REG102-25 Datasheet, PDF (10/25 Pages) Texas Instruments – 250mA Low-Dropout Regulator

ENABLE
The Enable pin is active high and compatible with standard
TTL-CMOS levels. Inputs below 0.5V (max) turn the regula-
tor off and all circuitry is disabled. Under this condition,
ground pin current drops to approximately 10nA. When not
used, the Enable pin can be connected to V_IN. When a pull-
up resistor is used, and operation below 1.8V is required, use
pull-up resistor values below 50kΩ.
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 REG102 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_N
=
29µVrms ^{R1 + R2}
R2
=
29µVrms •
V_OUT
V_REF
(1)
As the value of V_REF is 1.26V, this relationship reduces to:
V_N = 23
µVrms
V
•
V_OUT
(2)
Connecting a capacitor, C_NR, from the Noise Reduction (NR)
pin to ground forms a low-pass filter for the voltage refer-
ence. Adding C_NR (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 approxi-
mately a factor of 2.8 for V_OUT = 3.3V. This noise reduction
effect is shown in Figure 5 and as RMS Noise Voltage vs C_NR
in the Typical Characteristics section.
RMS NOISE VOLTAGE vs C_NR
110
100
90
80
REG102-3.3
70
60 ^REG102-2.5
REG102-5.0
50
40
30
C_OUT = 0µF
10Hz < BW < 100kHz
20
0.1
10
100
1k
10k
C_NR (pF)
FIGURE 5. Output Noise versus Noise Reduction Capacitor.
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 REG102 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.
The REG102 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, con-
nected from the output to the adjust pin can 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
Output
V_OUT
R₁
Adj
(adjustable
R_{2 versions)}
REG102
NOTE: R₁ and R₂ are internal
on fixed output versions.
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REG102
SBVS024F