MAX16909RAUE+ datasheet(13/20 Pages) MAXIM

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Maxim Integrated

MAX16909

36V, 220kHz to 1MHz Step-Down Converter

with Low Operating Current

The inductor value must be chosen so that the maximum

inductor current does not reach the device’s minimum

current limit. The optimum operating point is usually

found between 25% and 35% ripple current. When pulse

skipping (FSYNC low and light loads), the inductor value

also determines the load-current value at which PFM/

PWM switchover occurs.

Find a low-loss inductor having the lowest possible

DC resistance that fits in the allotted dimensions. Most

inductor manufacturers provide inductors in standard

values, such as 1.0FH, 1.5FH, 2.2FH, 3.3FH, etc. Also

look for nonstandard values, which can provide a bet-

ter compromise in LIR across the input voltage range. If

using a swinging inductor (where the no-load inductance

decreases linearly with increasing current), evaluate

the LIR with properly scaled inductance values. For

the selected inductance value, the actual peak-to-peak

inductor ripple current (DIINDUCTOR) is defined by:

OUT SUP

OUT

INDUCTOR

SUP

V(V

−

∆=

××

where DIINDUCTOR is in A, L is in H, and fSW is in Hz.

Ferrite cores are often the best choices, although pow-

dered iron is inexpensive and can work well at 200kHz.

The core must be large enough not to saturate at the

peak inductor current (IPEAK):

INDUCTOR

PEAK

LOAD(MAX)

∆

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

V(V

−

IRMS has a maximum value when the input voltage

equals twice the output voltage (VSUP = 2VOUT), so

IRMS(MAX) = ILOAD(MAX)/2.

Choose an input capacitor that exhibits less than +10NC

self-heating temperature rise at the RMS input current for

optimal long-term reliability.

The input-voltage ripple is composed of DVQ (caused

by the capacitor discharge) and DVESR (caused by the

equivalent series resistance (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

−×

∆=

××

and

OUT

D(1 D)

−

∆×

and

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 ESR to

meet output ripple and load-transient requirements, yet

have high enough ESR to satisfy stability 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 capacitor’s ESR dominates the

output-voltage ripple. So the size of the output capaci-

tor depends on the maximum ESR required to meet the

output-voltage ripple (VRIPPLE(P-P)) specifications:

VRIPPLE(P-P) = ESR × ILOAD(MAX) × 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.

Table 1. Inductor Size Comparison

INDUCTOR SIZE

SMALLER

LARGER

Lower price

Smaller ripple

Smaller form factor

Higher efficiency

Faster load response

Larger fixed-frequency

range in skip mode

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