lt3011emse Linear Technology Corporation, lt3011emse Datasheet - Page 12

• Manufacturer Part Number: lt3011emse
• Description: 50ma, 3v To 80v Low Dropout Micropower Linear Regulator With Pwrgd
• Manufacturer: Linear Technology Corporation

LT3011
APPLICATIONS INFORMATION
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 52°C/W to
64°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 follow:
12
I
I
V
I
P = 50mA • (30V – 5V) + (1mA • 30V) = 1.28W
1.28W • 58°C/W = 74°C
T
P1 (48V
+ (200μA • 48V) = 0.23W
P2 (48V
+ (1mA • 48V) = 2.20W
P3 (72V
+ (200μA • 72V) = 0.35W
P1 (72V
+ (1mA • 72V) = 3.42W
OUT(MAX)
OUT(MAX)
GND
JMAX
IN(MAX)
at (I
= 50°C + 74°C = 124°C
IN
IN
IN
IN
= 30V
OUT
• (V
= 50mA
, 5mA load) = 5mA • (48V – 5V)
, 50mA load) = 50mA • (48V – 5V)
, 5mA load) = 5mA (72V – 5V)
, 50mA load) = 50mA (72V – 5V)
= 50mA, V
IN(MAX)
– V
IN
OUT
= 30V) = 1mA
) + (I
GND
• V
IN(MAX)
)
Operation at the different power levels is as follows:
With a thermal resistance in the range of 52°C/W to 64°C/W,
this translates to a junction temperature rise above ambi-
ent of 33°C to 41°C.
High Temperature Operation
Care must be taken when designing LT3011 applications to
operate at high ambient temperatures. The LT3011 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 LT3011. 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 the 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.
Leakage in capacitors, or from solder fl ux left after insuf-
fi cient board cleaning, adversely affects the low quies-
cent current operation. Consider junction temperature
increase due to power dissipation in both the junction
and nearby components to ensure maximum specifi ca-
tions are not violated for the LT3011E/LT3011H/LT3011I
or external components.
Protection Features
The LT3011 incorporates several protection features which
make it ideal for use in battery-powered circuits. In ad-
dition to the normal protection features associated with
monolithic regulators, such as current limiting and thermal
limiting, the device is protected against reverse-input
voltages, and reverse voltages from output-to-input.
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)
3011f