Since the value of VREF is 1.295V, this relationship reduces to:
10.0
µVrms
VN = 35
• VOUT
V
Connecting a capacitor, CNR, from the Noise-Reduction
(NR) pin to ground, can reduce the output noise voltage.
Adding CNR, as shown in Figure 7, forms a low-pass filter
for the voltage reference. For CNR = 10nF, the total noise in
the 10Hz to 100kHz bandwidth is reduced by approximately
a factor of 3.5, as shown in Figure 8.
1.0
COUT = 0, CFB = 0
COUT = 0, CFB = 0.01µF
COUT = 10µF, CFB = 0.01µF
0.1
10
100
1000
10000
100000
45
Frequency
FIGURE 9. Output Noise Density on Adjustable Versions.
35
The REG103 utilizes an internal charge pump to develop an
internal supply voltage sufficient to drive the gate of the
DMOS pass element above VIN. The charge-pump switch-
ing noise (nominal switching frequency = 2MHz) is not
measurable at the output of the regulator.
COUT = 0
COUT = 10µF
25
0.001
0.01
0.1
1
DROP-OUT VOLTAGE
CNR (µF)
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 VIN to VOUT, the
regulator’s input-to-output resistance is the RdsON of the
DMOS pass element (typically 230mΩ). For static (DC)
loads, the REG103 will typically maintain regulation down
to VIN to VOUT voltage drop of 115mV at full-rated output
current. In Figure 10, the bottom line (DC dropout) shows
the minimum VIN to VOUT 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, CFB,
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.
VIN
NR
Low Noise
Charge Pump
(fixed output
versions only)
CNR
VREF
(optional)
(1.295V)
DMOS
Output
VOUT
Over Current
Over Temp
Protection
R1
R2
ENABLE
Adj
(Adjustable
Versions)
REG103
NOTE: R1 and R2 are internal
on fixed output versions.
ERROR
FIGURE 7. Block Diagram.
REG103
10
SBVS010D
BB [ BURR-BROWN CORPORATION ]
