LT3571 datasheet(10/16 Pages) LINER

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LT3571

3571fa

APPLICATIONS INFORMATION

Inductor Selection

The inductors used with the LT3571 should have a

saturation current rating of 0.4A, or greater. If the device

is used in an application where the input supply will be

hot-plugged, the saturation current rating should be equal

to, or greater than, the peak inrush current. For best loop

stability, the inductor value selected should provide a

ripple current of 80mA or more. For a given VIN and VOUT,

the inductor value to use in continuous conduction mode

(CCM) is estimated by the formula:

L = D• VIN

ƒ •80mA

where:

D =

VOUT +1– VIN

VOUT +1

and f is the switching frequency.

To achieve low output voltage ripple, a small value inductor

should be selected to force the LT3571 to operate in

discontinuous conduction mode (DCM). The inequality

is true when the LT3571 is operating in discontinuous

conduction mode.

L < D• VIN

ƒ •ILIMIT

where ILIMIT is the switch current limit. Operating in DCM

reduces the maximum load current and the conversion

efﬁciency.

Capacitor Selection

Low ESR capacitors should be used at the output to

minimize the output voltage ripple. Use only X5R and X7R

types, because they retain their capacitance over wider

voltage and temperature ranges than other types. High

output voltages typically require less capacitance for loop

stability. Typically, use a 1μF capacitor for output voltage

less than 25V, and a 0.22μF capacitor for output voltage

beyond 25V. Place the output capacitor as close as possible

to the VOUT lead and to the GND of the IC.

Either ceramic or solid tantalum capacitors may be used

for the input decoupling capacitor, which should be placed

as close as possible to the LT3571. A 1μF capacitor is

sufﬁcient for most applications.

Phase Lead Capacitor

A small value capacitor (i.e., 10pF to 22pF) can be added

in parallel with the resistor between the output and the FB

pin to reduce output perturbation due to a load step and

to improve transient response. This phase lead capacitor

introduces a pole-zero pair to the feedback that boosts the

phase margin near the crossover frequency. The APD is

very sensitive to a noisy bias supply. To lowpass ﬁlter noise

from the internal reference and error ampliﬁer, a 0.1μF

phase lead capacitor can be used. The corner frequency

of the noise ﬁlter is R1 • CPL.

APD Current Monitor

The power supply switching noise associated with a

switching power supply can interfere with the photodiode

DC measurement. To suppress this noise, a 0.1μF capacitor

is recommended at the APD pin. An additional series

resistor is necessary to ensure enough high frequency

compensation at the APD pin over the full operating range

of the LT3571, as shown in Figure 1. An additional output

lowpass ﬁlter, a 10k resistor and a 10nF capacitor in

parallel at MON pin can further reduce the power supply

noise, and other wide band noise, which might limit the

measurement accuracy of low current levels.

For applications requiring fast current monitor response

time, an RC lowpass ﬁlter at the MONIN pin is used to

replace the 0.1μF capacitor at the APD pin, as illustrated

in Figure 3.

Figure 3. RC Filter at MONIN Pin

MONIN

CFILT

3571 F03

VOUT

LT3571

APD

RSENSE

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