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LT1585A Datasheet(PDF) 6 Page - ON Semiconductor

Part No. LT1585A
Description  5A LOW DROPOUT FAST RESPONSE POSITIVE ADJUSTABLE AND FIXED VOLTAGE REGULATOR
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Maker  ONSEMI [ON Semiconductor]
Homepage  http://www.onsemi.com
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 6 page
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LT1585A
6
MOTOROLA ANALOG IC DEVICE DATA
Vout
Vin
LT1585A
ADJ
IN
OUT
+
+
R2
R1
C2
10
mF
C1
10
mF
Figure 11. Basic Adjustable Regulator
Vout = Vref (1 + R2/R1) + Iadj (R2)
Iadj
55
mA
Vref
Load Regulation
It is not possible to provide true remote load sensing
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.
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 4 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
The connection shown in Figure 11 does not multiply RP
by the divider ratio. As an example, RP 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.
Vin
LT1585A
ADJ
IN
OUT
R2*
R1*
Figure 12. Connection for
Best Load Regulation
FL
* Connect R1 to Case
* Connect R2 to Load
PD
Parasitic
Line Resistance
Thermal Considerations
The LT1585A 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
junc tion–to–ambient. Thes e sourc e s 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.
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.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.
For example, using an LT1585ACT (TO–220 package)
with an output adjusted to 3.3 V and assuming:
Vin (Max Continuous) = 5.25V (5V + 5%), Vout = 3.3V,
TJ IOUT = 5A.
TA = 70°C, qHEAT SINK = 3°C/W
qCASE–TO–HEAT SINK = 1°C/W (with Thermal Compound)
Power dissipation under these conditions is equal to:
PD = (Vin – Vout) (Iout) = (5.25 – 3.3) (5) = 9.75W
Junction temperature will be equal to:
TJ = TA + PD (qHEAT SINK + qCASE–TO–HEAT SINK + qJC)
For the Control Section:
TJ = 70°C + 9.75W (3°C/W + 1°C/W + 0.7°C/W)
TJ = 115.8°C
115.8
°C < 125°C = TJMAX
(Control Section Commercial Range)
For the Power Transistor:
TJ = 70°C + 9.75W (3°C/W + 1°C/W + 3°C/W)
TJ = 138.3°C
138.3
°C < 150°C = TJMAX
(Control Section Commercial Range)
In both cases the junction temperature is below the
maximum rating for the respective sections, ensuring reliable
operation.




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