MAX1966 データシートの表示（PDF） - Maxim Integrated

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MAX1966

Low-Cost Voltage-Mode PWM Step-Down Controllers

Maxim Integrated 

Low-Cost Voltage-Mode PWM

Step-Down Controllers

and keep a 0.1µF capacitor between VL and GND

close to the chip. The VIN-to-VL dropout voltage is typi-

cally 70mV at 25mA current, so when VVIN is less than

5V, VVL is typically VVIN - 70mV.

The internal linear regulator can source a minimum of

25mA to supply the IC and power the low-side and

high-side FET drivers.

Duty-Factor Limitations for Low

VOUT/VVIN Ratios

The MAX1966/MAX1967s’ output voltage is adjustable

down to 0.8V. However, the minimum duty factor may

limit the ability to supply low-voltage outputs from high-

voltage inputs. With high-input voltages, the required

duty factor is approximately:

( ) VOUT + RDS(ON) × ILOAD / VVIN

where RDS(ON) x ILOAD is the voltage drop across the

synchronous rectifier. The MAX1966/MAX1967s’ mini-

mum duty factor is 10%, so the maximum input voltage

(VVIN(DFMAX)) that can supply a given output voltage is:

( ) VVIN(DFMAX) ≤ 10 VOUT + RDS(ON) × ILOAD

If the circuit cannot attain the required duty factor dic-

tated by the input and output voltages, the output volt-

age still remains in regulation. However, there may be

intermittent or continuous half-frequency operation as

the controller attempts to lower the average duty factor

by deleting pulses. This can increase output voltage

ripple and inductor current ripple, which increases

noise and reduces efficiency. Furthermore, circuit sta-

bility is not guaranteed.

Applications Information

Design Procedure

Component selection is primarily dictated by the follow-

ing criteria:

1) Input Voltage Range: The maximum value

(VVIN(MAX)) must accommodate the worst-case

high-input voltage. The minimum value (VVIN(MIN))

must account for the lowest input voltage after

drops due to connectors, fuses, and switches are

considered. In general, lower input voltages pro-

vide the best efficiency.

2) Maximum Load Current: There are two current val-

ues to consider. Peak load current (ILOAD(MAX))

determines the instantaneous component stresses

and filtering requirements and is key in determining

output capacitor requirements. ILOAD(MAX) also

determines the required inductor saturation rating

and the design of the current-limit circuit. Con-

tinuous load current (ILOAD) determines the thermal

stresses, input capacitor, and MOSFETs, as well as

the RMS ratings of other heat-contributing compo-

nents such as the inductor.

3) Inductor Value: This choice provides tradeoffs

between size, transient response, and efficiency.

Higher inductance value results in lower inductor

ripple current, lower peak current, lower switching

losses, and, therefore, higher efficiency at the cost

of slower transient response and larger size. Lower

inductance values result in large ripple currents,

smaller size, and poorer efficiency, while also pro-

viding faster transient response. Except for low-cur-

rent applications, most circuits exhibit a good

balance between efficiency and economics with a

minimum inductor value that causes the circuit to

operate at the edge of continuous conduction

(where the inductor current just touches zero with

every cycle at maximum load). Inductor values lower

than this grant no further size-reduction benefit.

Table 1 shows representative values for some typical

applications up to 5A. With proper component selec-

tion, outputs of 20A or more are practical with the

MAX1966/MAX1967. The components listed in Table 1

were selected assuming a minimum cost design goal.

The MAX1966/MAX1967 can effectively operate with a

wide range of components.

Setting the Output Voltage

An output voltage between 0.8V and (0.9V x VVIN) can

be configured by connecting FB pin to a resistive

divider between the output and GND (Figures 3 and 4).

Select resistor R2 in the 1kΩ to 10kΩ range. R1 is then

given by:



R1 = R2

VOUT

VFB



− 1

where VFB = 0.8V.

Inductor Selection

Determine an appropriate inductor value with the fol-

lowing equation:

( ) L = VOUT × VVIN

×

VIN − VOUT

fOSC × LIR ×

ILOAD(MAX)

where LIR is the ratio of inductor ripple current to aver-

age continuous current at a minimum duty cycle.

Choosing LIR between 20% to 50% results in a good

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