ML4894

7

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.

In high current applications, the MOSFET’s power

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.

The worst case power dissipation for the P-MOS switch

occurs at the minimum input voltage and is determined

as follows:

PI

R

PMOS

RMS ON

DS ON

−

=×

()

()

2

(11)

where:

I

V

V

II

I

I

RM S ON

IN MAX

SENSE( MAX

SENSE( MAX

L PEAK MAX

L PEAK MAX

()

()

)

)

(())

(())

2

22

5

3

=×

+×

+

F

HG

I

KJ

The worst case power dissipation for the N-MOS switch

occurs at the maximum input voltage and is determined

using:

PI

R

N MOS

RMS OFF

DS ON

−

=×

()

(

)

2

(12)

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:

II

I

L PEAK MIN

SENSE MIN

L MIN

((

))

(

)

(

)

=+ ∆

(8)

I

V

R

I

LPEAK MIN

SENSE MIN

SENSE

LMIN

af

ch

af

af

=+ ∆

I

V

R

I

LPEAK MIN

SENSE

LMIN

af

ch

af

=+

006

.

∆

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:

PI

R

R

RMS OFF

SENSE

SENSE

=×

()

2

(9)

where:

I

V

V

II

I

I

RMS OFF

OUT

IN MAX

SENSE(MAX

SENSE(MAX

L PEAK MAX

L PEAK MAX

()

()

)

)

((

))

(())

2

22

1

3

=−

×

+×

+

F

HG

I

KJ

The final parameter that should be specified is the

winding resistance of the inductor. In general, the

winding resistance should be as low as possible,

preferably in the low m

Ω range. Since the inductor is in

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

approximated by:

PI

R

Cu

OUT

L

=×

2

(10)

A good rule of thumb is to allow 2 m

Ω of winding

resistance per

µH of inductance.

MOSFET SELECTION

The switching MOSFETs must be logic level types with

t

L

L

Figure 5. Buck Regulator Inductor Current