6

The actual power dissipation of the regulator circuit can be determined using by the equation:

P

= (V

- V

) I

+ V

I

D

IN

OUT

OUT

IN GND

Substituting P

for P

and solving for the operating conditions that are critical to the application will give

D(max)

D

the maximum operating conditions for the regulator circuit. For example, when operating the GM6156 at room

temperature with a minimum footprint layout, the maximum input voltage for a set output current can be

determined as follows:

The junction-to-ambient thermal resistance for the minimum footprint is 220°C/ W, from Table 1. The maximum

power dissipation must not be exceeded for proper operation. Using the output voltage of 3.3V and an output

current of 150mA, the maximum input voltage can be determined. From the Electrical Characteristics table, the

maximum ground current for150mA output current is 2500µ or 2.5mA.

455mW = (V

- 3.3V) 150mA + V

• 2.5mA

IN

IN

455mW = V

• 150mA - 495mA + V

• 2.5mA

IN

IN

950mW = V

• 152.5mA

IN

V

= 6.23V

IN(max)

Therefore, a 3.3V application at 150mA of output current can accept a maximum input voltage of 6.2V in a

SOT-25 package. For a full discussion of heat sinking and thermal effects on voltage regulators.

Fixed Regulator Applications

Figure 3. Ultra-Low-Noise Fixed Voltage Application

Figure 4. Low-Noise Fixed Voltage Application

Adjustable Regulator Applications

The GM6156 can be adjusted to a specific output voltage by using two externa resistors (Figure 5). The resistors set the

output voltage based on the following equation:

P

=

D(max)

P

= 455mW

D(max)

(125°C - 25°C)

220°C/W

Figure3 includes a 470pF capacitor for low-noise

operation and shows EN (pin 3) connected to IN (pin 1)

for an application where enable/ shutdown is not

required. C

= 2.2µF minimum.

OUT

1

2

3

4

5

2.2µF

470pF

Enable

Shutdown

Figure 4 is an example of a low-noise configuration

where C

is not required. C

= 1µF minimum.

BYP

OUT

V

= 1.242V X (

+1)

OUT

R2

R1

Parameter

R

Recommended

JA

q

Minimum Footprint

R

1" Square

JA

q

Copper Clad

q

SOT-23-5

220°C/ W

170°C/ W

130°C/ W

Table 1. SOT-25 Thermal Resistance

1

2

3

4

5

1.0µF

EN

BYP