Datasheet

LT3032 Series
20
3032fd
APPLICATIONS INFORMATION
Calculating Junction Temperature
Example: Given a positive output voltage of 3.3V, a posi-
tive input voltage of 4V to 6V, output current range from
10mA to 150mA, negative output voltage of –3.3V, negative
input voltage of –5V to –6V, a negative output current of
–100mA, and a maximum ambient temperature of 50°C,
what will the maximum junction temperature be for a
2500mm
2
board with topside copper of 1000mm
2
?
The power in each side equals:
P
SIDE
= (V
IN(MAX)
– V
OUT
)(I
OUT(MAX)
)+(V
IN(MAX)
•I
GND
)
where,
I
OUTP(MAX)
= 150mA
V
INP(MAX)
= 6V
I
GND
at (I
OUTP
= 150mA, V
INP
= 6V) = 3.7mA
I
OUTN(MAX)
= –100mA
V
INN(MAX)
= –6V
I
GND
at (I
OUTN
= –100mA, V
INN
= –6V) = –1.5mA
The total power equals:
P
TOTAL
= P
POSITIVE
+ P
NEGATIVE
So,
P
POSITIVE
= 150mA(6V – 3.3V) + 3.7mA(6V) = 0.43W
P
NEGATIVE
= –100mA(–6V+3.3V)–1.5mA(–6V) =
0.28W
P
TOTAL
= 0.43W + 0.28W = 0.71W
Junction Temperature equals:
T
J
= T
A
+ P
TOTAL
• θ
JA
(using tables)
T
J
= 50°C + 0.71W • 33°C/W = 73.4°C
In this case, the junction temperature is below the maxi-
mum rating, ensuring reliable operation.
Protection Features
The LT3032 incorporates several protection features that
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 LT3032 is protected against reverse input
voltages and reverse output voltages on both channels.
Current limit protection and thermal overload protection
protect the device against current overload conditions at
the outputs of the part. For normal operation, the junction
temperature should not be allowed to exceed 125°C.
The positive input of the LT3032 withstands 20V reverse
voltage. The negative input also withstands reverse volt-
age, but the negative input may not be more than 0.5V
(one V
BE
) higher than the OUTN and SHDNN pins. This
provides protection against batteries that are plugged in
backwards.
The outputs of the LT3032 can be pulled to opposing volt-
ages without damaging the part. The outputs may be pulled
to the opposing polarity with a load that is common mode
between the two and one regulator starts before the other;
in this condition, it does not matter which regulator started
fi rst. Both sides are capable of having the output pulled to
the opposing polarity and both will still start and operate.
If an input is left open circuit or grounded, the corre-
sponding output can be pulled to its opposing polarity by
as much as 20V. The output will act like an open circuit;
no current will fl ow into or out of the pin. If the input is
powered by a voltage source, the output will source the
short-circuit current and will protect itself by thermal
limiting. In this case, grounding the respective SHDNP/
SHDNN pin will turn off that side of the LT3032 and stop
the output from sourcing current.
The ADJP pin can be pulled above or below ground by
±7V without damage to the device. If the input is left open
circuit or grounded, the ADJP pin acts like an open circuit
when pulled below ground and like a large resistor (typically
100k) in series with a diode when pulled above ground.