MAX15021 datasheet(13/24 Pages) MAXIM

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Effective Input-Voltage Range

Although the MAX15021’s regulators can operate from

input supplies ranging from 2.5V to 5.5V, the input-volt-

age range can be effectively limited by the

MAX15021’s duty-cycle limitations for a given output

voltage (VOUT_). The maximum input voltage

(VPVIN_MAX) can be effectively limited by the control-

lable minimum on-time (tON(MIN)):

where tON(MIN) is 0.06µs (typ).

The minimum input voltage (VPVIN_MIN) can be effec-

tively limited by the maximum controllable duty cycle

and is calculated using the following equation:

where VOUT_ is the regulator output voltage and

tOFF(MIN) is the 0.06µs (typ) controllable off-time.

Inductor Selection

Three key inductor parameters must be specified for

operation with the MAX15021: inductance value (L),

peak inductor current (IPEAK), and inductor saturation

current (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. A

lower inductance minimizes size and cost and

improves large-signal and transient response.

However, efficiency is reduced due to higher peak cur-

rents and higher peak-to-peak output-voltage ripple for

the same output capacitor. A higher inductance

increases efficiency by reducing the ripple current;

however, resistive losses due to extra wire turns can

exceed the benefit gained from lower ripple current lev-

els especially when the inductance is increased without

also allowing for larger inductor dimensions. Choose

the inductor’s peak-to-peak current,

ΔIP-P, in the range

of 20% to 50% of the full load current; as a rule of

thumb 30% is typical.

Calculate the inductance, L, using the following equation:

where VPVIN_ is the input supply voltage, VOUT_ is the

regulator output voltage, and fSW is the switching fre-

quency. Use typical values for VPVIN_ and VOUT_ so

that efficiency is optimum for typical conditions. The

switching frequency (fSW) is programmable between

500kHz and 4MHz (see the

Oscillator section).

The peak-to-peak inductor current (

ΔIP-P), which

reflects the peak-to-peak output ripple, is largest at the

maximum input voltage. See the

Output-Capacitor

Selection section to verify that the worst-case output

current ripple is acceptable.

Select an inductor with a saturation current, ISAT, high-

er than the maximum peak current to avoid runaway

current during continuous output short-circuit condi-

tions. Also, confirm that the inductor’s thermal perfor-

mances and projected temperature rise above ambient

does not exceed its thermal capacity. Many inductor

manufacturers provide bias/load current versus tem-

perature rise performance curves (or similar) to obtain

this information.

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 withstand

the input ripple current and keep the input-voltage rip-

ple within design requirements.

The input-voltage 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 which peaks at the end of the

on-cycle. Calculate the required input capacitance and

ESR for a specified ripple using the following equations:

ILOAD(MAX) is the maximum output current,

ΔIP-P is the

peak-to-peak inductor current, and VPVIN_ is the input

supply voltage, VOUT_ is the regulator output voltage,

and fSW is the switching frequency.

ESR

[mV]

[A]

[V]

V [V] f

[MHz]

[A]

[V] V

[V]

[V] f

[MHz] L

ESR

LOAD(MAX)

PVIN_

LOAD(MAX)

OUT_

PVIN_

QSW

PVIN_

OUT_

PVIN_

[]

⎛

⎝⎜

⎞

⎠⎟

⎛

⎝

⎜

⎞

⎠

⎟

−

() ×

××

−

[]

μ=

×−

××

−

[V] (V

[V] V

[V])

[V] f

[MHz]

OUT_

PVIN_

OUT_

PVIN_

[V]

1 (t

[ s] f

[MHz])

PVIN_MIN

OUT_

OFF(MIN)

≥

−×

[V]

[ s] f

[MHz]

PVIN_MAX

OUT_

ON(MIN)

≤

Dual, 4A/2A, 4MHz, Step-Down DC-DC

Regulator with Tracking/Sequencing Capability

MAX15021

Maxim Integrated

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