MAX8745 datasheet(29/36 Pages) MAXIM

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High-Efficiency, Quad Output, Main Power-

Supply Controllers for Notebook Computers

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(see stability requirements), the filter capacitor’s ESR

dominates the output voltage ripple. So the output capaci-

tor’s size depends on the maximum ESR required to meet

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

VRIPPLE(P-P) = RESRILOAD(MAX)LIR

In Idle Mode, the inductor current becomes discontinu-

ous, with peak currents set by the Idle Mode current-

sense threshold (VIDLE = 0.2VLIMIT). In Idle Mode, the

no-load output ripple may be determined as follows:

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 (this is true of tanta-

lums, OS-CONs, polymers, and other electrolytics).

When using low-capacity filter capacitors, such as

ceramic capacitors, size is usually determined by the

capacity needed to prevent VSAG and VSOAR from

causing problems during load transients. Generally,

once enough capacitance is added to meet the over-

shoot requirement, undershoot at the rising load edge

is no longer a problem (see the VSAG and VSOAR equa-

tions in the Transient Response section). However, low-

capacity filter capacitors typically have high ESR zeros

that may affect the overall stability (see the Output-

Capacitor Stability Considerations section).

Output-Capacitor Stability Considerations

Stability is determined by the value of the ESR zero rel-

ative to the switching frequency. The boundary of insta-

bility is given by the following equation:

where:

For a typical 300kHz application, the ESR zero frequen-

cy must be well below 95kHz, preferably below 50kHz.

Tantalum and OS-CON capacitors in widespread use

at the time of publication have typical ESR zero fre-

quencies of 25kHz. In the design example used for

inductor selection, the ESR needed to support 25mVP-P

ripple is 25mV/1.5A = 16.7m

Ω. One 220µF/4V Sanyo

polymer (TPE) capacitor provides 15m

Ω (max) ESR.

This results in a zero at 48kHz, well within the bounds

of stability.

For low-input voltage applications where the duty cycle

exceeds 50% (VOUT/VIN

≥ 50%), the output ripple volt-

age should not be greater than twice the internal slope-

compensation voltage:

VRIPPLE

≤ 0.02 x VOUT

where VRIPPLE equals

ΔIINDUCTOR x RESR. The worst-

case ESR limit occurs when VIN = 2 x VOUT, so the

above equation may be simplified to provide the follow-

ing boundary condition:

RESR

≤ 0.04 x L x fSW

Do not put high-value ceramic capacitors directly

across the feedback sense point without taking precau-

tions to ensure stability. Large ceramic capacitors can

have a high ESR zero frequency and cause erratic,

unstable operation. However, it is easy to add enough

series resistance by placing the capacitors a couple of

inches downstream from the feedback sense point,

which should be as close as possible to the inductor.

Unstable operation manifests itself in two related but

distinctly different ways: short/long pulses and cycle

skipping resulting in lower frequency operation.

Instability occurs due to noise on the output or because

the ESR is so low that there is not enough voltage ramp

in the output voltage signal. This “fools” the error com-

parator into triggering too early or into skipping a cycle.

Cycle skipping is more annoying than harmful, resulting

in nothing worse than increased output ripple.

However, it can indicate the possible presence of loop

instability due to insufficient ESR. Loop instability can

result in oscillations at the output after line or load

steps. Such perturbations are usually damped, but can

cause the output voltage to rise above or fall below the

tolerance limits.

The easiest method for checking stability is to apply a

very fast zero-to-max load transient and carefully

observe the output-voltage-ripple envelope for over-

shoot and ringing. It may help to simultaneously moni-

tor the inductor current with an AC current probe. Do

not allow more than three cycles of ringing after the ini-

tial step-response under/overshoot.

Input Capacitor Selection

The input capacitor must meet the ripple current

requirement (IRMS) imposed by the switching currents.

For an out-of-phase regulator, the total RMS current in

the input capacitor is a function of the load currents,

ESR

ESR OUT

ESR

OSC

≤

RIPPLE P P

IDLE ESR

SENSE

(– ) =

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