MAX1644 データシートの表示（PDF） - Maxim Integrated

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MAX1644

2A, Low-Voltage, Step-Down Regulator with Synchronous Rectification and Internal Switches

Maxim Integrated 

2A, Low-Voltage, Step-Down Regulator with

Synchronous Rectification and Internal Switches

fPWM = switching frequency in PWM mode

(IOUT > 0.2A)

Select RTOFF according to the formula:

RTOFF = (tOFF - 0.07µs) (150kΩ / 1.26µs)

Recommended values for RTOFF range from 39kΩ to

470kΩ for off-times of 0.4µs to 4µs.

Inductor Selection

Three key inductor parameters must be specified:

inductor value (L), peak current (IPEAK), and DC resis-

tance (RDC). The following equation includes a con-

stant, denoted as LIR, which is the ratio of peak-

to-peak inductor AC current (ripple current) to maxi-

mum DC load current. A higher value of LIR allows

smaller inductance but results in higher losses and rip-

ple. A good compromise between size and losses is

found at approximately a 25% ripple-current to load-

current ratio (LIR = 0.25), which corresponds to a peak

inductor current 1.125 times higher than the DC load

current:

L = VOUT × tOFF

IOUT × LIR

where: IOUT = maximum DC load current

LIR = ratio of peak-to-peak AC inductor current

to DC load current, typically 0.25

The peak inductor current at full load is 1.125 · IOUT if

the above equation is used; otherwise, the peak current

is calculated by:

IPEAK

=

IOUT +

VOUT ×

2×

tOFF

L

Choose an inductor with a saturation current at least as

high as the peak inductor current. To minimize loss,

choose an inductor with a low DC resistance.

Capacitor Selection

The input filter capacitor reduces peak currents and

noise at the voltage source. Use a low-ESR and low-

ESL capacitor located no further than 5mm from IN.

Select the input capacitor according to the RMS input

ripple-current requirements and voltage rating:

IRIPPLE = ILOAD

( ) VOUT VIN − VOUT

VIN

The output filter capacitor affects the output voltage rip-

ple, output load-transient response, and feedback loop

stability. For stable operation, the MAX1644 requires a

minimum output ripple voltage of VRIPPLE ≥ 2% · VOUT

(with 2% load regulation setting).

The minimum ESR of the output capacitor should be:

ESR > 1% × L

tOFF

Stable operation requires the correct output filter

capacitor. When choosing the output capacitor, ensure

that:

COUT ≥ (tOFF / VOUT) ✕ (64µFV / µs)

With an AC load regulation setting of 1%, the COUT

requirement doubles, and the minimum ESR of the out-

put capacitor is halved.

Integrator Amplifier

An internal transconductance amplifier fine tunes the

output DC accuracy. A capacitor, CCOMP, from COMP

to VCC compensates the transconductance amplifier.

For stability, choose:

CCOMP ≥ 470pF

A large capacitor value maintains a constant average

output voltage but slows the loop response to changes

in output voltage. A small capacitor value speeds up

the loop response to changes in output voltage but

decreases stability. Choose the capacitor values that

result in optimal performance.

Setting the AC Loop Gain

The MAX1644 allows selection of a 1% or 2% AC load-

regulation error when the adjustable output voltage

mode is selected (Table 2). A 2% setting is automati-

cally selected in preset output voltage mode (FBSEL

connected to VCC or unconnected). A 2% load-regula-

tion error setting reduces output filter capacitor require-

ments, allowing the use of smaller and less expensive

capacitors. Selecting a 1% load-regulation error

reduces transient load errors, but requires larger capac-

itors.

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