LT1175
applicaTions inForMaTion
capacitors do not fail during a “shorting out” surge, only
voltage when the regulator output is being pulled high. If
a 4.8V output is pulled to 5V, for instance, the load on the
primary regulator would be (5V – 4.8V)/2kΩ = 100µA.
This also means that if the internal pass transistor leaks
50µA, the output voltage will be (50µA)(2kΩ) = 100mV
high. Thisconditionwillnotoccurundernormaloperating
conditions, but could occur immediately after an output
short circuit had overheated the chip.
during a “charge up” surge.
The output capacitor should be located within several
inchesoftheregulator.Ifremotesensingisused,theoutput
capacitor can be located at the remote sense node, but the
GND pin of the regulator should also be connected to the
remote site. The basic rule is to keep SENSE and GND pins
close to the output capacitor, regardless of where it is.
Operating at very large input-to-output differential volt-
ages (>15V) with load currents less than 5mA requires an
output capacitor with an ESR greater than 1Ω to prevent
low level output oscillations.
Thermal Considerations
The LT1175 is available in a special 8-pin surface mount
packagewhichhasPins1and8connectedtothedieattach
paddle.ThisreducesthermalresistancewhenPins1and8
are connected to expanded copper lands on the PC board.
Table2showsthermalresistanceforvariouscombinations
of copper lands and backside or internal planes. Table 2
also shows thermal resistance for the 5-pin DD surface
mount package and the 8-pin DIP and package.
Input Capacitor
The LT1175 requires a separate input bypass capacitor
only if the regulator is located more than six inches from
the raw supply output capacitor. A 1µF or larger tantalum
capacitor is suggested for all applications, but if low ESR
capacitors such as ceramic or film are used for the out-
put and input capacitors, the input capacitor should be
at least three times the value of the output capacitor. If a
solid tantalum or aluminum electrolytic output capacitor
is used, the input capacitor is very noncritical.
Table 2. Package Thermal Resistance (°C/W)
LAND AREA
DIP
140
110
ST
90
70
SO
100
80
Q
Minimum
60
50
Minimum with
Backplane
2
1cm Top Plane with
100
80
64
50
75
60
35
27
Backplane
High Temperature Operation
2
10cm Top Plane
with Backplane
The LT1175 is a micropower design with only 45µA qui-
escent current. This could make it perform poorly at high
temperatures (>125°C), where power transistor leakage
might exceed the output node loading current (5µA to
15µA). To avoid a condition where the output voltage
driftsuncontrolledhighduringahightemperatureno-load
condition, the LT1175 has an active load which turns on
when the output is pulled above the nominal regulated
voltage. This load absorbs power transistor leakage and
maintains good regulation. There is one downside to this
feature,however.Iftheoutputispulledhighdeliberately,as
itmightbewhentheLT1175isusedasabackuptoaslightly
higher output from a primary regulator, the LT1175 will act
as an unwanted load on the primary regulator. Because of
this, the active pull-down is deliberately “weak.” It can be
modeled as a 2k resistor in series with an internal clamp
Tocalculatedietemperature, maximumpowerdissipation
or maximum input voltage, use the following formulas
with correct thermal resistance numbers from Table 2.
For through-hole TO-220 applications use θ = 50°C/W
JA
without a heat sink and θ = 5°C/W + heat sink thermal
JA
resistance when using a heat sink.
Die Temp = TA + θJA V − V
I
(
OUT )( LOAD
)
IN
TMAX − TA
θJA
TMAX − TA
Maximum Power Dissipation =
Maximum Input Voltage
for Thermal Considerations
=
+ VOUT
θJA
I
LOAD
1175fe
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Linear [ Linear ]
