REG101-25 Datasheet, PDF (10/23 Pages) Texas Instruments – 100mA Low-Dropout Regulator

ENABLE
The Enable pin is active HIGH and compatible with stan-
dard TTL-CMOS levels. Inputs below 0.5V (max) turn the
regulator off and all circuitry is disabled. Under this condi-
tion, ground pin current drops to approximately 10nA. When
a pull-up resistor is used, and operation down to V_IN = 1.8V
is required, use values < 50kΩ.
OUTPUT NOISE
A precision band-gap reference is used for the internal
reference voltage, V_REF. This reference is the dominant
noise source within the REG101 and it 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_N
= 29µVrms ^{R1 + R2}
R2
= 29µVrms • ^VOUT
V_REF
Since the value of V_REF is 1.267V, this relationship reduces to:
V_N
=
23
µVrms
V
•
V_OUT
Connecting a capacitor, C_NR, from the Noise Reduction (NR)
pin to ground, as shown in Figure 4, 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_O = 3.3V. This noise reduction effect is
shown in Figure 5 and as “RMS Noise Voltage vs CNR” in the
Typical Characteristics section.
Noise can be further reduced by carefully choosing an
output capacitor, C_OUT. Best overall noise performance is
RMS NOISE VOLTAGE vs C_NR
110
100
REG101-5.0
90
REG101-3.3
80
REG101-2.5
70
60
50
40
30
C_NR = 0µF
10Hz < BW < 100kHz
20
1
10
100
1k
10k
C_NR (pF)
FIGURE 5. Output Noise versus Noise Reduction Capacitor.
achieved with very low (< 0.22µF) or very high (> 2.2µF)
values of C_OUT. See “RMS Noise Voltage vs C_OUT” in the
Typical Characteristics section.
The REG101 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 over most values of
C_OUT and I_OUT.
The REG101 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. See
the typical characteristics for output noise performance.
NR
(fixed output
versions only)
C_NR
(optional)
V_REF
(1.26V)
Enable
FIGURE 4. Block Diagram.
10
V_IN
Low Noise
Charge Pump
Over Current
Over Temp
Protection
DMOS
Pass
Transistor
V_OUT
R₁
Adj
(Adjustable
R_{2 Versions)}
REG101
NOTE: R₁ and R₂ are internal
on fixed output versions.
REG101
SBVS026D