LT1123 LINER [Linear Technology], LT1123 Datasheet - Page 9

• Manufacturer Part Number: LT1123
• Description: Low Dropout Regulator Driver
• Manufacturer: LINER [Linear Technology]

A
THERMAL LIMITING
The thermal limit of the LT1123 can be used to protect both
the LT1123 and the PNP pass transistor. This is accom-
plished by thermally coupling the LT1123 to the power
transistor. There are clip type heat sinks available for the
TO-92 package that will allow the LT1123 to be mounted
to the same heat sink as the PNP pass transistor. One
example is manufactured by IERC (part #RUR67B1CB).
The LT1123 should be mounted as close as possible to the
PNP. If the output of the regulator circuit can be shorted,
heat sinking must be adequate to limit the rate of tempera-
ture rise of the power device to approximately 50 C/
minute. This can be accomplished with a fairly small heat
sink, on the order of 3 – 4 square inches of surface area.
DESIGN EXAMPLE
Given the following operating requirements:
current. This can be found from the Maximum Dropout
Voltage curve. 50mA of drive current will guarantee 0.4V
dropout at an output current of 2A. This satisfies our
requirements.
on 50mA of drive current and a minimum input voltage of
5.5V, we can select R
graph the value of R
next lowest 5% value which is 47 .
circuit.
Worst case power in the LT1123 will be equal to:
PPLICATI
1. The first step is to determine the required drive
2. The next step is to determine the value of R
3. We can now look at the thermal requirements of the
5.5V < V
I
Max ambient temp. = 70 C
V
I
R
OUT MAX
DRIVE
V
OUT
D
IN MAX
= 47
= 5V
= 50mA
4R
O
IN
= 1.5A
– V
D
D
< 7V
U
D
is equal to 50 , so we should use the
BE
from the graph of Figure 4. From the
S
2
I FOR ATIO
U
W
U
D
. Based
Given: V
Then: P
Assuming a thermal resistance of 150 C/W, the maximum
junction temperature rise above ambient will be equal to
(P
tion temperature will be equal to the maximum ambient
temperature plus the junction temperature rise above
ambient. In this case we have (maximum ambient = 70 C)
plus (junction temperature rise = 33 C) is equal to 103 C.
This is well below the maximum operating junction tem-
perature of 125 C for the LT1123.
The power rating for R
Figure 8 using V
should be sized to dissipate a minimum of 1/2W.
The worst case power dissipation, for normal operation, in
the MJE1123 will be equal to:
The maximum operating junction temperature of the
MJE1123 is 150 C. The difference between the maximum
operating junction temperature of 150 C and the maxi-
mum ambient temperature of 70 C is 80 C. The device
must be mounted to a heat sink which is sized such that
the thermal resistance from the junction of the MJE1123
to ambient is less than 80 C/3W = 26.7 C/W.
It is recommended that the LT1123 be thermally coupled
to the MJE1123 so that the thermal limit circuit of the
LT1123 can protect both devices. In this case the ambient
temperature for the LT1123 will be equal to the tempera-
ture of the heat sink. The heat sink temperature, under
normal operating conditions, will have to be limited such
that the maximum operating junction temperature of the
LT1123 is not exceeded.
Refer to Linear Technology’s list of Suggested Manufac-
turers of Specialized Components for information on
where to find the required heat sinks, resistors and capaci-
tors. This listing is available through Linear Technology’s
marketing department.
MAX
(V
IN MAX
)(150 C/W) = 33 C. The maximum operating junc-
IN MAX
MAX
– V
(LT1123) = 0.22W.
OUT
= 7V, V
IN
)(I
= 7V and R
OUT MAX
BE
D
can be found from the plot of
= 0.6V, R
) = (7V – 5V)(1.5A) = 3W
D
= 47 . From the plot, R
D
= 47
LT1123
9
D