MIC2174 datasheet(13/24 Pages) MICREL

13 / 24 page

Micrel, Inc.

MIC2174

September 2009

M9999-090409-B

Application Information

MOSFET Selection

The MIC2174 controller works from power stage input

voltages of 3V to 40V and has an external 3V to 5.5V VIN

to provide power to turn the external N-Channel power

MOSFETs for the high- and low-side switches. For

applications where VIN < 5V, it is necessary that the

power MOSFETs used are sub-logic level and are in full

conduction mode for VGS of 2.5V. For applications when

VIN > 5V; logic-level MOSFETs, whose operation is

specified at VGS = 4.5V must be used.

There are different criteria for choosing the high-side and

low-side MOSFETs. These differences are more

significant at lower duty cycles such as a 12V to 1.8V

conversion. In such an application, the high-side

MOSFET is required to switch as quickly as possible to

minimize transition losses, whereas the low-side

MOSFET can switch slower, but must handle larger

RMS currents. When the duty cycle approaches 50%,

the current carrying capability of the high-side MOSFET

starts to become critical.

It is important to note that 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 and in calculating the value of current limit.

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

MIC2174 gate-drive circuit. At 300kHz switching

frequency and above, the gate charge can be a

significant source of power dissipation in the MIC2174.

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:

side]

G[high

(avg)

(5)

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 = VIN.

fSW = Switching Frequency (300kHz)

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

because an internal body diode or 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:

ISS

side]

G[low

(avg)

(6)

Since the current from the gate drive comes from the VIN,

the power dissipated in the MIC2174 due to gate drive is:

(avg))

(avg)

side]

G[low

side]

G[high

GATEDRIVE

(7)

A convenient figure of merit for switching MOSFETs is

the on resistance times 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 MIC2174. Also, the RDS(ON) of the low-side

MOSFET will determine the current limit value. Please

refer to “Current Limit” subsection in “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).

CONDUCTION

(8)

DS(ON)

SW(RMS)

CONDUCTION

(9)

AC(on)

)

AC(off

(10)

where:

RDS(ON) = on-resistance of the MOSFET switch

D = Duty Cycle = VOUT / VHSD

Making the assumption that the turn-on and turn-off

transition times are equal; the transition times can be

approximated by:

HSD

OSS

ISS

(11)

where:

CISS and COSS are measured at VDS = 0

IG = gate-drive current

The total high-side MOSFET switching loss is:

HSD

)

(12)

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