MIC2159 datasheet(9/17 Pages) MICREL

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Micrel

MIC2159

Application information

MOSFET Selection

The MIC2159 controller works from input voltages of 3V

to 14.5V and has an internal 5V regulator to provide

power to turn the external N-Channel power MOSFETs

for high- and low-side switches. For applications where

VIN < 5V, the internal VDD regulator operates in dropout

mode, and 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. For the lower (<5v) applications, the

VDD supply can be connected directly to Vin to help

increase the driver voltage to the MOSFET.

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 and in calculating the value of current-sense

(CS) resistor. 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 MIC2159 gate-drive circuit. At 400kHz switching

frequency and above, the gate charge can be a

significant source of power dissipation in the MIC2159.

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:

IG[high-side](avg)=QG x FS

Where:

IG[high-side](avg)=Average high-side MOSFET gate

current

QG = total gate charge for the high-side MOSFET

taken from manufacturer’s data sheet for VGS =

5V.

FS = Switching Frequency (400kHz)

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

because the 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:

IG[low-side](avg) = CISS × VGS x FS

Since the current from the gate drive comes from the

input voltage, the power dissipated in the MIC2159 due

to gate drive is:

PGATEDRIVE = VIN.(IG[high-sde](avg) + IG[low-side](avg))

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 MIC2159.

Parameters that are important to MOSFET switch

selection are:

•

Voltage rating

•

On-resistance

•

Total gate charge

The voltage ratings for the top and bottom MOSFET 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 parasitic

elements.

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:

RMS

CONDUCTION

⋅

)

(

)

(

)

(

off

RSW = on-resistance of the MOSFET switch

OUT

cyle

duty

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

times

are

equal;

the

transition

times

can

approximated by:

OSS

ISS

⋅

where:

CISS and COSS are measured at VDS = 0

IG = gate-drive current (1.4A for the MIC2159)

The total high-side MOSFET switching loss is:

(

)

⋅

Where:

tT = Switching transition time (~20ns)

VD = Freewheeling diode drop (0.5v)

FS = Switching Frequency (400kHz)

The low-side MOSFET switching losses are negligible

and can be ignored for these calculations.

Inductor Selection

Values for inductance, peak, and RMS currents are

required to select the output inductor. The input and

output voltages and the inductance value determine the

peak-to-peak inductor ripple current. Generally, higher

October 2006

M9999-101206

MIC2159 Datasheet(PDF) 9 Page - Micrel Semiconductor

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