MIC2168A

Micrel, Inc.

M9999-062205

8

April 2005

that the power MOSFETs used are low threshold and are in

It is important to note the on-resistance of a MOSFET in-

creases with increasing temperature. A 75°C rise in junction

temperature will increase the channel resistance of the MOS-

FET by 50% to 75% of the resistance specified at 25°C. This

change in resistance must be accounted for when calculating

MOSFET power dissipation and in calculating the value of

current-sense (CS) resistor. Total gate charge is the charge

required to turn the MOSFET on and off under specified

plied by the MIC2168A gate drive circuit. At 1MHz switching

frequency and above, the gate charge can be a significant

source of power dissipation in the MIC2168A. At low output

load, this power dissipation is noticeable as a reduction in

efficiency. The average current required to drive the high-

side MOSFET is:

I

Q

f

G[high

I

Q

I

Q

-

I

Q

I

Q

side](avg)

I

Q

I

=

×

I

Q

I

G

S

where:

current.

G[high-side](avg)

G[high-side](avg)

because the freewheeling diode is conducting during this

time. The switching loss for the low-side MOSFET is usu-

ally negligible. Also, the gate-drive current for the low-side

MOSFET is more accurately calculated using CISS at

For the low-side MOSFET:

I

C

V

f

G[low

I

C

I

C

-side](avg)

I

C

I

GS

V

f

V

V

f

V

f

=

×

I

C

I

ISS

V

f

V

f

Since the current from the gate drive comes from the input

voltage, the power dissipated in the MIC2168A due to gate

drive is:

P

V

GATEDRIVE

P

V

P

=

+

P

V

P

IN

(

)

I

I

I

I

G[high

G[high --side](avg)

side](avg)

G[low

G[low --side](avg)

side](avg)

=

+

=

+

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

A convenient figure of merit for switching MOSFETs is the on

numbers translate into higher efficiency. Low gate-charge

DS(ON)

DS(ON)

logic-level MOSFETs are a good choice for use with the

MIC2168A.

Parameters that are important to MOSFET switch selection

are:

• Voltage rating

• On-resistance

• Total gate charge

The voltage ratings for the top and bottom MOSFET are

essentially equal to the input voltage. A safety factor of 20%

for voltage spikes due to circuit parasitics.

The power dissipated in the switching transistor is the sum

and the switching losses that occur during the period of time

when the MOSFETs turn on and off (P

when the MOSFETs turn on and off (P ).

P

P

P

SW

P

P

P

AC

=

+

P

P

P

CONDUCTION

where:

P

I

R

CONDUCTION

P

I

P

I

SW(rms)

SW

2

=

×

P

I

P

I

SW(rms)

P

P

P

AC

P

P

P

AC(on)

=

+

P

P

P

AC(off)

D =

V

V

O

IN









duty cycle

Makingtheassumptiontheturn-onandturn-offtransitiontimes

are equal; the transition times can be approximated by:

t

C

V

C

V

I

T

ISS

C

V

C

OSS

C

V

C

G

=

×

+

C

V

C

GS

C

V

C

V

where:

The total high-side MOSFET switching loss is:

P

(V

V ) I

t

f

AC

P

(

P

D

P

) I

T

t

f

t

t

f

t

f

=

+

P

(

P

(V

V

V

V

V

V

V

V

V

V

V

V

×

×

) I

) I

D

P

D

P

) I

) I

) I

T

K

T

t

f

t

f

where:

The low-side MOSFET switching losses are negligible and

can be ignored for these calculations.

Inductor Selection

Values for inductance, peak, and RMS currents are required

to select the output inductor. The input and output voltages

and the inductance value determine the peak-to-peak induc-

tor ripple current. Generally, higher inductance values are

used with higher input voltages. Larger peak-to-peak ripple

currents will increase the power dissipation in the inductor

and MOSFETs. Larger output ripple currents will also require

more output capacitance to smooth out the larger ripple cur-

rent. Smaller peak-to-peak ripple currents require a larger

inductance value and therefore a larger and more expensive

inductor. A good compromise between size, loss and cost is

to set the inductor ripple current to be equal to 20% of the

maximum output current. The inductance value is calculated

by the equation below.

L

V

(V m

V

)

V m

f

0.2 I

OUT

V

(

V

(

IN

V m

V m

OUT

V

)

V

)

IN

V m

V m

S

ff

OUT

=

×

−

V

(

V

(V m

V m

V m

V m

V m

V m

× ×

ff

SS

ffff

×

(

)

V m

V max

ax

(

)

V m

V max

ax

×

−

×

−

V m

V m

V m

V max

ax

ax

ax

(

)

V m

V max

ax

(

)

max

max

where:

0.2 = ratio of AC ripple current to DC output current

The peak-to-peak inductor current (AC ripple current) is: