ADP3303A

–7–

REV. A

The proper formula to compute R1 and R2 is:

R

kV

R

k

V

SEL

SEL

1

44

1 189

2

44

1

1 189

=

×

=

−









ΩΩ

.

,

.

Where

V

The output voltage can be selected from 2.2 V to 10 V. R1 is

connected from the OUT pin to the FB pin and R2 is connected

from the FB pin to GND. As an example, the Feedback Resistor

Selection Table shows the feedback resistor values for 3 V and

5 V output voltages.

Table I. Feedback Resistor Selection Table

R1

R2

V

OUT

(1% Resistor)

(1% Resistor)

3 V

110 k

Ω

73.2 k

Ω

5 V

187 k

Ω

57.6 k

Ω

OUTPUT CURRENT LIMITING

Short circuit protection is provided by limiting the pass transis-

tors base drive current. Maximum output current is limited to

200 mA.

THERMAL OVERLOAD PROTECTION

The ADP3303A is protected against damage due to excessive

power dissipation by its thermal overload protection circuit,

which limits the die temperature to a maximum of 165

°C.

Under extreme conditions (i.e., high ambient temperature and

power dissipation), where die temperature starts to rise above

165

°C, the output current is reduced until the die temperature

has dropped to a safe level. The output current is restored when

the die temperature is reduced.

Current and thermal limit protections are intended to protect

the device against accidental overload conditions. For normal

operation, device power dissipation should be externally limited

so that junction temperatures will not exceed 125

°C.

CALCULATING JUNCTION TEMPERATURE

Device power dissipation is calculated as follows:

Where

and

Assuming

The proprietary thermal coastline TSSOP-14 package of the

ADP3303A, in conjunction with the recommended PCB layout

shown in Figure 21, yields a thermal resistance of 96

°C/W. As a

result, the die temperature rise for the example circuit is:

∆T = T

If the maximum ambient temperature is 50

°C, this yields a

below the 125

°C maximum operating junction temperature

rating.

PRINTED CIRCUIT BOARD LAYOUT CONSIDERATION

The rate at which heat is transferred is directly proportional to

the temperature differential between the die and PC board.

Once heat is transferred to the PC board, it should be dissipated

to the air or other medium.

Surface mount components rely on the conductive traces or

pads to transfer heat away from the device. Appropriate PC

board layout technique should be used to remove heat from

immediate vicinity of the package.

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’s with thicker cop-

per 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 solder mask or silk screen the heat dissipating traces.

Black anodizing will significantly improve heat reduction by

means of increased radiation.

Figure 22 shows the recommended board layout for the

ADP3303A. Although it is not critical, make sure R1 is con-

nected right at the pin or the point you want to regulate in order

to realize a proper kelvin connection. This will improve overall

precision and stability. The same consideration is valid for the

R2 connection to the ground pin, but a short connection is

strongly suggested. No other components can be connected to

the FB pin except an optional 10 nF–100 nF capacitor (C

parallel to R1 that serves as a noise reduction capacitor.

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 0.3 V or below, or tying it to ground, will turn the

output OFF. In shutdown mode, quiescent current is reduced

to less than 1

µA.

INPUT–OUTPUT DROPOUT VOLTAGE AND DROPOUT

DETECTOR

The ADP3303A maintains a regulated output with an input

voltage as low as 150 mV above the nominal output voltage.

Input voltage falling below this level will generate an error signal

indicating that the error amplifier output is reaching its satu-

rated state and will not be able to drive the pass transistor any

harder. Lowering the input voltage any further will result in

output voltage reduction and loss of regulation.

The input voltage threshold which generates the error output

signal depends on the load current. At the rated output current,

it is slightly lower than the nominal output voltage plus the

dropout voltage. However, the threshold is much lower at

lighter loads.

APPLICATION CIRCUITS

Crossover Switch

The circuit in Figure 23 shows that two ADP3303As 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 volt-

ages from the Ordering Guide.