MAX746CSLASHD datasheet(12/16 Pages) MAXIM

12 / 16 page

High-Efficiency, PWM, Step-Down,

N-Channel DC-DC Controller

______________________________________________________________________________________

Next, determine the value of RSENSE such that:

VLIMIT(min)

125mV

RSENSE = _____________ = ________

IPK

For example, to obtain 5V at 3A, IPK = 3.3A and

RSENSE = 125mV/3.3A = 38m

Ω.

The sense resistor should have a power rating greater

than (IPK2) (RSENSE) with an adequate safety margin.

With a 3A load current, IPK = 3.3A and RSENSE = 38m

Ω.

The power dissipated by the resistor (assuming an 80%

duty cycle) is 331mW. Metal-film resistors are recom-

mended. Do not use wire-wound resistors because

their inductance will adversely affect circuit operation.

The duty cycle (for continuous conduction) is determined

from the following equation:

VOUT + VDIODE

Duty Cycle (%) = _____________________ x 100%

V+ - VSW + VDIODE

where VSW is the voltage drop across the external

N-FET and sense resistor. VSW can be approximated

as [ILOAD x (rDS(ON) + RSENSE)].

Inductor Selection

Once the sense-resistor value is determined, calculate

the inductor value (L) using the following equation. The

correct inductor value ensures proper slope compen-

sation. Continuing from the equations above:

(RSENSE)(VOUT)

L = ______________________

(VRAMP(max))(fOSC)

(38mΩ)(5V)

= _____________________ = 38µH

(50mV)(100kHz)

where VRAMP(max) is the 50mV peak value of the slope-

compensation linear ramp signal.

Although 38µH is the calculated value, the component

used may have a tolerance of ±30% or more.

Inductors with molypermalloy powder (MPP), Kool Mµ,

or ferrite are recommended. Inexpensive iron-powder

core inductors are not suitable, due to their increased

core losses, especially at switching frequencies in the

100kHz range. MPP and Kool Mµ cores have low per-

meability, allowing larger currents.

For highest efficiency, use a coil with low DC resis-

tance. To minimize radiated noise, use a toroid, a pot

core, or a shielded coil.

It is customary to select an inductor with a saturation

rating that exceeds the peak current set by RSENSE,

but inductors are often specified very conservatively.

If the inductor’s core losses do not cause excessive

temperature rise (inductor wire insulation is usually

rated for +125°C) and the associated efficiency loss-

es are minimal, inductors with lower current ratings

are acceptable.

In the 3.3V Standard Application Circuit (Figure 1b), the

inductor selected has a 2.2A current rating even

though the peak current is 3.3A. This inductor was

selected for two reasons: it is the highest-rated readily

available surface-mount inductor of its size, and lab

tests have verified that the core-loss increase is mini-

mal. With a 3A load current, the inductor current does

not begin showing significant losses due to saturation

until the supply voltage increases to 10V (the maximum

supply for this circuit is 6V).

GND

V+

VIN

VOUT

R4 = 10k

Ω TO 60kΩ

OUT

(

)

R5 = R4

-1

VREF = 2.0V NOMINAL

* SEE COMPENSATION CAPACITOR SECTION.

VOUT

VREF

MAX746

EXT

OUT

VOUT

26.1k (1%)

R4b

22.6k (1%)

R4a

17.4k (1%)

5V/3.3V

VOUT = VREF

(

)

R4a

R4a + R4b

(

)

MAX746

SELECT WITH FET OFF:

VOUT = VREF

VREF = 2.0V NOMINAL

Figure 6. Adjustable Output Circuit

Figure 7. 3.3V/5V Ajustable Output Circuit

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