MIC2168

Micrel

M9999-111803

8

November 2003

low-side switches. For applications where V

internal V

necessary that the power MOSFETs used are low threshold

and are in full conduction mode for V

tions when V

is specified at V

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

increases with increasing temperature. A 75

°C rise in junc-

tion temperature will increase the channel resistance of the

MOSFET 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 operating conditions (V

charge is supplied by the MIC2168 gate drive circuit. At 1MHz

switching frequency and above, the gate charge can be a

significant source of power dissipation in the MIC2168. At low

output load, this power dissipation is noticeable as a reduc-

tion in efficiency. The average current required to drive the

high-side MOSFET is:

IQ

f

G[high-side](avg)

G

S

=×

where:

I

current.

Q

manufacturer’s data sheet for V

The low-side MOSFET is turned on and off at V

because the freewheeling diode is conducting during this

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

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

MOSFET is more accurately calculated using CISS at

V

For the low-side MOSFET:

IC

V

f

G[low-side](avg)

ISS

GS

S

=×

×

Since the current from the gate drive comes from the input

voltage, the power dissipated in the MIC2168 due to gate

drive is:

PV

I

I

GATEDRIVE

IN G[high-side](avg)

G[low-side](avg)

=+

()

A convenient figure of merit for switching MOSFETs is the on

resistance times the total gate charge R

numbers translate into higher efficiency. Low gate-charge

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

MIC2168.

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%

should be added to the V

for voltage spikes due to circuit parasitics.

The power dissipated in the switching transistor is the sum of

the conduction losses during the on-time (P

the switching losses that occur during the period of time when

the MOSFETs turn on and off (P

PP

P

SW

CONDUCTION

AC

=+

where:

PI

R

CONDUCTION

SW(rms)

SW

2

=×

PP

P

AC

AC(off)

AC(on)

=+

R

D

duty cycle

V

V

O

IN

=









Making the assumption the turn-on and turn-off transition

times are equal; the transition times can be approximated by:

t

CV

C

V

I

T

ISS

GS

OSS

IN

G

=

×+

×

where:

C

I

The total high-side MOSFET switching loss is:

P(V

V ) I

t

f

AC

IN

D

PK

T

S

=+

×

×

×

where:

t

V

f

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

current. 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 max

V

)

V max

f

0.2 I

max

OUT

IN

OUT

IN

S

OUT

=

×−

××

×

()

()

()

where:

f

0.2 = ratio of AC ripple current to DC output current

V