MIC2104 datasheet(27/42 Pages) MICROCHIP

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DS20005899A-page 27

MIC2103/4

MIC2103/4. 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:

EQUATION 5-2:

The low-side MOSFET is turned on and off at VDS = 0

because

internal

body

diode

external

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

VDS = 0 instead of gate charge.

For the low-side MOSFET:

EQUATION 5-3:

Because the current from the gate drive comes from

the VDD, which is the output of the internal linear

regulator powered by VIN, the power dissipated in the

MIC2103/4 due to gate drive is:

EQUATION 5-4:

A convenient figure of merit for switching MOSFETs is

the on resistance multiplied by 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 MIC2103/4. Also, the

RDS(ON) of the low-side MOSFET will determine the

current-limit value. Please refer to the Current-Limit

subsection in the Functional Description for more

details.

Parameters that are important to MOSFET switch

selection are:

• Voltage rating

• On-resistance

• Total gate charge

The voltage ratings for the high-side and low-side

MOSFETs are essentially equal to the power stage

input voltage VHSD. A safety factor of 20% should be

added to the VDS(max) of the MOSFETs to account for

voltage spikes due to circuit parasitic elements.

The power dissipated in the MOSFETs is the sum of the

conduction losses during the on-time (PCONDUCTION)

and the switching losses during the period of time when

the MOSFETs turn on and off (PAC).

EQUATION 5-5:

The high-side MOSFET and low-side MOSFET RMS

currents can be calculated by Equation 5-6:

EQUATION 5-6:

G HIGH

SIDE AVG



–





Where:

IG(HIGH-SIDE(AVG)) = Average high-side MOSFET gate

current.

QG = Total gate charge for the high-side MOSFET

taken from the manufacturer’s data sheet for

VGS = VDD.

fSW = Switching frequency.

G LOW

SIDE AVG



–



ISS



GATEDRIVE

G HIGH

SIDE AVG



–



G LOW

SIDE AVG



–





CONDUCTION

SW RMS



DS ON





AC OFF



AC ON



Where:

ISW(RMS) = RMS current of the MOSFET switch.

RDS(ON) = On-resistance of the MOSFET switch.

SWHS RMS



OUT MAX







Where:

D = Duty cycle = VOUT/VHSD.

SWLS RMS



OUT MAX



1 D

–







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