ADP3309

–7–

REV. 0

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.

Error Flag Dropout Detector

The ADP3309 will maintain its output voltage over a wide

range of load, input voltage and temperature conditions. If the

output is about to lose regulation, for example, by reducing the

supply voltage below the combined regulated output and drop-

out voltages, the

ERR pin will be activated. The ERR output is

an open collector that will be driven low.

Once set, the

ERRor flag’s hysteresis will keep the output low

until a small margin of operating range is restored either by

raising the supply voltage or reducing the load.

APPLICATION CIRCUITS

Crossover Switch

The circuit in Figure 21 shows that two ADP3309s 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.

OUTPUT SELECT

4V

0V

ADP3309-2.7

OUT

IN

SD

GND

ADP3309-3.3

+

+

IN

OUT

C1

1.0 F

C2

0.47 F

SD

GND

Figure 21. Crossover Switch

Higher Output Current

The ADP3309 can source up to 100 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.

ADP3309-3.3

OUT

IN

SD

GND

+

MJE253*

C1

47 F

C2

10 F

*AAVID531002 HEATSINK IS USED

ERR

R1

50

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.

+

C1

100 F

10V

L1

6.8 H

D1

1N5817

C2

100 F

10V

SW1

SW2

GND

FB

ADP3000-ADJ

R1

120

R2

30.1k

1%

Q1

2N3906

ADP3309-3.3

IN

OUT

GND

SD

R3

124k

1%

R4

274k

Q2

2N3906

C3

2.2 F

3.3V@100mA

Figure 23. Constant Dropout Post Regulator