECIFICATION RC1587
REV. 1.5 7
Figure 11. Basic Regulator Circuit
Load Regulation
It is not possible to provide true remote load sensing because
the RC1587 series are three-terminal devices. Load regula-
tion 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 returned
to the negative side of the load. This is illustrated in
Figure 12.
Figure 12. 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 regu-
lation is obtained when the top of resistor divider R1 con-
nects directly to the regulator output and not to the load.
Figure 13 illustrates this point.
If R1 connects to the load, then the effective resistance
between the regulator and the load would be:
R
P
× (1 + R2/R1), R
P
= Parasitic Line Resistance
The connection shown in Figure 13 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 repre-
sents a significant percentage of the overall regulation. 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.
Figure 13. Connection for Best Load Regulation
Thermal Considerations
The RC1587 series protect themselves under overload condi-
tions with internal power and thermal limiting circuitry.
However, for normal continuous load conditions, do not
exceed maximum junction temperature ratings. It is impor-
tant to consider all sources of thermal resistance from junc-
tion-to-ambient. These sources include 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 tempera-
ture and ensure safe operating temperatures.
For example, look at using an RC1587T to generate 3A @
1.5V ± 2% from a 3.3V source (3.2V to 3.6V).
Assumptions:
•V
IN
= 3.6V worst case
•V
OUT
= 1.46V worst case
•I
OUT
= 3A continuous
•T
A
= 70°C
• θ
Case-to-Ambient
= 3°C/W (assuming both a heatsink and
a thermally conductive material)
The power dissipation in this application is:
P
D
= (V
IN
– V
OUT
) * (I
OUT
) = (3.6 – 1.46) * (3) = 6.42W
From the specification table:
T
J
= T
A
+ (P
D
) * (θ
Case-to-Ambient
+ θ
JC
)
= 70 + (6.42) * (3 + 3) = 109°C
The junction temperature is below the maximum rating.
RC1587
ADJ
I
ADJ
35µA
R2
R1
C2
22µF
V
OUT
V
REF
+
C1
10µF
+
65-1587-14
IN
V
OUT
= V
REF
(1 + R2/R1) + I
ADJ
(R2)
OUT
V
IN
RC1587-1.5, -3.3
GND
R
L
65-1587-17
R
P
PARASITIC
LINE RESISTANCE
IN OUT
V
IN
RC1587
ADJ
R2*
CONNECT R1 TO CASE
CONNECT R2 TO LOAD
*
R1*
R
L
65-1587-15
R
P
PARASITIC
LINE RESISTANCE
IN OUT
V
IN
