MIC2131 datasheet(15/20 Pages) MICREL

15 / 20 page

Micrel, Inc.

MIC2130/1

April 2008

M9999-042108-C

and fall times). Whereas the low-side MOSFET can

switch slower, but must handle larger RMS currents.

When duty cycle approaches 50%, the current carrying

capability of the upper MOSFET starts to become critical

also and can sometimes require external high current

drivers to achieve the necessary switching speeds.

MOSFET loss = Static loss + Transition loss

Static loss (Ps) = IFETRMS

2 x RDS

Transition loss (Pt) = IOUT x (tr+tf) x VDSOFF x FSWITCH/2

tr + tf = Rise time + Fall time

Due to the worst case driver currents of the MIC2130/31,

the value of tr + tf simplifies to:

tr + tf (ns) =

∆Qg (nC)

∆Qg can be found in the MOSFET characteristic curves

VDSOFF = Voltage across MOSFET when it is off

IFETRMS =

)

(

⋅

IX = IOUT – IRIPPLE/2

IY = IOUT + IRIPPLE/2

D_on = TON x FSWITCH

high-side FET on time

D_off = TON x FSWITCH

low-side FET on time

D_on = D = VO/(VIN x eff) since it changes depending on

which MOSFET we are calculating losses for.

High-side FET TON = D_on/FSWITCH

The lower MOSFET is not on for the whole time that the

upper MOSFET is off due to the fixed 80ns high side

driver delay. Therefore, there is an 80ns term subtracted

from the lower FET on time equation.

Low-side FET TON = (1-D_on)/FSWITCH – 80ns

There are many MOSFET packages available which

have varying values of thermal resistance and can

therefore dissipate more power if there is sufficient

airflow or heat sink externally to remove the heat.

However, for this exercise we can assume a maximum

dissipation of 1.2W per MOSFET package. This can be

altered if the final design has higher allowable package

dissipation.

Look at lower MOSFET first:

Pdis_max = 1.2 W = Ps + Pt

For the low side FET, Pt is small because VDSOFF is

clamped to the forward voltage drop of the Schottky

diode. Therefore:

RDSON(max) ~ 1.2/IFETRMS

Example: For 12V to 1.8V @ 10A

RDSON(max) < 14mΩ

It is important to remember to use the RDSON(max) figure

for the MOSFET at the maximum temperature to help

prevent thermal runaway (as the temperature increases,

the RDSON increases).

∆Qg(max) should be limited so that the low side MOSFET

is off within the fixed 80ns delay before the high side

driver turns on.

High side MOSFET:

For the high side FET, the losses should ideally be

evenly spread between transition and static losses. Use

the C of the VIN range to balance the losses.

Pt = Pdis_max/2 = 0.6 = IOUT x ∆Qg x VINMID x FSWITCH/2

∆Qg(max) < 0.6 x 2 / (IOUT x VINMID x FSWITCH)

RDSon is calculated similarly for the high side MOSFET:

RDSON(max) ~ 0.6 / IFETRMS

Using previous example:

∆Qg(max) < 20nC

RDSON(max) < 35mΩ

Note that these are maximum values based upon

thermal limits and are not targeted at the highest

efficiency. Selection of lower values is recommended to

achieve higher efficiency designs.

Limits to watch out for:

QgTOTAL < 1500 nC/VIN

Total of both high side and low side MOSFET Qg value

at VGS = 5V for both channels.

Example: @ VIN(max) = 13.2V,

QgTOTAL < 1500/13.2 = 114nC

∆QgLOW < 120nC

Maximum turn on gate charge for the low side MOSFET

to ensure proper turn off before high side MOSFET is

switched on.

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