Rainbow-electronics MAX15046B User Manual download pdf (Page 14)

40V, High-Performance, Synchronous

Buck Controller

MAX15046

14 _____________________________________________________________________________________

Setting the Switching Frequency

An external resistor connecting RT to GND sets the

switching frequency (f

). The relationship between f

and R

is:

-7 2

SW SW

17.3 10

f (1x10 ) x (f )

where f

is in kHz and R

is in kI. For example, a

300kHz switching frequency is set with R

= 49.9kI.

Higher frequencies allow designs with lower inductor

values and less output capacitance. Peak currents and

R losses are lower at higher switching frequencies, but

core losses, gate-charge currents, and switching losses

increase.

Inductor Selection

Three key inductor parameters must be specified for

operation with the MAX15046: inductance value (L),

inductor saturation current (I

SAT

), and DC resistance

). To determine the inductance, select the ratio of

inductor peak-to-peak AC current to DC average cur-

rent (LIR) first. For LIR values that are too high, the RMS

currents are high, and therefore I

R losses are high.

Use high-valued inductors to achieve low LIR values.

Typically, inductor resistance is proportional to induc-

tance for a given package type, which again makes I

losses high for very low LIR values. A good compromise

between size and loss is a 30% peak-to-peak ripple cur-

rent to average-current ratio (LIR = 0.3). The switching

frequency, input voltage, output voltage, and selected

LIR determine the inductor value as follows:

OUT IN OUT

IN SW OUT

V (V - V )

V f I LIR

× × ×

where V

, V

OUT

, and I

OUT

are typical values. The

switching frequency is set by R

(see Setting the

Switching Frequency section). The exact inductor value

is not critical and can be adjusted to make trade-offs

among size, cost, and efficiency. Lower inductor val-

ues minimize size and cost, but also improve transient

response and reduce efficiency due to higher peak cur-

rents. On the other hand, higher inductance increases

efficiency by reducing the RMS current.

Find a low-loss inductor with the lowest possible DC

resistance that fits in the allotted dimensions. The

saturation current rating (I

SAT

) must be high enough to

ensure that saturation cannot occur below the maximum

current-limit value (I

CL(MAX)

), given the tolerance of the

on-resistance of the low-side MOSFET and of the LIM

reference current (I

LIM

). Combining these conditions,

select an inductor with a saturation current (I

SAT

) of:

≥ ×

SAT CL(TYP)

I 1.35 I

where I

CL(TYP)

is the typical current-limit set point. The

factor 1.35 includes R

DS(ON)

variation of 25% and 10%

for the LIM reference current error. A variety of inductors

from different manufacturers are available to meet this

requirement (for example, Vishay IHLP-4040DZ-1-5 and

other inductors from the same series).

Setting the Valley Current Limit

The minimum current-limit threshold must be high enough

to support the maximum expected load current with the

worst-case low-side MOSFET on-resistance value as the

DS(ON)

of the low-side MOSFET is used as the current-

sense element. The inductor’s valley current occurs at

LOAD(MAX)

minus one half of the ripple current. The

minimum value of the current-limit threshold voltage

ITH

) must be higher than the voltage on the low-side

MOSFET during the ripple-current valley,

ITH DS(ON,MAX) LOAD(MAX)

LIR

V R I 1

−

 

> × ×

 

 

where R

DS(ON,MAX)

in I is the maximum on-resistance

of the low-side MOSFET at maximum load current

LOAD(MAX)

and is calculated from the following equation:

DS(ON,MAX) DS(ON) MOSFET MAX AMB

R R [1 TC (T - T )]= × + ×

where R

DS(ON)

(in I is the on-resistance of the low-

side MOSFET at ambient temperature T

AMB

(in degrees

Celsius), TC

MOSFET

is the temperature coefficient of

the low-side MOSFET in ppm/NC, and T

MAX

(in degrees

Celsius) is the temperature at maximum load current

LOAD(MAX)

. Obtain the R

DS(ON)

and TC

MOSFET

from the

MOSFET data sheet.

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Rainbow-electronics MAX15046B User Manual Page 14

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