MAX8743(2004) 데이터 시트보기 (PDF) - Maxim Integrated

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MAX8743
(Rev.:2004)

Dual, High-Efficiency, Step-Down Controller with High Impedance in Shutdown

Maxim Integrated 

Dual, High-Efficiency, Step-Down

Controller with High Impedance in Shutdown

Table 4. Frequency Selection Guidelines

TON SETTING

VCC

FLOAT

REF

AGND

SIDE 1

FREQUENCY

(kHz)

235

345

485

620

SIDE 1

K-FACTOR

(µs)

4.24

2.96

2.08

1.63

SIDE 2

FREQUENCY

(kHz)

170

255

355

460

SIDE 2

K-FACTOR

(µs)

5.81

4.03

2.81

2.18

APPROXIMATE

K-FACTOR

ERROR (%)

±10

±10

±12.5

±12.5

Output Capacitor Selection

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.

Also, the capacitance value must be high enough to

absorb the inductor energy going from a full-load to no-

load condition without tripping the OVP circuit.

For CPU core voltage converters and other applica-

tions where the output is subject to violent load tran-

sients, the output capacitor’s size depends on how

much ESR is needed to prevent the output from dip-

ping too low under a load transient. Ignoring the sag

due to finite capacitance:

RESR

≤

I

VDIP

LOAD(MAX)

In non-CPU applications, the output capacitor’s size

depends on how much ESR is needed to maintain an

acceptable level of output voltage ripple:

RESR ≤ LIR

×

VP−P

I LOAD(MAX)

The actual microfarad 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 volt-

age rating rather than by capacitance value (this is true

of tantalums, OS-CONs®, and other electrolytics).

When using low-capacity filter capacitors such as

ceramic or polymer types, capacitor size is usually

determined by the capacity needed to prevent VSAG

and VSOAR from causing problems during load tran-

sients. Also, the capacitance must be great enough to

prevent the inductor’s stored energy from launching the

output above the overvoltage protection threshold.

Generally, once enough capacitance is added to meet

the overshoot requirement, undershoot at the rising

load edge is no longer a problem (see the VSAG and

VSOAR equations in the Transient Response section).

Output Capacitor Stability

Considerations

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

ative to the switching frequency. The point of instability

is given by the following equation:

where:

f ESR

≤

fSW

π

f ESR

=

2×

1

π × RESR ×

CF

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 15kHz. In the design example used for

inductor selection, the ESR needed to support 20mVP-P

ripple is 20mV/2A = 10mΩ. Three 470µF/6V Kemet

T510 low-ESR tantalum capacitors in parallel provide

10mΩ (max) ESR. Their typical combined ESR results in

a zero at 11.3kHz, well within the bounds of stability.

Do not put high-value ceramic capacitors directly

across the outputs without taking precautions to ensure

stability. Large ceramic capacitors can have a high-

ESR zero frequency and cause erratic, unstable opera-

tion. However, it is easy to add enough series

resistance by placing the capacitors a couple of inches

downstream from the inductor and connecting OUT_ or

the FB_ divider close to the inductor.

Unstable operation manifests itself in two related but

distinctly different ways: double-pulsing and feedback-

loop instability.

OS-CON is a registered trademark of Sanyo Electric Co., Ltd.

______________________________________________________________________________________ 19

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