Datasheet

REG113
9
SBVS031D
www.ti.com
OUTPUT NOISE
A precision bandgap reference is used to generate the
internal reference voltage, V
REF
. This reference is the domi-
nant noise source within the REG113 and generates approxi-
mately 29µVrms 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:
V Vrms
RR
R
Vrms
V
V
N
OUT
REF
=µ
+
=µ •29
2
29
12
(1)
Since the value of V
REF
is 1.26V, this relationship reduces to:
V
Vrms
V
V
N
OUT
=
µ
•23
(2)
Connecting a capacitor, C
NR
, from the Noise Reduction (NR)
pin to ground (as shown in Figure 3) 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 2.8 for V
OUT
= 3.3V. This noise reduction effect is
shown in Figure 4, and as
RMS Noise Voltage vs C
NR
in the
Typical Characteristics section.
Noise can be further reduced by carefully choosing an output
capacitor, C
OUT
. Best overall noise performance is achieved
with very low (< 0.22µF) or very high (> 2.2µF) values of C
OUT
(see the RMS Noise Voltage vs C
OUT
typical characteristic).
The REG113 uses 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 switching
noise (nominal switching frequency = 2MHz) is not measur-
able at the output of the regulator over most values of I
OUT
and C
OUT
.
DROPOUT VOLTAGE
The REG113 uses an N-channel DMOS as the pass ele-
ment. When (V
IN
– V
OUT
) is less than the dropout voltage
(V
DROP
), the DMOS pass device behaves like a resistor;
therefore, for low values of (V
IN
– V
OUT
), the regulator input-
to-output resistance is the Rds
ON
of the DMOS pass element
(typically 600m
Ω ). For static (DC) loads, the REG113 will
FIGURE 4. Output Noise versus Noise Reduction Capacitor.
0.1 100 1k10 10k
110
100
90
80
70
60
50
40
30
20
Noise Voltage (Vrms)
C
NR
(pF)
RMS NOISE VOLTAGE vs C
NR
REG113-5.0
REG113-3.3
REG113-2.5
C
OUT
= 0µF
10Hz < BW < 100kHz
FIGURE 3. Block Diagram.
Over-Current
Over Temp
Protection
V
REF
(1.26V)
Low-Noise
Charge Pump
DMOS
Output
V
OUT
NR
GND
Enable
REG113
V
IN
C
NR
(optional)
typically maintain regulation down to a (V
IN
– V
OUT
) voltage
drop of 250mV at full rated output current. In Figure 5, the
bottom line (DC dropout) shows the minimum V
IN
to V
OUT
voltage drop required to prevent dropout under DC load
conditions.
FIGURE 5. Transient and DC Dropout.
0 50 100 150 200 250 350300 400
600
500
400
300
200
100
0
Dropout Voltage (mV)
I
OUT
(mA)
DROPOUT VOLTAGE vs I
OUT
Dropout for 0mA to I
OUT
Transient
DC Dropout