REG102-25 datasheet(10/25 Pages) TI1

10 / 25 page

REG102

SBVS024F

www.ti.com

FIGURE 4. Block Diagram.

FIGURE 5. Output Noise versus Noise Reduction Capacitor.

0.1

100

10k

110

100

NR (pF)

RMS NOISE VOLTAGE vs C

REG102-5.0

REG102-3.3

REG102-2.5

OUT = 0µF

10Hz < BW < 100kHz

Over-Current

Over Temp

Protection

REF

(1.26V)

Low-Noise

Charge Pump

DMOS

Output

NOTE: R

1 and R2 are internal

on fixed output versions.

OUT

Adj

(adjustable

versions)

(fixed output

versions only)

Enable

REG102

(optional)

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 VIN. When a pull-

up resistor is used, and operation below 1.8V is required, use

pull-up resistor values below 50k

Ω.

OUTPUT NOISE

A precision bandgap reference is used to generate the

internal reference voltage, VREF. This reference is the domi-

nant noise source within the REG102 and generates approxi-

mately 29

µVrms 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:

Vrms

OUT

REF

=µ

•

(1)

As the value of VREF is 1.26V, this relationship reduces to:

Vrms

OUT

•

(2)

Connecting a capacitor, CNR, from the Noise Reduction (NR)

pin to ground forms a low-pass filter for the voltage refer-

ence. Adding CNR (as shown in Figure 4) forms a low-pass

filter for the voltage reference. For CNR = 10nF, the total noise

in the 10Hz to 100kHz bandwidth is reduced by approxi-

mately a factor of 2.8 for VOUT = 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, COUT. Best overall noise performance is achieved

with very low (< 0.22

µF) or very high (> 2.2µF) values of C

OUT

(see the

RMS Noise Voltage vs COUT 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 VIN. The charge-pump switching

noise (nominal switching frequency = 2MHz) is not measur-

able at the output of the regulator over most values of IOUT

and COUT.

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, CFB, 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).

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