LT3040 datasheet(20/30 Pages) AD

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LT3040

Rev. 0

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the LT3040’s FS and VIN pins. The low pass filter (LPF)

created by this bypass resistor along with the VIN pin

capacitor bypasses the noise contribution from the volt-

age source. To ensure fast start-up of LT3040 with this

LPF in place, the LT3040’s fast start-up circuit charges

the VIN pin capacitor with a 10mA (typical) current source.

One problem that conventional linear regulators face is

that the resistor divider setting the output voltage gains up

the reference noise. In contrast, the LT3040’s unity-gain

follower architecture presents no gain from the VIN pin to

the output. Therefore, if a capacitor bypasses the voltage

source noise, then the output noise is independent of the

programmed output voltage. The resultant output noise

is then set just by the error amplifier’s noise — typically

4nV/√Hz from 10kHz to 1MHz and 1.2µVRMS in a 10Hz

to 100kHz bandwidth using a 4.7µF VIN pin capacitor.

Paralleling multiple LT3040s further reduces noise by

√N, for N parallel regulators.

Refer to the Typical Performance Characteristics section

for noise spectral density and RMS integrated noise over

various load currents and VIN pin capacitances.

VIN Pin (BYPASS) Capacitance: Noise, PSRR,

Transient Response and Soft-Start

In addition to reducing output noise, using a VIN pin

bypass capacitor also improves PSRR and transient per-

formance. Note that any bypass capacitor leakage may

deteriorate the LT3040’s DC regulation. Capacitor leakage

of 100nA results in a 100µV DC error for a 1k imped-

ance at the VIN pin. The use of a good quality low leakage

ceramic capacitor is recommended.

Using a VIN bypass capacitor may soft-start the output

and limit inrush current. If fast start functionality is not

used and a filter resistor (RIN) is used (refer to Figure 9)

then the RC time constant, formed by the VIN pin resistor

and capacitor, controls the soft start time. Ramp up rate

from 0% to 90% of nominal VOUT is:

tss = 2.3 • RIN • CIN

If the fast start functionality is used (see next section)

then the soft-start time is dependent on the amount of the

APPLICATIONS INFORMATION

capacitor used and the voltage input at the FS pin. In this

case the ramp up rate to within 10mV of nominal VOUT is:

tss(FS) = CIN • VFS/10mA

Figure 9.

V+

RFAULT

COUT

–

VIN

GND ILIM

EN/UV

V+

VIN

FAULT

RIN

RILIM

CIN

CV+

LT3040

VOUT

RLOAD

QDRV

VFB

QPWR

OUT

3040 TA09

Low Pass Filter with Fast-Startup Disabled

Fast Start

For ultralow noise applications that require low 1/f noise,

a VIN filter with a very low frequency pole may be nec-

essary. This significantly increases the start-up time in

accordance with the RC time constant of the VIN filter.

The LT3040 incorporates fast start-up circuitry that

sources the VIN pin with a typical current of 10mA during

start-up to rapidly charge the filter capacitor, decreasing

start-up time.

The 10mA current source turns on if the FS to VIN differ-

ential is greater than 100mV and remains engaged if the

FS pin is greater than the VIN pin by 7mV. This current

decreases to 0mA as VIN pin charges to the FS pin volt-

age. If the regulator is in current limit, dropout, thermal

shutdown or if supply voltage is below minimum V+, the

fast start function is disabled.

While using the fast start functionality, if the VIN slew

rate approaches the FS slew rate during startup, the VIN

pin voltage may catch up to the FS pin while FS is ris-

ing. This will shut off the fast start current until FS rises

~100mV above VIN. This is intended operation. This turn

on and turn off will cause stair stepping at the VIN pin

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