MIC2198BML datasheet(10/14 Pages) MICREL

10 / 14 page

MIC2198

Micrel, Inc.

MIC2198

October 2005

where:

IG[high-side](avg) =

average high-side MOSFET gate current.

QG = total gate charge for the high-side MOSFET

taken from manufacturer’s data sheet

with VGS = 5V.

fs = 500kHz

The low-side MOSFETis turned on and off at VDS=0because

the freewheeling diode is conducting during this time. The

switching losses for the low-side MOSFET is usually negli-

gible. Also, the gate drive current for the low-side MOSFET

is more accurately calculated using CISS at VDS = 0 instead

of gate charge.

For the low-side MOSFET:

G[low-side](avg)

ISS

Since the current from the gate drive comes from the input

voltage, the power dissipated in the MIC2198 due to gate

drive is:

V 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 (RDS(on) × QG).

Lower numbers translate into higher efﬁciency. Low gate-

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

the MIC2198. Power dissipation in the MIC2198 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 VDS(max) of the MOSFETs to account 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 (PCONDUCTION)

and the switching losses that occur during the period of time

when the MOSFETs turn on and off (PAC).

CONDUCTION

where:

CONDUCTION

SW(rms)

AC(off)

AC(on)

RSW = on-resistance of the MOSFET switch.

Makingtheassumptiontheturn-onandturnofftransitiontimes

are equal, the transition time can be approximated by:

ISS

OSS

where:

CISS and COSS are measured at VDS = 0.

IG = gate drive current (1A for the MIC2198)

The total high-side MOSFET switching loss is:

PAC = (VIN + VD) × IPK × tT ×fS

where:

tT = switching transition time (typically 20ns to 50ns).

VD = freewheeling diode drop, typically 0.5V.

fS it the switching frequency, nominally 500kHz

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 VIN is low (maximum duty cycle). The

low-side MOSFETs RMS current is greatest when VIN is high

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

MOSFETs see occurs during short circuit conditions, where

the output current is equal to IOVERCURRENT(max). (See the

“Sense Resistor” section). The calculations below are for

normal operation. To calculate the stress under short circuit

conditions, substitute IOVERCURRENT(max) for IOUT(max). Use

the formula below to calculate D under short circuit condi-

tions.

0.063 1.8 10

SHORTCIRCUIT

−

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

D I

SW(high side)(rms)

OUT(max)

−













1 D I

SW(low side)(rms)

OUT(max)

−

(

)













where:

D = duty cycle of the converter

OUT

= 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 VIN less than 10V and 85%

can be used for VIN greater than 10V. The efﬁciency can be

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.

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