MIC2182 datasheet(22/40 Pages) MICROCHIP

22 / 40 page

MIC2182

DS20006644A-page 22



2022 Microchip Technology Inc. and its subsidiaries

current for the low-side MOSFET is more accurately

calculated using CISS at VDS = 0V instead of gate

charge.

The gate drive current for the low-side MOSFET:

EQUATION 5-11:

Because the current from the gate drive comes from

the input voltage, the power dissipated in the MIC2182

due to gate drive is:

EQUATION 5-12:

A convenient figure of merit for switching MOSFETs is

the on-resistance times the total gate charge (RDS(ON)

x 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 4-9 and Figure 4-10 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 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).

EQUATION 5-13:

Making the assumption the turn-on and turn-off

transition times are equal, the transition time can be

approximated by:

EQUATION 5-14:

The total high-side MOSFET switching loss is:

EQUATION 5-15:

Because the low-side MOSFET body diode is forward

biased before the low-side MOSFET is turned on and

after the low-side MOSFET is turned off, this keeps the

voltage across the low-side MOSFET to about 0.5V

during switching transitions, the low-side MOSFET

switching losses are negligible and can be ignored.

IGLS AVG





CISS VGS fSW



Where:

CISS = Input capacitance of the low-side MOSFET at

VDS = 0V.

PD GDRV





VIN IGHS AVG





IGLS AVG











Where:

PD(GDRV) = Power Dissipated Due to Gate Drive

PD SW





PCONDUCTION PAC

Where:

PCONDUCTION = ISW(RMS)2 x RDS(ON)

PAC =

PAC(OFF) + PAC(ON)

RDS(ON) =

On-Resistance of the MOSFET

Switch

ISW(RMS) =

RMS current of the MOSFET switch

CISS VGS COSS V



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

Where:

CISS and COSS are Measured at VDS = 0V

IG = Gate Drive Current (1A for the MIC2182)

PAC SWHS





VIN VD





IL AVG





tT fSW



Where:

IL(AVG) = Average inductor current

tT =

Switching transition time

typically 20 ns to 50 ns)

VD =

Freewheeling diode drop, typically 0.5V

fSW =

Switching frequency, normally 300 kHz

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