ADP322/ADP323

Data Sheet

Rev. E | Page 20 of 24

Note that the measured noise reduction is less than the theoretical

noise reduction. Figure 57 shows the noise spectral density of an

adjustable ADP323 set to 500 mV and 2.5 V with and without the

noise reduction network. The output noise with the noise reduction

network is approximately the same for both voltages, especially

beyond 10 Hz. The noise of the 500 mV and 2.5 V outputs without

the noise reduction network differs by a factor of 5 up to approxi-

mately 10 kHz. Above 20 kHz, the closed loop gain of the error

amplifier is limited by its open loop gain characteristic. Therefore,

the noise contribution from 20 kHz to 100 kHz is less than what

it can be if the error amplifier had infinite bandwidth. This is

also the reason why the noise is less than what might be expected

simply based on the dc gain, that is, 39.5 µV rms vs. 70 µV rms.

1

10

100

1k

10k

1

10

100

1k

10k

100k

1M

10M

FREQUENCY (Hz)

2.5V WITH NR

2.5V WITHOUT NR

500mV

Figure 57. 500 mV and 2.5 V Output Voltage with and Without Noise

Reduction Network

CURRENT-LIMIT AND THERMAL OVERLOAD

PROTECTION

The ADP322/ADP323 are protected against damage due to

excessive power dissipation by current and thermal overload

protection circuits. The ADP322/ADP323 are designed to

current limit when the output load reaches 300 mA (typical).

When the output load exceeds 300 mA, the output voltage is

reduced to maintain a constant current limit.

Thermal overload protection is built in, which limits the

junction temperature to a maximum of 155°C (typical). Under

extreme conditions (that is, high ambient temperature and

power dissipation) when the junction temperature starts to

rise above 155°C, the output is turned off, reducing the output

current to zero. When the junction temperature drops below

140°C, the output is turned on again and the output current

is restored to its nominal value.

Consider the case where a hard short from VOUTx to GND

occurs. At first, the ADP322/ADP323 limits current so that only

300 mA is conducted into the short. If self heating of the junction

is great enough to cause its temperature to rise above 155°C,

thermal shutdown activates, turning off the output and reducing

the output current to zero. As the junction temperature cools

and drops below 140°C, the output turns on and conducts 300 mA

into the short, again causing the junction temperature to rise

above 155°C. This thermal oscillation between 140°C and 155°C

causes a current oscillation between 0 mA and 300 mA that

continues as long as the short remains at the output.

Current and thermal limit protections are intended to protect

the device against accidental overload conditions. For reliable

operation, device power dissipation must be externally limited

so that junction temperatures do not exceed 125°C.

THERMAL CONSIDERATIONS

In most applications, the ADP322/ADP323 do not dissipate a lot

of heat due to high efficiency. However, in applications with a high

ambient temperature and high supply voltage to output voltage

differential, the heat dissipated in the package is large enough

that it can cause the junction temperature of the die to exceed

the maximum junction temperature of 125°C.

When the junction temperature exceeds 155°C, the converter

enters thermal shutdown. It recovers only after the junction

temperature decreases below 140°C to prevent any permanent

damage. Therefore, thermal analysis for the chosen application

is very important to guarantee reliable performance over all

conditions. The junction temperature of the die is the sum of

the ambient temperature of the environment and the temperature

rise of the package due to the power dissipation, as shown in

Equation 2.

To guarantee reliable operation, the junction temperature of the

ADP322/ADP323 must not exceed 125°C. To ensure that the

junction temperature stays below this maximum value, the user

must be aware of the parameters that contribute to junction

temperature changes. These parameters include ambient tem-

perature, power dissipation in the power device, and thermal

number is dependent on the package assembly compounds used

and the amount of copper to which the GND pins of the package

ADP322/ADP323 for various PCB copper sizes.

ADP322/ADP323 Triple LDO (°C/W)

49.5

100

83.7

500

68.5

1000

64.7

1

Device soldered to JEDEC standard board.