MIC49150-0.9WR datasheet(11/13 Pages) MICREL

11 / 13 page

Micrel, Inc.

MIC49150

November 2006

M9999-111306

∆T = TJ(max) – TA(max)

T J(max) = 125°C

TA(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:

PD = VIN × IIN + VBIAS × IBIAS – VOUT × IOUT

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:

PD = (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.

PD = 513mW

From Figure 2, the minimum current of copper required

to operate this application at a ∆T of 75°C is less than

100mm

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

The θJA 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 VOUT and

the adjust pin should not exceed 10kΩ. Larger values

can cause instability. The resistor values are calculated

by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

0.9

OUT

Where VOUT 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 VIN and pulled up

to the maximum supply voltage.

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