ADP7104CPZ-REDYKIT(RevB) View Datasheet(PDF) - Analog Devices
Part Name
Description
Manufacturer
ADP7104CPZ-REDYKIT Datasheet PDF : 28 Pages
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Data Sheet
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.
6
PG –40°C
PG –5°C
PG +25°C
5
PG +85°C
PG +125°C
4
3
2
1
0
4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 5.0
VOUT (V)
Figure 67. Typical Power-Good Threshold vs. Temperature, VOUT Rising
6
PG –40°C
PG –5°C
PG +25°C
5 PG +85°C
PG +125°C
4
3
2
1
0
4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 5.0
VOUT (V)
Figure 68. Typical Power-Good Threshold vs. Temperature, VOUT Falling
ADP7104
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.
VIN = 8V
CIN +
1µF
ON 100kΩ
OFF
100kΩ
VIN VOUT
ADJ
EN/
UVLO
RFB1
40.2kΩ
+ CNR
100nF
+
VOUT
COUT
=
5V
1µF
RFB2
13kΩ
RNR
13kΩ
100kΩ
GND PG
PG
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
Rev. B | Page 19 of 28
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