REG113 datasheet(9/16 Pages) TI

9 / 16 page

REG113

SBVS031D

www.ti.com

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 REG113 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)

Since 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 (as shown in Figure 3) 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 2.8 for VOUT = 3.3V. This noise reduction effect is

shown in Figure 4, 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 REG113 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.

DROPOUT VOLTAGE

The REG113 uses an N-channel DMOS as the pass ele-

ment. When (VIN – VOUT) is less than the dropout voltage

(VDROP), the DMOS pass device behaves like a resistor;

therefore, for low values of (VIN – VOUT), the regulator input-

to-output resistance is the RdsON 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

10k

110

100

NR (pF)

RMS NOISE VOLTAGE vs C

REG113-5.0

REG113-3.3

REG113-2.5

OUT = 0µF

10Hz < BW < 100kHz

FIGURE 3. Block Diagram.

Over-Current

Over Temp

Protection

REF

(1.26V)

Low-Noise

Charge Pump

DMOS

Output

OUT

GND

Enable

REG113

(optional)

typically maintain regulation down to a (VIN – VOUT) voltage

drop of 250mV at full rated output current. In Figure 5, the

bottom line (DC dropout) shows the minimum VIN to VOUT

voltage drop required to prevent dropout under DC load

conditions.

FIGURE 5. Transient and DC Dropout.

100

150

200

250

350

300

400

600

500

400

300

200

100

OUT (mA)

DROPOUT VOLTAGE vs I

OUT

Dropout for 0mA to I

OUT Transient

DC Dropout

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