The Enable Pin Current versus Applied Voltage relationship
is shown in Figure 4. When the Enable pin is connected to
VIN greater than 10V, a series resistor may be used to limit
the current.
Since the value of VREF is 1.295V, this relationship reduces to:
µVRMS
VN = 35
• VOUT
V
Connecting a capacitor, CNR, from the Noise-Reduction (NR)
pin to ground can reduce the output noise voltage. Adding
100
10
CNR, as shown in Figure 5, 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. This noise reduction effect is shown in
Figure 6.
1
0.1
45
REG104-3.3
0.01
0.001
0
2
4
6
8
10
12
14
16
Enable Voltage
35
FIGURE 4. Enable Pin Current versus Applied Voltage.
OUTPUT NOISE
COUT = 0
COUT = 10µF
A precision band-gap reference is used for the internal
reference voltage, VREF, for the REG104. This reference is
the dominant noise source within the REG104. It generates
approximately 45µVRMS in the 10Hz to 100kHz bandwidth at
the reference output. The regulator control loop gains up the
reference noise, so that the noise voltage of the regulator is
approximately given by:
25
0.001
0.01
0.1
1
CNR (µF)
FIGURE 6. Output Noise versus Noise Reduction Capacitor.
The REG104 adjustable version does not have the noise-
reduction pin available, however, the adjust pin is the
summing junction of the error amplifier. A capacitor, CFB
VOUT
VREF
R1 + R2
,
VN = 45µVRMS
= 45µVRMS •
R2
VIN
NR
(fixed output
versions only)
Low Noise
Charge Pump
CNR
(optional)
VREF
(1.295V)
DMOS
Output
VOUT
Over Current
Over Temp
Protection
R1
R2
Enable
Adj
(Adjustable
Versions)
REG104
NOTE: R1 and R2 are internal
on fixed output versions.
FIGURE 5. Block Diagram.
REG104
SBVS025G
9
BB [ BURR-BROWN CORPORATION ]
