lt3010hms8e-trpbf Linear Technology Corporation, lt3010hms8e-trpbf Datasheet - Page 11

• Manufacturer Part Number: lt3010hms8e-trpbf
• Description: 50ma, 3v To 80v Low Dropout Micropower Linear Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Continuous operation at large input/output voltage dif-
ferentials and maximum load current is not practical
due to thermal limitations. Transient operation at high
input/output differentials is possible. The approximate
thermal time constant for a 2500sq mm 3/32" FR-4 board
with maximum topside and backside area for one ounce
copper is 3 seconds. This time constant will increase as
more thermal mass is added (i.e. vias, larger board, and
other components).
For an application with transient high power peaks, average
power dissipation can be used for junction temperature
calculations as long as the pulse period is signifi cantly less
than the thermal time constant of the device and board.
Calculating Junction Temperature
Example 1: Given an output voltage of 5V, an input volt-
age range of 24V to 30V, an output current range of 0mA
to 50mA, and a maximum ambient temperature of 50°C,
what will the maximum junction temperature be?
The power dissipated by the device will be equal to:
where:
So:
The thermal resistance will be in the range of 40°C/W to
62°C/W depending on the copper area. So the junction
temperature rise above ambient will be approximately
equal to:
The maximum junction temperature will then be equal to
the maximum junction temperature rise above ambient
plus the maximum ambient temperature or:
I
I
V
I
P = 50mA • (30V – 5V) + (1mA • 30V) = 1.28W
1.31W • 50°C/W = 65.5°C
T
OUT(MAX)
OUT(MAX)
GND
JMAX
IN(MAX)
at (I
= 50°C + 65.5°C = 115.5°C
= 30V
OUT
• (V
= 50mA
= 50mA, V
IN(MAX)
– V
IN
OUT
= 30V) = 1mA
) + (I
GND
• V
IN(MAX)
)
Example 2: Given an output voltage of 5V, an input voltage
of 48V that rises to 72V for 5ms(max) out of every 100ms,
and a 5mA load that steps to 50mA for 50ms out of every
250ms, what is the junction temperature rise above ambi-
ent? Using a 500ms period (well under the time constant
of the board), power dissipation is as follows:
Operation at the different power levels is as follows:
With a thermal resistance in the range of 40°C/W to
62°C/W, this translates to a junction temperature rise above
ambient of 26°C to 38°C.
High Temperature Operation
Care must be taken when designing LT3010/LT3010-5
applications to operate at high ambient temperatures.
The LT3010/LT3010-5 works at elevated temperatures
but erratic operation can occur due to unforeseen varia-
tions in external components. Some tantalum capacitors
are available for high temperature operation, but ESR is
often several ohms; capacitor ESR above 3Ω is unsuit-
able for use with the LT3010/LT3010-5. Ceramic capacitor
manufacturers (Murata, AVX, TDK, and Vishay Vitramon
at this writing) now offer ceramic capacitors that are rated
to 150°C using an X8R dielectric. Device instability will
occur if output capacitor value and ESR are outside design
limits at elevated temperature and operating DC voltage
bias (see information on capacitor characteristics under
P1(48V in, 5mA load) = 5mA • (48V – 5V)
P2(48V in, 50mA load) = 50mA • (48V – 5V)
P3(72V in, 5mA load) = 5mA • (72V – 5V)
P4(72V in, 50mA load) = 50mA • (72V – 5V)
76% operation at P1, 19% for P2, 4% for P3, and
1% for P4.
P
+ 1%(3.42W) = 0.64W
EFF
= 76%(0.23W) + 19%(2.20W) + 4%(0.35W)
LT3010/LT3010-5
+ (200μA • 48V) = 0.23W
+ (200μA • 72V) = 0.35W
+ (1mA • 48V) = 2.20W
+ (1mA • 72V) = 3.42W
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