Since the value of V
REF
is 1.295V, this relationship reduces to:
µ
Vrms
V
N
=
35
•
V
OUT
V
Connecting a capacitor, C
NR
, from the Noise-Reduction
(NR) pin to ground, can reduce the output noise voltage.
Adding C
NR
, as shown in Figure 7, forms a low-pass filter
for the voltage reference. For C
NR
= 10nF, the total noise in
the 10Hz to 100kHz bandwidth is reduced by approximately
a factor of 3.5, as shown in Figure 8.
10.0
nV/√Hz
1.0
0.1
10
45
C
OUT
= 0, C
FB
= 0
C
OUT
= 0, C
FB
= 0.01µF
C
OUT
= 10µF, C
FB
= 0.01µF
100
1000
Frequency
10000
100000
Output Noise Voltage
(µV
RMS
10Hz - 100kHz)
FIGURE 9. Output Noise Density on Adjustable Versions.
35
25
0.001
C
OUT
= 0
C
OUT
= 10µF
0.01
C
NR
(µF)
0.1
1
The REG103 utilizes an internal charge pump to develop an
internal supply voltage sufficient to drive the gate of the
DMOS pass element above V
IN
. The charge-pump switch-
ing noise (nominal switching frequency = 2MHz) is not
measurable at the output of the regulator.
DROP-OUT VOLTAGE
The REG103 uses an N-channel DMOS as the “pass”
element. When the input voltage is within a few hundred
millivolts of the output voltage, the DMOS device behaves
like a resistor. Therefore, for low values of V
IN
to V
OUT
, the
regulator’s input-to-output resistance is the Rds
ON
of the
DMOS pass element (typically 230mΩ). For static (DC)
loads, the REG103 will typically maintain regulation down
to V
IN
to V
OUT
voltage drop of 115mV at full-rated output
current. In Figure 10, the bottom line (DC dropout) shows
the minimum V
IN
to V
OUT
voltage drop required to prevent
drop-out under DC load conditions.
FIGURE 8. Output Noise versus Noise-Reduction Capacitor.
The REG103 adjustable version does not have the noise-
reduction pin available, however, the adjust pin is the sum-
ming junction of the error amplifier. A capacitor, C
FB
,
connected from the output to the adjust pin will reduce both
the output noise and the peak error from a load transient.
Figure 9 shows improved output noise performance for two
capacitor combinations.
V
IN
NR
(fixed output
versions only)
C
NR
(optional)
V
REF
(1.295V)
Low Noise
Charge Pump
DMOS
Output
Over Current
Over Temp
Protection
V
OUT
R
1
Adj
(Adjustable
Versions)
ENABLE
R
2
REG103
NOTE: R
1
and R
2
are internal
on fixed output versions.
ERROR
FIGURE 7. Block Diagram.
10
REG103
SBVS010D
TI [ TEXAS INSTRUMENTS ]
