REG103FA-ASLASH500.B datasheet(10/28 Pages) TI2

10 / 28 page

REG103

SBVS010D

Since the value of VREF is 1.295V, this relationship reduces to:

Vrms

OUT

•

Connecting a capacitor, CNR, from the Noise-Reduction

(NR) pin to ground, can reduce the output noise voltage.

Adding CNR, as shown in Figure 7, 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 3.5, as shown in Figure 8.

FIGURE 7. Block Diagram.

Over Current

Over Temp

Protection

REF

(1.295V)

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

REG103

ERROR

(optional)

FIGURE 8. Output Noise versus Noise-Reduction Capacitor.

0.001

0.01

0.1

NR (µF)

OUT = 0

OUT = 10µF

FIGURE 9. Output Noise Density on Adjustable Versions.

100

1000

10000

100000

10.0

1.0

0.1

Frequency

OUT = 0, CFB = 0

OUT = 0, CFB = 0.01µF

OUT = 10µF, CFB = 0.01µF

The REG103 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,

connected from the output to the adjust pin will reduce both

the output noise and the peak error from a load transient.

Figure 9 shows improved output noise performance for two

capacitor combinations.

The REG103 utilizes 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 switch-

ing noise (nominal switching frequency = 2MHz) is not

measurable at the output of the regulator.

DROP-OUT VOLTAGE

The REG103 uses an N-channel DMOS as the “pass”

element. When the input voltage is within a few hundred

millivolts of the output voltage, the DMOS device behaves

like a resistor. Therefore, for low values of VIN to VOUT, the

regulator’s input-to-output resistance is the RdsON of the

DMOS pass element (typically 230m

Ω). For static (DC)

loads, the REG103 will typically maintain regulation down

to VIN to VOUT voltage drop of 115mV at full-rated output

current. In Figure 10, the bottom line (DC dropout) shows

the minimum VIN to VOUT voltage drop required to prevent

drop-out under DC load conditions.

REG103FA-A/500.B Fiches technique(PDF) 10 Page - Texas Instruments

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