Low-Cost Voltage-Mode PWM

Step-Down Controllers

_______________________________________________________________________________________

9

and keep a 0.1µF capacitor between VL and GND

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

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:

synchronous rectifier. The MAX1966/MAX1967s’ mini-

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

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

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-

determines the instantaneous component stresses

and filtering requirements and is key in determining

determines the required inductor saturation rating

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

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

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

Select resistor R2 in the 1k

Ω to 10kΩ range. R1 is then

given by:

Inductor Selection

Determine an appropriate inductor value with the fol-

lowing equation:

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

LV

VV

V

f

LIR

I

OUT

IN

OUT

VIN

OSC

LOAD MAX

=×

−

()

××

×

()

RR

V

V

OUT

FB

12

1

=−









VV

R

I

VIN DFMAX

OUT

DS ON

LOAD

()

(

)

≤+

×

()

10

VR

I

V

OUT

DS ON

LOAD

VIN

+×

()

()

/