ADP3308

–7–

REV. A

The following general guidelines will be helpful when designing

a board layout:

1. PC board traces with larger cross section areas will remove

more heat. For optimum results, use PC boards with thicker

copper and or wider traces.

2. Increase the surface area exposed to open air so heat can be

removed by convection or forced air flow.

3. Do not use solder mask or silk screen on the heat dissipating

traces because it will increase the junction to ambient

thermal resistance of the package.

Shutdown Mode

Applying a TTL high signal to the shutdown pin or tying it to

the input pin will turn the output ON. Pulling the shutdown pin

down to a TTL low signal or tying it to ground will turn the

output OFF. In shutdown mode, quiescent current is reduced

to less than 1

µA.

APPLICATION CIRCUITS

Crossover Switch

The circuit in Figure 21 shows that two ADP3308s can be used

to form a mixed supply voltage system. The output switches

between two different levels selected by an external digital input.

Output voltages can be any combination of voltages from the

Ordering Guide of the data sheet.

ADP3308-3.0

OUT

IN

GND

OUTPUT SELECT

4V

0V

SD

C1

1.0 F

ADP3308-3.3

OUT

IN

GND

SD

C2

0.47 F

Figure 21. Crossover Switch

D1

1N5817

C2

100 F

10V

L1

6.8 H

R1

120

C3

2.2 F

3.0V@50mA

C1

100 F

10V

R2

30.1k

1%

Q1

2N3906

Q2

2N3906

R4

274k

R3

124k

1%

ADP3000-ADJ

SW1

GND

SW2

FB

ADP3308-3.0

IN

SD

OUT

GND

Figure 23. Constant Dropout Post Regulator

Higher Output Current

The ADP3308 can source up to 50 mA without any heatsink or

pass transistor. If higher current is needed, an appropriate pass

transistor can be used, as in Figure 22, to increase the output

current to 1 A.

MJE253*

C1

47 F

R1

50

*AAVID531002 HEAT SINK IS USED

ADP3308-3.0

GND

SD

C2

10 F

IN

OUT

Figure 22. Higher Output Current Linear Regulator

Constant Dropout Post Regulator

The circuit in Figure 23 provides high precision with low drop-

out for any regulated output voltage. It significantly reduces the

ripple from a switching regulator while providing a constant

dropout voltage, which limits the power dissipation of the LDO

to 30 mW. The ADP3000 used in this circuit is a switching

regulator in the step-up configuration.