LT1585A ON Semiconductor, LT1585A Datasheet - Page 6

• Manufacturer Part Number: LT1585A
• Description: 5A LOW DROPOUT FAST RESPONSE POSITIVE ADJUSTABLE AND FIXED VOLTAGE REGULATOR
• Manufacturer: ON Semiconductor

Load Regulation
because the LT1585A are 3–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.
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 4 illustrates this point. If R1 connects to the load, the
effective resistance between the regulator and the load is :
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 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.
6
V out = V ref (1 + R2/R1) + I adj (R2)
It is not possible to provide true remote load sensing
For adjustable voltage devices, negative side sensing is a
R P (1 + R2/R1), R P = Parasitic Line Resistance
The connection shown in Figure 11 does not multiply R P
V in
V in
* Connect R1 to Case
*
10
Connect R2 to Load
C1
m
F
Figure 11. Basic Adjustable Regulator
+
IN
Figure 12. Connection for
LT1585A
Best Load Regulation
IN
ADJ
I adj
55
m
LT1585A
A
OUT
ADJ
OUT
Line Resistance
Parasitic
P D
V ref
R1*
R2*
R1
R2
+
C2
10
m
F
V out
FL
LT1585A
Thermal Considerations
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
junc tion–to–ambient. Thes e sourc es inc lude 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 section 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
temperature and heat sink resistance.
sections to ensure that both thermal limits are met.
IC junction to the bottom of the case directly below the
die.This is the lowest resistance path for heat flow. Proper
mounting ensures the best thermal flow from this area of the
package to the heat sink. It is strongly recommended to use
thermal compound at the case–to–heat sink interface. Use a
thermally conductive spacer if the case of the device must be
electrically isolated and include its contribution to the total
thermal resistance.
with an output adjusted to 3.3 V and assuming:
maximum rating for the respective sections, ensuring reliable
operation.
The LT1585A protects the device under overload
Calculate the maximum junction temperature for both
Junction–to–case thermal resistance is specified from the
For example, using an LT1585ACT (TO–220 package)
V in (Max Continuous) = 5.25V (5V + 5%), V out = 3.3V,
T J
T A = 70 C,
q
Power dissipation under these conditions is equal to:
P D = (V in – V out ) (I out ) = (5.25 – 3.3) (5) = 9.75W
Junction temperature will be equal to:
T J = T A + P D (
For the Control Section:
T J = 70 C + 9.75W (3 C/W + 1 C/W + 0.7 C/W)
T J
115.8 C < 125 C = T JMAX
(Control Section Commercial Range)
For the Power Transistor:
T J = 70 C + 9.75W (3 C/W + 1 C/W + 3 C/W)
T J
138.3 C < 150 C = T JMAX
(Control Section Commercial Range)
In both cases the junction temperature is below the
CASE–TO–HEAT SINK = 1 C/W (with Thermal Compound)
= 115.8 C
= 138.3 C
I OUT = 5A.
q
HEAT SINK = 3 C/W
q
MOTOROLA ANALOG IC DEVICE DATA
HEAT SINK +
q
CASE–TO–HEAT SINK +
q
JC )