November 2004

11

MIC2199

MIC2199

Micrel

reduction in efficiency. The average current required to drive

the high-side MOSFET is:

IQ

f

G[high-side](avg)

G

S

=×

where:

I

average high-side MOSFET gate current

Q

taken from manufacturer’s data sheet

with V

f

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

because the freewheeling diode is conducting during this

time. The switching losses for the low-side MOSFET is

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

MOSFET is more accurately calculated using C

0 instead of gate charge.

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 MIC2199 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

MIC2199. Power dissipation in the MIC2199 package limits

the maximum gate drive current.

Parameters that are important to MOSFET switch selection

are:

• Voltage rating

• On-resistance

• Total gate charge

The voltage rating of the MOSFETs are essentially equal to

the input voltage. A safety factor of 20% should be added to

the V

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

Making the assumption the turn-on and turnoff transition

times are equal, the transition time 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.

RMS Current and MOSFET Power Dissipation

Calculation

Under normal operation, the high-side MOSFETs RMS cur-

rent is greatest when V

low-side MOSFETs RMS current is greatest when V

(minimum duty cycle). However, the maximum stress the

MOSFETs see occurs during short circuit conditions, where

the output current is equal to I

normal operation. To calculate the stress under short circuit

conditions, substitute I

the formula below to calculate D under short circuit condi-

tions.

D

0.063 1.8 10

V

SHORTCIRCUIT

3

IN

=

−

××

−

The RMS value of the high-side switch current is:

ID

I

I

12

SW(high side)(rms)

OUT(max)

2

PP

2

−

=×

+

⎛

⎝

⎜

⎞

⎠

⎟

I1 D I

I

12

SW(low side)(rms)

OUT(max)

2

PP

2

−

=

−

()

+

⎛

⎝

⎜

⎞

⎠

⎟

where:

D = duty cycle of the converter

D

V

V

OUT

IN

=

×

η

η = efficiency of the converter.

Converter efficiency depends on component parameters,

which have not yet been selected. For design purposes, an

efficiency of 90% can be used for V

can be used for V

more accurately calculated once the design is complete. If the

assumed efficiency is grossly inaccurate, a second iteration

through the design procedure can be made.