MAX15046 Datasheet, PDF(14/23 Page) Maxim Integrated Products – 40V, High-Performance, Synchronous Buck Controller

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40V, High-Performance, Synchronous

Buck Controller

Setting the Switching Frequency

An external resistor connecting RT to GND sets the

switching frequency (fSW). The relationship between fSW

and RRT is:

RRT

fSW

17.3 Ã10 9

(1x10 -7) x (fSW 2)

where fSW is in kHz and RRT is in kI. For example, a

300kHz switching frequency is set with RRT = 49.9kI.

Higher frequencies allow designs with lower inductor

values and less output capacitance. Peak currents and

I2R 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 (ISAT), and DC resistance

(RDC). 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 I2R 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 I2R

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:

L = VOUT (VIN - VOUT )

VIN Ã fSW Ã IOUT Ã LIR

where VIN, VOUT, and IOUT are typical values. The

switching frequency is set by RT (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 (ISAT) must be high enough to

ensure that saturation cannot occur below the maximum

current-limit value (ICL(MAX)), given the tolerance of the

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

reference current (ILIM). Combining these conditions,

select an inductor with a saturation current (ISAT) of:

ISAT â¥ 1.35 ÃICL(TYP)

where ICL(TYP) is the typical current-limit set point. The

factor 1.35 includes RDS(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

RDS(ON) of the low-side MOSFET is used as the current-

sense element. The inductorâs valley current occurs at

ILOAD(MAX) minus one half of the ripple current. The

minimum value of the current-limit threshold voltage

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

MOSFET during the ripple-current valley,

VITH

RDS(ON,MAX)

Ã ILOAD(MAX)

ï£«ï£¬ï£1

LIR ï£¶

2 ï£·ï£¸

where RDS(ON,MAX) in I is the maximum on-resistance

of the low-side MOSFET at maximum load current

ILOAD(MAX) and is calculated from the following equation:

RDS(ON,MAX) = RDS(ON) Ã [1+ TCMOSFET Ã (TMAX - TAMB)]

where RDS(ON) (in I is the on-resistance of the low-

side MOSFET at ambient temperature TAMB (in degrees

Celsius), TCMOSFET is the temperature coefficient of

the low-side MOSFET in ppm/NC, and TMAX (in degrees

Celsius) is the temperature at maximum load current

ILOAD(MAX). Obtain the RDS(ON) and TCMOSFET from the

MOSFET data sheet.

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