MAX724 데이터 시트보기 (PDF) - Maxim Integrated

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MAX724

5A/2A Step-Down, PWM, Switch-Mode DC-DC Regulators

Maxim Integrated 

5A/2A Step-Down, PWM,

Switch-Mode DC-DC Regulators

AV(DC) = gM(400kΩ) ≈ 2000

fPOLE = 1/[2π(400kΩ)]CC

90° PHASE SHIFT

-AV(MID) = gM / (2π f CC)

fZERO = 1 / (2π RC CC)

Compensation Network

A series RC network connected from VC to ground

compensates the MAX724/MAX726. Compensation

RC values are shown in the applications circuits. RC

and CC shape error-amplifier gain as follows: At DC,

RC and CC have no effect, so the error-amplifier's

gain is the product of its transconductance (approxi-

mately 5000µmhos) and an internal 400kΩ load

impedance (rINT) at VC. So at DC, AV(DC) = gM(rINT) =

approximately 2000µmhos. RC and CC then add a

low-frequency pole and a high-frequency zero, as

shown in Figure 5.

AV(HI) = gMRC

FREQUENCY

Figure 5. Error-Amplifier Gain as Set by RC and CC at VC Pin

FEEDBACK RESISTOR

MAIN FILTER CAP

LF

CF

TO LOAD

Output Overshoot

The MAX724/MAX726 error-amplifier design minimizes

overshoot, but precautions against overshoot should

still be exercised in sensitive applications. Worst-case

overshoot typically occurs when recovering from an

output short because VC slews down from its highest

voltage. This can be checked by simply shorting and

releasing the output.

Reduce objectional overshoot by increasing the com-

pensation resistor (to 3kΩ or 4kΩ) at VC. This allows

the error-amplifier output, VC, to move more rapidly in

the negative direction. In some cases, loop stability

may suffer with a high-value compensation resistor. An

option, then, is to add output filter capacitance, which

reduces short-circuit recovery overshoot by limiting out-

put rise time. Lowering the compensation capacitor to

below 0.05µF may also help by allowing VC to slew fur-

ther before the output rises too far.

Figure 6. Optional LC Output Filter

0.2µs would be needed to provide a narrow enough

duty cycle that could control current when the output is

shorted. Since 0.6µs is too long (at 100kHz), the fOSC

is lowered to 20kHz once FB (and hence the output)

drops below about 1.3V (see Frequency vs. VFB Voltage

graph in the Typical Operating Characteristics). This

way, the MAX724/MAX726's 0.6µs minimum tON allows

a sufficiently small duty cycle (at the reduced fOSC) so

that current can still be limited.

Optional Output Filters

Though not shown in the application circuits in Figures

2, 7, and 8, additional filtering can easily be added to

reduce output ripple to levels below 2%. It is more

effective to add an LC type filter rather than additional

output capacitance alone. A small-value inductor (2µH

to 10µH) and between 47µF and 220µF of filter capaci-

tance should suffice (Figure 6). Although the inductor

does not need to be of high quality (it is not switching),

it must still be rated for the full load current.

When an LC filter is added, do not move the connection

of the feedback resistor to the LC output. It should be left

connected to the main output filter capacitor (C1 in Figure

2). If the feedback connection is moved to the LC filter

point, the added phase shift may impact stability.

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