ADP7104ACPZ-2.5-R7 datasheet(19/28 Pages) AD

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Data Sheet

ADP7104

Rev. B | Page 19 of 28

POWER-GOOD FEATURE

The ADP7104 provides a power-good pin (PG) to indicate

the status of the output. This open-drain output requires an

external pull-up resistor to VIN. If the part is in shutdown

mode, current-limit mode, or thermal shutdown, or if it falls

below 90% of the nominal output voltage, the power-good pin

(PG) immediately transitions low. During soft-start, the rising

threshold of the power-good signal is 93.5% of the nominal

output voltage.

The open-drain output is held low when the ADP7104 has suffi-

cient input voltage to turn on the internal PG transistor. The PG

transistor is terminated via a pull-up resistor to VOUT or VIN.

Power-good accuracy is 93.5% of the nominal regulator output

voltage when this voltage is rising, with a 90% trip point when

this voltage is falling. Regulator input voltage brownouts or

glitches trigger power no good signals if VOUT falls below 90%.

A normal power-down causes the power-good signal to go low

when VOUT drops below 90%.

Figure 67 and Figure 68 show the typical power-good rising and

falling threshold over temperature.

4.2

4.3

4.4

4.5

4.6

4.7

4.8

4.9

5.0

VOUT (V)

PG –40°C

PG –5°C

PG +25°C

PG +85°C

PG +125°C

Figure 67. Typical Power-Good Threshold vs. Temperature, VOUT Rising

4.2

4.3

4.4

4.5

4.6

4.7

4.8

4.9

5.0

VOUT (V)

PG –40°C

PG –5°C

PG +25°C

PG +85°C

PG +125°C

Figure 68. Typical Power-Good Threshold vs. Temperature, VOUT Falling

NOISE REDUCTION OF THE ADJUSTABLE ADP7104

The ultralow output noise of the fixed output ADP7104 is

achieved by keeping the LDO error amplifier in unity gain

and setting the reference voltage equal to the output voltage.

This architecture does not work for an adjustable output

voltage LDO. The adjustable output ADP7104 uses the more

conventional architecture where the reference voltage is fixed

and the error amplifier gain is a function of the output voltage.

The disadvantage of the conventional LDO architecture is that

the output voltage noise is proportional to the output voltage.

The adjustable LDO circuit may be modified slightly to reduce

the output voltage noise to levels close to that of the fixed

output ADP7104. The circuit shown in Figure 69 adds two

additional components to the output voltage setting resistor

divider. CNR and RNR are added in parallel with RFB1 to reduce

the ac gain of the error amplifier. RNR is chosen to be equal to

RFB2, this limits the ac gain of the error amplifier to approxi-

mately 6 dB. The actual gain is the parallel combination of RNR

and RFB1 divided by RFB2. This ensures that the error amplifier

always operates at greater than unity gain.

CNR is chosen by setting the reactance of CNR equal to RFB1 −

RNR at a frequency between 50 Hz and 100 Hz. This sets the

frequency where the ac gain of the error amplifier is 3 dB

down from its dc gain.

VOUT = 5V

VIN = 8V

VOUT

VIN

GND

ADJ

EN/

UVLO

100kΩ

100k

Ω

100k

Ω

COUT

1µF

CIN

1µF

OFF

RNR

13k

Ω

RFB2

13k

Ω

RFB1

40.2k

Ω

CNR

100nF

Figure 69. Noise Reduction Modification to Adjustable LDO

The noise of the LDO is approximately the noise of the fixed

output LDO (typically 15 µV rms) times the square root of the

parallel combination of RNR and RFB1 divided by RFB2. Based on

the component values shown in Figure 69, the ADP7104 has the

following characteristics:

•

DC gain of 4.09 (12.2 dB)

•

3 dB roll off frequency of 59 Hz

•

High frequency ac gain of 1.82 (5.19 dB)

•

Noise reduction factor of 1.35 (2.59 dB)

•

RMS noise of the adjustable LDO without noise reduction

of 27.8 µV rms

•

RMS noise of the adjustable LDO with noise reduc-

tion (assuming 15 µV rms for fixed voltage option) of

20.25 µV rms

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