Micrel, Inc.
∆T
= T
J(max)
– T
A(max)
T
J(max)
= 125°C
T
A(max)
= maximum ambient operating temp-
erature
For example, the maximum ambient temperature is
50°C, the
∆T
is determined as follows:
∆T
= 125°C – 50°C
∆T
= 75°C
Using Figure 2, the minimum amount of required copper
can be determined based on the required power
dissipation. Power dissipation in a linear regulator is
calculated as follows:
P
D
= V
IN
× I
IN
+ V
BIAS
× I
BIAS
– V
OUT
× I
OUT
Using a typical application of 750mA output current, 1.2V
output voltage, 1.8V input voltage and 3.3V bias voltage,
the power dissipation is as follows:
P
D
= (1.8V) × (730mA) + 3.3V(30mA) – 1.2V(750mA)
At full current, a small percentage of the output current is
supplied from the bias supply, therefore the input current
is less than the output current.
P
D
= 513mW
From Figure 2, the minimum current of copper required
to operate this application at a
∆T
of 75°C is less than
100mm
2
.
MIC49150
The
θ
JA
of this package is ideally 80°C/W, but it will vary
depending upon the availability of copper ground plane
to which it is attached.
Adjustable Regulator Design
The MIC49150 adjustable version allows programming
the output voltage anywhere between 0.9Vand 5V. Two
resistors are used. The resistor value between V
OUT
and
the adjust pin should not exceed 10kΩ. Larger values
can cause instability. The resistor values are calculated
by:
⎞
⎛
V
R1
=
R2
× ⎜
OUT
−
1
⎟
⎟
⎜
0.9
⎠
⎝
Where V
OUT
is the desired output voltage.
Enable
The
fixed
output voltage versions of the MIC49150
feature an active high enable input (EN) that allows on-
off control of the regulator. Current drain reduces to
“zero” when the device is shutdown, with only
microamperes of leakage current. The EN input has
TTL/CMOS compatible thresholds for simple logic
interfacing. EN may be directly tied to V
IN
and pulled up
to the maximum supply voltage.
Quick Method
Determine the power dissipation requirements for the
design along with the maximum ambient temperature at
which the device will be operated. Refer to Figure 3,
which shows safe operating curves for three different
ambient temperatures: 25°C, 50°C and 85°C. From
these curves, the minimum amount of copper can be
determined by knowing the maxi-mum power dissipation
required. If the maximum ambient temperature is 50°C
and the power dissipation is as above, 513mW, the
curve in Figure 3 shows that the required area of copper
is less than 100mm
2
.
November 2006
11
M9999-111306
MICREL [ MICREL SEMICONDUCTOR ]
