MAX20002CATPC datasheet(14/19 Pages) MAXIM

14 / 19 page

Forced-PWM and Skip Modes

In PWM mode of operation, the devices switch at

a constant frequency with variable on-time. In skip

mode of operation, the converter’s switching frequency

is load dependent. At higher load current, the switching

frequency does not change and the operating mode is

similar to the PWM mode. Skip mode helps improve

efficiency in light-load applications by allowing the

converters to turn on the high-side switch only when the

output voltage falls below a set threshold. As such, the

converters do not switch MOSFETs on and off as often

as in the PWM mode. Consequently, the gate charge and

switching losses are much lower in skip mode.

Inductor Selection

Three key inductor parameters must be specified for

operation with the devices: inductance value (L), inductor

saturation current (ISAT), and DC resistance (RDCR). To

select inductor value, the ratio of inductor peak-to-peak

AC current to DC average current (LIR) must be selected

first. A good compromise between size and loss is a 30%

peak-to-peak ripple current to average-current ratio (LIR

= 0.3). The switching frequency, input voltage, output volt-

age, and selected LIR then determine the inductor value

as follows:

SUP

OUT

SUP

OUT

) V

LIR

−×

××

where VSUP, VOUT, and IOUT are typical values (so that

efficiency is optimum for typical conditions). The switch-

ing frequency is set by RFOSC (see TOC 8 in the Typical

Operating Characteristics section).

Input Capacitor

The input filter capacitor reduces peak currents drawn

from the power source and reduces noise and voltage

ripple on the input caused by the circuit’s switching.

The input capacitor RMS current requirement (IRMS) is

defined by the following equation:

OUT

SUP

OUT

RMS

LOAD(MAX)

SUP

x(V

- V

)

IRMS has a maximum value when the input voltage

equals twice the output voltage:

SUP

OUT

= ×

therefore:

LOAD(MAX)

RMS

SUP

Choose an input capacitor that exhibits less than +10°C

self-heating temperature rise at the RMS input current for

optimal long-term reliability.

The input-voltage ripple is comprised of ΔVQ (caused

by the capacitor discharge) and ΔVESR (caused by the

ESR of the capacitor). Use low-ESR ceramic capacitors

with high ripple-current capability at the input. Assume

the contribution from the ESR and capacitor discharge

equal to 50%. Calculate the input capacitance and ESR

required for a specified input voltage ripple using the

following equations:

ESR

OUT

ESR

∆

where:

SUP

OUT

SUP

- V

) V

∆=

××

and:

OUT

D(1- D)

∆×

OUT

SUPSW

where: IOUT is the maximum output current and D is the

duty cycle.

Output Capacitor

The output filter capacitor must have low enough equiva-

lent series resistance (ESR) to meet output-ripple and

load-transient requirements. The output capacitance must

be high enough to absorb the inductor energy while

transitioning from full-load to no-load conditions without

tripping the overvoltage-fault protection. When using

high-capacitance, low-ESR capacitors, the filter capaci-

tor’s ESR dominates the output-voltage ripple, so the size

of the output capacitor depends on the maximum ESR

required to meet the output-voltage ripple (VRIPPLE(P-P))

specifications:

RIPPLE(P-P)

LOAD(MAX)

ESR I

LIR

The actual capacitance value required relates to the

physical size needed to achieve low ESR, as well as

to the chemistry of the capacitor technology. Thus, the

capacitor is usually selected by ESR and voltage rating

rather than by capacitance value.

www.maximintegrated.com

Maxim Integrated │ 14

MAX20002/MAX20003

36V, 220kHz to 2.2MHz, 2A/3A Fully

Integrated Step-Down Converters

with 15μA Operating Current

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