LT3592IMSE datasheet(12/24 Pages) LINER

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LT3592

3592fa

APPLICATIONS INFORMATION

As the input voltage increases, the inductor current ramps

up more quickly, the number of skipped pulses increases,

and the output voltage ripple increases. For operation

above VIN(MAX), the only component requirement is that

they be adequately rated for operation at the intended

voltage levels.

The LT3592 is robust enough to survive prolonged opera-

tion under these conditions as long as the peak inductor

current does not exceed 1.2A. Inductor saturation due to

high current may further limit performance in this operat-

ing regime.

Inductor Selection and Maximum Output Current

A good ﬁrst choice for the inductor value is:

= 1.2A •

OUT + 0.2V + VD

()

where VD is the forward voltage drop of the catch diode

(~0.4V), f is the switching frequency in MHz and L is in μH.

With this value, there will be no subharmonic oscillation for

applications with 50% or greater duty cycle. For low duty

cycle applications in which VIN is more than three times

VOUT, a good guide for the minimum inductor value is

= 1.7 •

VIN VOUT 0.2V

()

VIN VSW + VD

•

VOUT + 0.2V + VD

()

where VSW is the switch voltage drop (about 0.3V at

500mA). The inductor’s RMS current rating must be greater

than your maximum load current and its saturation current

should be about 30% higher. For robust operation in fault

conditions, the saturation current should be above 1.5A. To

keep efﬁciency high, the series resistance (DCR) should be

less than 0.1Ω. Table 2 lists several inductor vendors.

Of course, such a simple design guide will not always re-

sult in the optimum inductor for your application. A larger

value provides a higher maximum load current and reduces

output voltage ripple at the expense of a slower transient

response. If your load is lower than 500mA, then you can

decrease the value of the inductor and operate with higher

ripple current. This allows you to use a physically smaller

inductor, or one with a lower DCR resulting in higher efﬁ-

ciency. There are several graphs in the Typical Performance

Characteristics section of this data sheet that show the

maximum load current as a function of input voltage and

inductor value for several popular output voltages. Low

inductance may result in discontinuous mode operation,

which is acceptable, but further reduces maximum load

current. For details of the maximum output current and

discontinuous mode operation, see Linear Technology

Application Note 44.

Catch Diode

Depending on load current, a 500mA to 1A Schottky di-

ode is recommended for the catch diode, D1. The diode

must have a reverse voltage rating equal to or greater

than the maximum input voltage. The ON Semiconductor

MBRA140T3 and Central Semiconductor CMMSH1-40 are

good choices, as they are rated for 1A continuous forward

current and a maximum reverse voltage of 40V.

Input Filter Network

For applications that only require a capacitor, bypass VIN

with a 1μF or higher ceramic capacitor of X7R or X5R

type. Y5V types have poor performance over tempera-

ture and applied voltage and should not be used. A 1μF

ceramic capacitor is adequate to bypass the LT3592 and

will easily handle the ripple current. However, if the input

power source has high impedance, or there is signiﬁcant

inductance due to long wires or cables, additional bulk

capacitance might be necessary. The can be provided

with a low performance (high ESR) electrolytic capacitor

in parallel with the ceramic device.

Some applications, such as those in automobiles, may

require extra ﬁltering due to EMI/EMC requirements. In

these applications, very effective EMI ﬁltering can be pro-

vided by a capacitor to ground right at the source voltage,

a series ferrite bead, and a pi ﬁlter composed of a capacitor

to ground, a series inductor, and another capacitor directly

from the device pin to ground (see the Block Diagram for

an example). Typical values for the ﬁlter components are

10nF for C2C, a ferrite bead that is ~220Ω at 100MHz for

L2, 3.3μF for C2B, 10μH for L3, and 1μF for C2A.

Step-down regulators draw current from the input sup-

ply in pulses with very fast rise and fall times. The input

capacitor is required to reduce the resulting voltage ripple

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