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LT3065 Series

3065fc

For more information www.linear.com/LT3065

applicaTions inForMaTion

Table3shows1%resistordividervaluesforsomecommon

output voltages with a resistor divider current of 10μA.

Table 3. Output Voltage Resistor Divider Values

VOUT (V)

R1 (kΩ)

R2 (kΩ)

1.2

60.4

1.5

88.7

1.8

118

2.5

60.4

191

237

3.3

61.9

280

432

Bypass Capacitance and Output Voltage Noise

The LT3065 regulator provides low output voltage noise

over a 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 high quality low leak-

age capacitor is recommended. This capacitor bypasses

the internal reference of the regulator, providing a low

frequency noise pole for the internal reference. With the

use of 10nF for CREF/BYP, output voltage noise decreases

to as low as 25μVRMS when the output voltage is set for

0.6V. For higher output voltages (generated by using a

feedback resistor divider), the output voltage noise gains

up proportionately when using CREF/BYP.

To lower the higher output voltage noise, connect a

feedforward capacitor (CFF) from VOUT to the ADJ pin. A

high quality, low leakage capacitor is recommended. This

capacitor bypasses the error amplifier of the regulator,

providing an additional low frequency noise pole. With

the use of 10nF for both CFF and CREF/BYP, output voltage

noise decreases to 25μVRMS when the output voltage is

set to 5V by a 10μA feedback resistor divider. If the cur-

rent in the feedback resistor divider is doubled, CFF must

also be doubled to achieve equivalent noise performance.

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.

Higher values of output voltage noise can occur if care

is not exercised with regard to circuit layout and testing.

Crosstalk from nearby traces induces unwanted noise

onto the LT3065’s output. Power supply ripple rejection

must also be considered. The LT3065 regulator does not

have unlimited power supply rejection and passes a small

portion of the input noise through to the output.

Using a feedforward capacitor (CFF) connected between

VOUTandADJhastheaddedbenefitofimprovingtransient

response for output voltages greater than 0.6V. With no

feedforward capacitor, the settling time increases as the

output voltage increases above 0.6V. Use the equation

in Figure 4 to determine the minimum value of CFF to

achieve a transient response that is similar to the 0.6V

output voltage performance regardless of the chosen

output voltage (See Figure 5 and Transient Response in

the Typical Performance Characteristics section).

Figure 4. Feedforward Capacitor for Fast Transient Response

Figure 5. Transient Response vs Feedforward Capacitor

100µs/DIV

VOUT = 5V

COUT = 10µF

IFB-DIVIDER = 10µA

1nF

10nF

LOAD CURRENT

500mA/DIV

100pF

3065 F05

3065 F04

SHDN

OUT

ADJ

GND REF/BYP

LT3065

VIN

VOUT

CREF/BYP

CFF

COUT

CFF ≥

10nF

10µA

• IFB_DIVIDER

(

)

IFB_DIVIDER =

VOUT

R1+R2

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