MIC2182 datasheet(17/28 Pages) MICREL

17 / 28 page

June 2000

MIC2182

Micrel

charge can be a significant source of power dissipation in the

MIC2182. At low output load this power dissipation is notice-

able as a reduction in efficiency. The average current re-

quired to drive the high-side MOSFET is:

G[high-side](avg)

=×

where:

G[high-side](avg) =

average high-side MOSFET gate current

G = total gate charge for the high-side MOSFET

taken from manufacturer’s data sheet

with V

GS = 5V.

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

DS = 0

because the freewheeling diode is conducting during this

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

usually negligable. Also, the gate drive current for the low-

side MOSFET is more accurately calculated using C

ISS at

DS = 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 MIC2182 due to gate

drive is:

gate drive

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

DS(on) × QG). Lower

numbers translate into higher efficiency. Low gate-charge

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

MIC2182. Power dissipation in the MIC2182 package limits

the maximum gate drive current. Refer to Figure 10 for the

MIC2182 gate drive limits.

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

DS(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 (P

conduction) and the

switching losses that occur during the period of time when the

MOSFETs turn on and off (P

AC).

conduction

where:

(rms)

conduction

=×

AC(off)

AC(on)

SW = on-resistance of the MOSFET switch.

Making the assumption the turn-on and turnoff transition

times are equal, the transition time can be approximated by:

ISS

OSS

×+

where:

ISS and COSS are measured at VDS = 0.

G = gate drive current (1A for the MIC2182)

The total high-side MOSFET switching loss is:

P(V

V ) I

where:

T = switching transition time

(typically 20ns to 50ns)

D = freewheeling diode drop, typically 0.5V.

S it the switching frequency, nominally 300kHz

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 MOSFET’s RMS

current is greatest when V

IN is low (maximum duty cycle). The

low-side MOSFET’s RMS current is greatest when V

IN is high

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

MOSFETs see occurs during short circuit conditions, where

the output current is equal to I

overcurrent(max). (See the Sense

Resistor section). The calculations below are for normal

operation. To calculate the stress under short circuit condi-

tions, substitute I

overcurrent(max) for IOUT(max). Use the formula

below to calculate D under short circuit conditions.

0.063 1.8 10

short circuit

=−

−

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

(rms)

SW(highside)

OUT(max)

=×













(rms)

1 D I

SW(low side)

OUT(max)

=−

()













where:

D = duty cycle of the converter

OUT

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

IN less than 10V and 85%

can be used for V

IN 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.

For the high-side switch, the maximum dc power dissipation

is:

(rms)

switch1(dc)

DS(on)1

SW1

=×

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