MAX5083ATE datasheet(11/18 Pages) MAXIM

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OUT

−













where VFB = 1.23V.

Inductor Selection

Three key inductor parameters must be specified for oper-

ation with the MAX5082/MAX5083: inductance value (L),

peak inductor current (IPEAK), and inductor saturation cur-

rent (ISAT). The minimum required inductance is a function

of operating frequency, input-to-output voltage differential,

and the peak-to-peak inductor current (ΔIP-P). Higher

ΔIP-P allows for a lower inductor value while a lower ΔIP-P

requires a higher inductor value. A lower inductor value

minimizes size and cost and improves large-signal and

transient response, but reduces efficiency due to higher

peak currents and higher peak-to-peak output voltage

ripple for the same output capacitor. On the other hand,

higher inductance increases efficiency by reducing the

ripple current. Resistive losses due to extra wire turns can

exceed the benefit gained from lower ripple current levels

especially when the inductance is increased without also

allowing for larger inductor dimensions. A good compro-

mise is to choose ΔIP-P equal to 40% of the full load cur-

rent. Calculate the inductor using the following equation:

OUT IN

OUT

IN SW

P P

)

−

×∆

VIN and VOUT are typical values so that efficiency is

optimum for typical conditions. The switching frequency

(fSW) is fixed at 250kHz or can vary between 150kHz

and 350kHz when synchronized to an external clock

(see the Oscillator/Synchronization Input (SYNC) sec-

tion). The peak-to-peak inductor current, which reflects

the peak-topeak output ripple, is worst at the maximum

input voltage. See the Output Capacitor Selection section

to verify that the worst-case output ripple is acceptable.

The inductor saturating current (ISAT) is also important

to avoid runaway current during continuous output short

circuit. Select an inductor with an ISAT specification higher

than the maximum peak current limit of 3.5A.

Input Capacitor Selection

The discontinuous input current of the buck converter

causes large input ripple currents and therefore the input

capacitor must be carefully chosen to keep the input

voltage ripple within design requirements. The input volt-

age ripple is comprised of ΔVQ (caused by the capacitor

discharge) and ΔVESR (caused by the ESR of the input

capacitor). The total voltage ripple is the sum of ΔVQ and

ΔVESR. Calculate the input capacitance and ESR required

for a specified ripple using the following equations:

ESR

OUT_MAX

Q SW

ESR

D(1 D)

−

∆













∆×

where

OUT

IN SW

OUT

) V

and

−

∆=

××

IOUT_MAX is the maximum output current, D is the duty

cycle, and fSW is the switching frequency.

The MAX5082/MAX5083 includes internal and external

UVLO hysteresis and soft-start to avoid possible unin-

tentional chattering during turn-on. However, use a bulk

capacitor if the input source impedance is high. Use

enough input capacitance at lower input voltages to avoid

possible undershoot below the undervoltage lockout

threshold during transient loading.

Output Capacitor Selection

The allowable output voltage ripple and the maximum

deviation of the output voltage during load steps deter-

mine the output capacitance and its ESR. The output

ripple is mainly composed of ΔVQ (caused by the capaci-

tor discharge) and ΔVESR (caused by the voltage drop

across the equivalent series resistance of the output

capacitor). The equations for calculating the peak-to-peak

output voltage ripple are:

P-P

OUT

ESR

P P

16 C

ESR

∆

∆=

××

∆

×∆

Normally, a good approximation of the output voltage rip-

ple is ΔVRIPPLE ≈ ΔVESR + ΔVQ. If using ceramic capaci-

tors, assume the contribution to the output voltage ripple

from ESR and the capacitor discharge to be equal to 20%

and 80%, respectively. ΔIP-P is the peak-to-peak inductor

current (see the Input Capacitors Selection section) and

fSW is the converter’s switching frequency.

MAX5082/MAX5083

1.5A, 40V, MAXPower Step-Down

DC-DC Converters

www.maximintegrated.com

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