1.5A, 40V, MAXPower Step-Down

DC-DC Converters

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11

Inductor Selection

Three key inductor parameters must be specified for

operation with the MAX5082/MAX5083: inductance

tance is a function of operating frequency, input-to-out-

put voltage differential, and the peak-to-peak inductor

current (

value while a lower

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 especial-

ly when the inductance is increased without also allow-

ing for larger inductor dimensions. A good compromise

is to choose

Calculate the inductor using the following equation:

mum for typical conditions. The switching frequency

350kHz when synchronized to an external clock (see the

Oscillator/Synchronization Input (SYNC) section). The

peak-to-peak inductor current, which reflects the peak-to-

peak 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

away current during continuous output short circuit.

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 voltage ripple is comprised of

capacitor discharge) and

the input capacitor). The total voltage ripple is the sum

of

ESR required for a specified ripple using the following

equations:

where

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

capacitor discharge) and

age drop across the equivalent series resistance of the

output capacitor). The equations for calculating the

peak-to-peak output voltage ripple are:

Normally, a good approximation of the output voltage

ripple is

capacitors, assume the contribution to the output volt-

age ripple from ESR and the capacitor discharge to be

∆

∆

∆∆

V

I

1

VI

Q

P- P

OUT

SW

ESR

P- P

=

××

=×

6C

f

ESR

()

∆I

VV

V

VL

D

V

V

P-P

IN

OUT

OUT

IN

SW

OUT

IN

=

×

××

=

−

f

and

ESR

V

I

ID

D

f

=

+

⎛

⎝⎜

⎞

⎠⎟

=

×

×

−

∆

∆

∆

ESR

OUT_MAX

P-P

IN

OUT_MAX

QSW

I

C

1

V

2

()

()

L

f

=

××

−

VV

V

VI

OUT

IN

OUT

IN

SW

P-P

∆

R

R

V

V

4

3

1

=

⎡

⎣

⎢

⎤

⎦

⎥

−

OUT

FB