LT3040 datasheet(19/30 Pages) AD

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LT3040

Rev. 0

For more information www.analog.com

One measure of stability is the closed loop response of

the buffer. Figure 8 shows the closed loop response for

LT3040 with a 4.7µF output capacitor.

Figure 8.

V+ = 2V

VIN = 1.25V

CIN = 10nF

COUT = 4.7µF

ILOAD = 200mA

ILOAD = 1mA

FREQUENCY (Hz)

100

10k

100k

10M

–30

–20

–10

3040 F08

LT3040 Closed Loop Response

Stability and Supply Capacitance

The LT3040 is stable with a minimum 4.7µF V+ pin capaci-

tor. ADI recommends using low ESR ceramic capacitors.

In cases where long wires connect the power supply to

the LT3040’s supply and ground terminals, the use of low

value supply capacitors combined with a large load cur-

rent can result in instability. The resonant LC tank circuit

formed by the wire inductance and the supply capacitor is

the cause and not because of LT3040’s instability.

The self-inductance, or isolated inductance, of a wire

is directly proportional to its length. The wire diameter,

however, has less influence on its self-inductance. For

example, the self-inductance of a 2-AWG isolated wire

with a diameter of 0.26” is about half the inductance

of a 30-AWG wire with a diameter of 0.01”. One foot of

30-AWG wire has 465nH of self-inductance.

Several methods exist to reduce a wire’s self-inductance.

One method divides the current flowing towards the

LT3040 between two parallel conductors. In this case,

placing the wires further apart reduces the inductance;

up to a 50% reduction when placed only a few inches

apart. Splitting the wires connect two equal inductors in

parallel. However, when placed in close proximity to each

other, their mutual inductance adds to the overall self-

inductance of the wires — therefore a 50% reduction is

not possible in such cases. The second and more effec-

tive technique to reduce the overall inductance is to place

the forward and return current conductors (the supply

and ground wires) in close proximity. Two 30-AWG wires

separated by 0.02” reduce the overall inductance to about

one-fifth of a single wire.

If a battery mounted in close proximity powers the

LT3040, a 4.7µF supply capacitor suffices for stability.

However, if a distantly located supply powers the LT3040,

use a larger value input capacitor. Use a rough guideline

of 1µF (in addition to the 4.7µF minimum) per 8” of wire

length. The minimum supply capacitance needed to stabi-

lize the application also varies with the output capacitance

as well as the load current. Placing additional capacitance

on the LT3040’s output helps. However, this requires sig-

nificantly more capacitance compared to additional supply

bypassing. Series resistance between the power supply

and the LT3040 V+ input also helps stabilize the applica-

tion; as little as 0.1Ω to 0.5Ω suffices. This impedance

dampens the LC tank circuit at the expense of dropout

voltage. A better alternative is to use a higher ESR tan-

talum or electrolytic capacitor at the LT3040 supply in

parallel with a 4.7µF ceramic capacitor.

Output Noise

The LT3040 offers many advantages with respect to noise

performance. Traditional buffers or linear regulators have

several sources of noise. The most critical noise sources

for a traditional regulator are its voltage reference, error

amplifier, noise from the resistor divider network used for

setting output voltage and the noise gain created by this

resistor divider. Many low noise regulators pin out their

voltage reference to allow for noise reduction by bypass-

ing the reference voltage.

Unlike most linear regulators, the LT3040 does not use

an internal voltage reference. The external voltage source

(Voltage Reference/Voltage Output DAC) will dominate

the output voltage noise. The resultant voltage noise

equals the external reference noise which in turn is RMS

summed with the error amplifier’s noise. The typical noise

contribution from LT3040’s error amplifier is 4nV/√Hz.

In order to achieve this noise specification, the external

voltage source can use a filter resistor connected between

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