MIC2155 datasheet(24/33 Pages) MICREL

24 / 33 page

Micrel, Inc.

MIC2155/2156

May 2009

M9999-052709-A

(408) 944-0800

during the MOSFET’s conduction time. If operating at

4.5VIN, without connecting VDD to VIN, the gate drive

voltage to the high-side FET could be as low as 3.2V.

MOSFETs with low VGS enhanced gates should be used

in this situation.

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

increases at high junction temperature. A 75°C rise in

junction temperature will increase the channel resistance

of the MOSFET by 40% 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

MIC2155 gate drive circuit. Gate charge can be a

significant source of power dissipation in the controller

due to the high switching frequencies and generally

large MOSFETs that are driven. At low output load this

power dissipation is noticeable as a reduction in

efficiency. The average current required to drive the

MOSFETs is:

IDD

where:

Qg is the total gate charge for all high and low

side MOSFETs. This information should be

obtained from the manufacturer’s data sheet with

a 5V VGS.

Since the current from the gate drive comes from the

input voltage, the power dissipated in the MIC2155 due

to gate drive is:

DRIVE

GATE

A convenient figure of merit for switching MOSFETs is

the on-resistance times the total gate charge (RDSON ×

Qg). Lower numbers translate into higher efficiency. Low

gate charge, logic level MOSFETs are a good choice for

use with the MIC2155. The internal LDO that supplies

VDD is rated for 75mA. Exceeding this value could

damage the regulator or cause excessive power

dissipation in the IC. Refer to the “Supply Voltages and

Internal Regulator” section of this specification for

additional information.

Parameters that are important to MOSFET switch

selection are:

• Voltage rating

• On resistance

• Total Gate Charge

The VDS voltage rating of the MOSFETs is 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).

CONDUCTION

where:

)

(

)

off

(

SWITCH

)

rms

(

SWITCH

CONDUCTION

RSWITCH is the on resistance of the MOSFET switch.

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

times are equal, the total AC switching loss is:

PEAK

)

(

Where:

Tt is the switching transition time (typically 15ns

to 30ns)

fS it the switching frequency of each phase

RMS Current and MOSFET Power Dissipation

Calculation

Under normal operation, the high side MOSFET’s RMS

current is greatest when VIN is low (maximum duty

cycle). The low side MOSFET’s RMS current is greatest

when VIN is high (minimum duty cycle). However, the

MOSFET sees maximum stress during short circuit

conditions, where the output current is equal to the

maximum overcurrent level. The calculations below are

for normal operation. To calculate the stress under short

circuit conditions, substitute the maximum overcurrent

level for IOUT(max).

The RMS value of the high side switch current is:

()

)

(

)

(

(max)

OUT

)

SIDE

LOW

(

RMS

(max)

OUT

)

SIDE

HIGH

(

RMS

−

where:

D is the duty cycle of the converter

IPP is the individual inductor ripple current

OUT

and

η is the efficiency of the converter.

Converter efficiency also depends on other component

parameters that have not yet been selected. For design

purposes, an efficiency estimate of 85%-90% can be

used. The efficiency can be more accurately calculated

once the design is complete. If the assumed efficiency is

grossly inaccurate, a second iteration through the design

procedure should be made.

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