ML4894 データシートの表示（PDF） - Micro Linear Corporation

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ML4894

Synchronous Buck Controller

Micro Linear Corporation 

IL

VIN – VOUT

L

VOUT

L

ILPK

∆IL

IOUT = ISENSE + 1/2 ∆IL

ISENSE =

VSENSE

RSENSE

TON

TOFF

t

Figure 5. Buck Regulator Inductor Current

ML4894

For reliable operation, the inductor current rating should

exceed the value calculated by 10%-20%.

For future reference, determine the peak inductor current

at the minimum input voltage:

IL(PEAK(MIN)) = ISENSE(MIN) + ∆IL(MIN)

(8)

c a fh a f a f IL PEAK MIN

= VSENSE MIN

R SENSE

+ ∆IL MIN

c a fh a f IL PEAK MIN

=

0.06V

R SENSE

+ ∆IL MIN

ON resistance - gate charge product provides a good

figure of merit by which to compare various MOSFETs,

the lower the figure the better. The internal gate drivers of

the ML4894 can drive over 100nC of total gate charge,

but 60nC to 70nC is a more practical limit to ensure good

switching times.

The drain-source breakdown voltage rating is determined

by the input voltage. For input voltages up to 10V, a drain

to source rating of 20V is acceptable. For input voltages

up to 15V, a drain to source rating of 30V is

recommended. For a more reliable design, look for

MOSFETs that are avalanche rated.

Now the sense resistor’s power rating can be determined.

The sense resistor must be able to carry the peak current

in the inductor during the OFF-time:

PRSENSE = IRMS(OFF) 2 × R SENSE

(9)

where:

In high current applications, the MOSFET’s power

dissipation often becomes a major design factor. The I2R

losses generate the largest portion of heat in the MOSFET

package. Make sure that the MOSFETs are within their

rated junction temperature at the maximum ambient

temperature by calculating the temperature rise using the

thermal resistance specifications.

FG IJ I 2

H K RMS(OFF)

=

V

1− OUT

VIN( MAX)

×

I2

SENSE(MAX )

+ ISENSE(MAX)

× IL(PEAK(MAX))

+

I2

L( PEAK( MAX))

3

The worst case power dissipation for the P-MOS switch

occurs at the minimum input voltage and is determined

as follows:

The final parameter that should be specified is the

winding resistance of the inductor. In general, the

PP−MOS = IRMS(ON)2 × RDS(ON)

(11)

winding resistance should be as low as possible,

preferably in the low mΩ range. Since the inductor is in

where:

series with the load at all times, the copper losses can be

approximated by:

FHG IKJ IRMS(ON) 2 =

5V

VIN( MAX )

×

I2

SENSE( MAX)

+ ISENSE( MAX )

× IL(PEAK( MAX ))

+

I2

L( PEAK( MAX ))

3

PCu = IOUT 2 × RL

(10)

The worst case power dissipation for the N-MOS switch

occurs at the maximum input voltage and is determined

A good rule of thumb is to allow 2 mΩ of winding

using:

resistance per µH of inductance.

PN−MOS = IRMS(OFF)2 × RDS(ON)

(12)

MOSFET SELECTION

The switching MOSFETs must be logic level types with

the ON resistance specified at VGS = 4.5V. In general, the

7

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