12
LT3027
3027fa
The power dissipated by each channel of the device will be
equal to:
I
OUT(MAX)
(V
IN(MAX)
– V
OUT
) + I
GND
(V
IN(MAX)
)
where (for the first channel):
I
OUT(MAX)
= 100mA
V
IN(MAX)
= 6V
I
GND
at (I
OUT
= 100mA, V
IN
= 6V) = 2mA
so:
P1 = 100mA(6V – 3.3V) + 2mA(6V) = 0.28W
and (for the second channel):
I
OUT(MAX)
= 50mA
V
IN(MAX)
= 6V
I
GND
at (I
OUT
= 50mA, V
IN
= 6V) = 1mA
so:
P2 = 50mA(6V – 2.5V) + 1mA(6V) = 0.18W
The thermal resistance will be in the range of 40°C/W to
60°C/W depending on the copper area. So the junction
temperature rise above ambient will be approximately
equal to:
(0.28W + 018W)(60°C/W) = 27.8°C
The maximum junction temperature will then be equal to
the maximum junction temperature rise above ambient
plus the maximum ambient temperature or:
T
JMAX
= 50°C + 27.8°C = 77.8°C
Protection Features
The LT3027 regulator incorporates several protection fea-
tures which makes it ideal for use in battery-powered cir-
cuits. In addition to the normal protection features asso-
ciated with monolithic regulators, such as current limiting
and thermal limiting, the devices are protected against
reverse input voltages and reverse voltages from output to
input.
Current limit protection and thermal overload protection are
intended to protect the device against current overload
conditions at the output of the device. For normal opera-
tion, the junction temperature should not exceed 125°C.
The input of the device will withstand reverse voltages of
APPLICATIONS INFORMATION
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20V. Current flow into the device will be limited to less than
1mA (typically less than 100µA) and no negative voltage will
appear at the output. The device will protect both itself and
the load. This provides protection against batteries which
can be plugged in backward.
The output of the LT3027 can be pulled below ground
without damaging the device. If the input is left open circuit
or grounded, the output can be pulled below ground by 20V.
The output will act like an open circuit; no current will flow
out of the pin. If the input is powered by a voltage source,
the output will source the short-circuit current of the de-
vice and will protect itself by thermal limiting. In this case,
grounding the SHDN pins will turn off the device and stop
the output from sourcing the short-circuit current.
The ADJ pins can be pulled above or below ground by as
much as 7V without damaging the device. If the input is left
open circuit or grounded, the ADJ pins will act like an open
circuit when pulled below ground and like a large resistor
(typically 100k) in series with a diode when pulled above
ground.
In situations where the ADJ pins are connected to a resis-
tor divider that would pull the pins above their 7V clamp
voltage if the output is pulled high, the ADJ pin input cur-
rent must be limited to less than 5mA. For example, a re-
sistor divider is used to provide a regulated 1.5V output from
the 1.22V reference when the output is forced to 20V. The
top resistor of the resistor divider must be chosen to limit
the current into the ADJ pin to less than 5mA when the ADJ
pin is at 7V. The 13V difference between output and ADJ
pin divided by the 5mA maximum current into the ADJ pin
yields a minimum top resistor value of 2.6k.
In circuits where a backup battery is required, several dif-
ferent input/output conditions can occur. The output volt-
age may be held up while the input is either pulled to ground,
pulled to some intermediate voltage or is left open circuit.
When the IN pins of the LT3027 are forced below the cor-
responding OUT pins or the OUT pins are pulled above the
IN pins, input current will typically drop to less than 2µA.
This can happen if the input of the device is connected to
a discharged (low voltage) battery and the output is held
up by either a backup battery or a second regulator circuit.
The state of the SHDN pins will have no effect on the reverse
output current when the output is pulled above the input.
