MIC2182 datasheet(21/40 Pages) MICROCHIP

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

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DS20006644A-page 21

MIC2182

EQUATION 5-6:

5.2

Current-Sense Resistor Selection

Low inductance power resistors, such as metal film

resistors should be used. Most resistor manufacturers

make low inductance resistors with low temperature

coefficients, designed specifically for current-sense

applications. Both resistance and power dissipation

must be calculated before the resistor is selected. The

value of RCS is chosen based on the maximum output

current and the minimum current-limit threshold voltage

level. The power dissipated is based on the maximum

peak current limit at the maximum current-limit

threshold voltage.

EQUATION 5-7:

The maximum overcurrent at the maximum

current-limit threshold voltage is:

EQUATION 5-8:

The maximum power dissipated in the sense resistor

is:

EQUATION 5-9:

5.3

MOSFET Selection

External N-channel logic-level power MOSFETs must

be used for the high-side and low-side switches. The

MOSFET gate-to-source drive voltage of the MIC2182

is regulated by an internal 5V VDD regulator. Logic-level

MOSFETs, whose operation is specified at VGS = 4.5V

must be used.

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

increases with increasing temperature. A 75°C rise in

junction 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.

Total gate charge is the charge required to turn the

MOSFET on and off under specified operating

conditions (VDS and VGS). The gate charge is supplied

by the MIC2182 gate drive circuit. At 300 kHz switching

frequency and above, the gate charge can be a

significant source of power dissipation in the MIC2182.

At low output load, this power dissipation is noticeable

as a reduction in efficiency. The average current re-

quired to drive the high-side MOSFET is:

EQUATION 5-10:

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

because the freewheeling diode is conducting during

this time. The switching losses for the low-side

MOSFET is usually negligible. Also, the gate drive

RWINDING HOT





RWINDING 20C





1 0.0042

THOT T20C

–













Where:

THOT =

Temperature of the wire

under operating load

T20°C =

Ambient room temperature

RWINDING(20°C) = Room temperature winding

resistance (usually specified

by the manufacturer)

RCS

75mV

IOUT MAX





--------------------------



IOVERCURRENT MAX





135mV

RCS

-----------------

PD RCS





IOVERCURRENT MAX





2 R



IGHS AVG





QG fSW



Where:

IGHS(AVG) = Average High-Side MOSFET Gate

Current

QG =

Total Gate Charge for the High-Side

MOSFET Taken from Manufac-

turer’s Data Sheet with VGS = 5V

fSW =

Switching Frequency

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