LT3081
21
3081fc
For more information www.linear.com/LT3081
applicaTions inForMaTion
The second technique for reducing power dissipation,
shown in Figure 13, uses a resistor in parallel with the
LT3081. This resistor provides a parallel path for current
flow, reducing the current flowing through the LT3081.
This technique works well if input voltage is reasonably
constant and output load current changes are small. This
technique also increases the maximum available output
current at the expense of minimum load requirements.
R
P
dissipates 1.52W of power. As with the first technique,
choose appropriate wattage resistors to handle and dis-
sipate the
power properly. With this configuration, the
LT3081
supplies only 0.86A. Therefore, load current can
increase by 0.64A to a total output current of 2.14A while
keeping the LT3081 in its normal operating range.
High Temperature Operation
Care must be taken when designing the LT3081H/
LT3081MP applications to operate at high ambient tem
-
peratures. The LT3081
H/LT3081MP operates at high
temperatures, but erratic operation can occur due to un-
foreseen variations
in
external components. Some tantalum
capacitors are available for high temperature operation, but
ESR is often several ohms; capacitor ESR above 0.5Ω is
unsuitable for use with the LT3081H/LT3081MP. Multiple
ceramic capacitor manufacturers
now offer
ceramic capaci-
tors that are rated to 150°C using an X8R dielectric. Check
each
passive component for absolute value and voltage
ratings over the operating temperature range.
Leakages in capacitors or from solder flux left after insuf
-
ficient board cleaning adversely affects low current nodes,
such
as the SET, I
MON
, and TEMP pins. Consider junction
temperature increase due to power dissipation in both
the junction and nearby components to ensure maximum
specifications are not violated for the LT3081H/LT3081MP
or external components.
Protection Features
The LT3081 incorporates several protection features ideal
for harsh industrial and automotive environments, among
other applications. In addition to normal monolithic regula
-
tor protection features such as current limiting and thermal
limiting, the LT3081 protects itself against reverse-input
voltages, reverse-output voltages, and large OUT-to-SET
pin voltages.
Current limit protection and thermal overload protection
protect the IC against output current overload conditions.
For normal operation, do not exceed the rated absolute
maximum junction temperature. The thermal shutdown
circuit’s temperature threshold is typically 165°C and
incorporates about 5°C of hysteresis.
3081 F13
IN
SET OUT
+
–
LT3081
50µA
R
SET
V
OUT
V
IN
C2
C1
R
P
Figure 13. Reducing Power Dissipation Using a Parallel Resistor
As an example, assume: V
IN
= 5V, V
IN(MAX)
= 5.5V, V
OUT
= 3.3V, V
OUT(MIN)
= 3.2V, I
OUT(MAX)
= 1.5A and I
OUT(MIN)
= 0.7A. Also, assuming that R
P
carries no more than 90%
of I
OUT(MIN)
= 630mA.
Calculating R
P
yields:
R
P
=
= 3.65Ω
(5% Standard value = 3.6Ω)
The maximum total power dissipation is:
(5.5V – 3.2V) • 1.5A = 3.5W
However, the LT3081 supplies only:
1.5A –
= 0.86A
Therefore, the LT3081’s power dissipation is only:
P
DISS
= (5.5V – 3.2V) • 0.86A = 1.98W
