LT3012 LINER [Linear Technology], LT3012 Datasheet - Page 11

• Manufacturer Part Number: LT3012
• Description: 250mA, 4V to 80V Low Dropout Micropower Linear Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
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:
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:
I
I
V
I
P = 50mA • (30V – 5V) + (1mA • 30V) = 1.28W
1.31W • 50°C/W = 65.5°C
T
P2(48V in, 50mA load) = 50mA • (48V – 5V)
P1(48V in, 5mA load) = 5mA • (48V – 5V)
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
+ (200μA • 48V) = 0.23W
OUT
+ (1mA • 48V) = 2.20W
= 30V) = 1mA
) + (I
GND
• V
IN(MAX)
)
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 LT3012 applications to
operate at high ambient temperatures. The LT3012 works
at elevated temperatures but erratic operation can occur
due to unforeseen variations in external components. Some
tantalum capacitors are available for high temperature
operation, but ESR is often several ohms; capacitor ESR
above 3Ω is unsuitable for use with the LT3012. 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 Output Capacitance and Transient Response). Check
each passive component for absolute value and voltage
ratings over the operating temperature range.
Leakages in capacitors or from solder fl ux left after
insuficient board cleaning adversely affects low
quiescent current operation. The output voltage resistor
divider should use a maximum bottom resistor value of
124k to compensate for high temperature leakage, setting
divider current to 10μA. Consider junction temperature
increase due to power dissipation in both the junction and
nearby components to ensure maximum specifi cations are
not violated for the device or external components.
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
EFF
= 76%(0.23W) + 19%(2.20W) + 4%(0.35W)
+ 1%(3.42W) = 0.64W
+ (200μA • 72V) = 0.35W
+ (1mA • 72V) = 3.42W
LT3012
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