LT3063 datasheet(13/22 Pages) LINER

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LT3063 Series

3063fa

For more information www.linear.com/LT3063

APPLICATIONS INFORMATION

Bypass Capacitance, Output Voltage Noise and

Transient Response

The LT3063 regulator provides low output voltage noise

over the 10Hz to 100kHz bandwidth while operating at full

load with the addition of a bypass capacitor (CREF/BYP)

from the REF/BYP pin to GND. A good quality low leak-

age capacitor is recommended. This capacitor bypasses

the reference of the regulator, providing a low frequency

noise pole for the internal reference. With the use of 10nF

for CREF/BYP, the output voltage noise decreases to as

low as 30µVRMS when the output voltage is set for 0.6V.

For higher output voltages (generated by using a resistor

divider), the output voltage noise gains up accordingly

when using CREF/BYP by itself.

To lower the output voltage noise for higher output volt-

ages, include a feedforward capacitor (CFF) from VOUT

to the ADJ pin. A good quality, low leakage capacitor is

recommended.Thiscapacitorbypassestheerroramplifier

oftheregulator,providingalowfrequencynoisepole.With

the use of 5nF for both CFF and CREF/BYP, output voltage

noise decreases to 30µVRMS when the output voltage is

set to 5V by a 5µA feedback resistor divider. If the cur-

rent in the feedback resistor divider is doubled, CFF must

also be doubled to achieve equivalent noise performance.

Higher values of output voltage noise may be measured

if care is not exercised with regard to circuit layout and

testing.Crosstalkfromnearbytracescaninduceunwanted

noiseontotheLT3063’soutput.Powersupplyripplerejec-

tion must also be considered. The LT3063 regulator does

not have unlimited power supply rejection and will pass

a small portion of the input noise through to the output.

Using a feedforward capacitor (CFF) from VOUT to the ADJ

pin has the added benefit of improving transient response

foroutputvoltagesgreaterthan0.6V.Withnofeedforward

capacitor, the settling time will increase as the output

voltage is raised above 0.6V. Use the equation in Figure 2

to determine the minimum value of CFF to achieve a

transient response that is similar to 0.6V output voltage

performance regardless of the chosen output voltage (see

Figure3andTransientResponseintheTypicalPerformance

Characteristics section).

Feedforward capacitance can also be used in fixed-voltage

parts; the feedforward capacitor is connected from OUT

to ADJ in the same manner. In this case, the current in

the internal feedback resistor divider is 5μA.

During start-up, the internal reference soft-starts if a refer-

ence bypass capacitor is present. Regulator startup time

is directly proportional to the size of the bypass capacitor,

slowing to 6ms with a 10nF bypass capacitor (See SHDN

Transient Response vs REF/BYP Capacitor in the Typical

Performance Characteristics section). The reference by-

pass capacitor is actively pulled low during shutdown to

reset the internal reference.

Start-up time is also affected by the use of a feedforward

capacitor. Start-up time is directly proportional to the size

of the feedforward capacitor and output voltage, and is

inversely proportional to the feedback resistor divider cur-

rent, slowing to 15ms with a 4.7nF feedforward capacitor

and a 10µF output capacitor for an output voltage set to

5V by a 5µA feedback resistor divider.

3063 F02

SHDN

OUT

ADJ

GND REF/BYP

LT3063

VIN

VOUT

CREF/BYP

CFF

COUT

CFF

10nF

10µA

• IFB–DIVIDER

(

)

IFB–DIVIDER =

VOUT

+R2

Figure 2. Feedforward Capacitor for Fast Transient Response

Figure 3. Transient Response vs Feedforward Capacitor

100µs/DIV

VOUT = 5V

COUT = 10µF

IFB-DIVIDER = 5µA

1nF

10nF

LOAD CURRENT

200mA/DIV

10pF

3063 F03

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