ADP7182 Data Sheet
Rev. E | Page 24 of 32
NOISE REDUCTION OF THE ADJUSTABLE ADP7182
The ultralow output noise of the fixed output ADP7182 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 ADP7182 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 can be modified slightly to reduce
the output voltage noise to levels close to that of the fixed output of
the ADP7182. The circuit shown in Figure 87 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 nearly equal to RFB2; this
limits the ac gain of the error amplifier to approximately 6 dB.
The actual gain is the parallel combination of RNR and RFB1 divided
by RFB2. This resistance 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 10 Hz and 100 Hz. This capacitance sets
the frequency where the ac gain of the error amplifier is 3 dB down
from its dc gain.
R
FB2
13kΩ
R
FB1
147kΩ
GND
EN ADJ
VIN VOUT
ADP7182
ON
ON
–2V
OFF 0V
2V
IN
= –16V V
OUT
= –15V
C
OUT
2.2µF
C
NR
100nF
C
IN
2.2µF
R
NR
13kΩ
10703-085
Figure 87. Noise Reduction Modification to Adjustable LDO
The noise of the LDO is approximately the noise of the fixed output
LDO (typically 18 μV rms) times RFB2, divided by the parallel
combination of RNR and RFB1. Based on the component values
shown in Figure 87, the ADP7182 has the following
characteristics:
DC gain of 12.3 (21.8 dB)
3 dB roll-off frequency of 10.8 Hz
High frequency ac gain of 1.92 (5.67 dB)
Noise reduction factor of 6.41 (16.13 dB)
Measured rms noise of the adjustable LDO at −200 mA
without noise reduction of 220 μV rms
Measured rms noise of the adjustable LDO at −200 mA
with noise reduction circuit of 35 μV rms
Calculated rms noise of the adjustable LDO with noise
reduction (assuming 18 μV rms for fixed voltage option) of
34.5 μV rms
The noise of the LDO is approximately the noise of the fixed output
LDO (typically 18 μV rms) times the high frequency ac gain. The
following equation shows the calculation with the values shown
in Figure 87.
kΩ/13
kΩ1/147kΩ1/13
1
1 × μV 18 (2)
Figure 88 shows the difference in noise spectral density for the
adjustable ADP7182 set to −15 V with and without the noise
reduction network. In the 100 Hz to 30 kHz frequency range,
the reduction in noise is significant.
100k
1
10
100
1k
10k
1 100M10M1M100k10k1k10010
NOISE SPECTRAL DENSITY (nV Hz)
FREQUENCY (Hz)
–15V ADJ
–15V ADJ NR
10703-086
Figure 88. −15 V Adjustable ADP7182 with and without the
Noise Reduction Network (CNR and RNR)
CURRENT-LIMIT AND THERMAL OVERLOAD
PROTECTION
The ADP7182 is protected against damage due to excessive power
dissipation by current-limit and thermal overload protection
circuits. The ADP7182 is designed to limit current when the
output load reaches −350 mA (typical). When the output load
exceeds −350 mA, the output voltage is reduced to maintain a
constant current limit.
Thermal overload protection is included, which limits the junction
temperature to a maximum of 150°C (typical). Under extreme
conditions (that is, high ambient temperature and power
dissipation) when the junction temperature starts to rise above
150°C, the output is turned off, reducing the output current to
0 mA. When the junction temperature falls below 135°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 VOUT to ground
occurs. At first, the ADP7182 limits current so that only −350 mA
is conducted into the short. If self-heating of the junction is great
enough to cause its temperature to rise above 150°C, thermal
shutdown is activated, turning off the output and reducing the
output current to 0 mA. As the junction temperature cools and
falls below 135°C, the output turns on and conducts −350 mA
into the short, again causing the junction temperature to rise
above 150°C. This thermal oscillation between 135°C and 150°C
causes a current oscillation between −350 mA and 0 mA that
continues as long as the short remains at the output.