MIC2155 datasheet(25/33 Pages) MICREL

25 / 33 page

Micrel, Inc.

MIC2155/2156

May 2009

M9999-052709-A

(408) 944-0800

For the high-side switch, the maximum DC power

dissipation is:

()2

)

rms

(

DSON

)

(

SWITCH

For the low-side switch, the DC power dissipation is:

()2

)

rms

(

DSON

)

(

SWITCH

The switching loss for each of the high-side MOSFETs

is:

)

peak

(

The total power dissipation for each MOSFET is:

)

(

SWITCH

total

FET

External Schottky Diode

A freewheeling diode in parallel with the low-side FET is

needed to keep the inductor current flow continuous

while both MOSFETs are turned off (dead time). Dead

time is necessary to prevent current from flowing

unimpeded through both MOSFETs. An external

Schottky diode is not necessary for circuit operation

since the low-side MOSFET contains a parasitic body

diode. An external diode will improve efficiency due to its

lower forward voltage drop as compared to the internal

parasitic diode in the FET. It may also decrease high

frequency noise because the schottky diode junction

does not suffer from reverse recovery.

If the MOSFET body diode is used, it must be rated to

handle the peak and average current. The body diode

may have a relatively slow reverse recovery time and a

relatively high forward voltage drop. The power lost in

the diode is proportional to the forward voltage drop of

the diode. As the high-side MOSFET starts to turn on,

the body diode becomes a short circuit for the reverse

recovery period, dissipating additional power. The diode

recovery and the circuit inductance will cause ringing

during the high-side MOSFET turn on. If the internal FET

diode is used, power dissipated during the dead time

should be added to the PDISS of the low-side MOSFET.

An external Schottky diode conducts at a lower forward

voltage preventing the body diode in the MOSFET from

turning on. The lower forward voltage drop dissipates

less power than the body diode. The lack of a reverse

recovery mechanism in a Schottky diode causes is less

ringing and power loss. Depending on the circuit

components and operating conditions, an external

Schottky diode may give a ½% to 1% improvement in

efficiency.

This power dissipation is calculated below

OUT

)

ave

(

Where td is the dead time when both MOSFETs are off.

The reverse voltage requirement of the diode is:

RRM

DIODE

The power dissipated by the diode is:

AVE

DIODE

where VF is the forward voltage at the peak diode

current.

Snubber Design

A snubber is used to damp out high frequency ringing

caused by parasitic inductance and capacitance in the

buck converter circuit. A snubber is needed for each of

the two phases in the converter. Figure 22 shows a

simplified schematic of one of the buck converter

phases. Stray capacitance consists mostly of the two

MOSFET’s output capacitance (COSS). The stray

inductance is mostly package and etch inductance. The

arrows show the resonant current path when the high

side MOSFET turns on. This ringing causes stress on

the semiconductors in the circuit as well as increased

EMI.

COUT

CIN

LSTRAY1

LSTRAY2

LSTRAY3

LSTRAY4

Sync_buck

Controller

VDC

COSS1

COSS2

–

Figure 22. Output Parasitics

One method of reducing the ringing is to use a resistor

and capacitor to lower the Q of the resonant circuit. The

circuit in Figure 23 shows the resistor in between the

switch node and ground. Capacitor Cs is used to block

DC and minimize the power dissipation in the resistor.

This capacitor value should be between 5 and 10 times

the parasitic capacitance of the MOSFET COSS. A

capacitor that is too small will have high impedance and

prevent the resistor from damping the ringing. A

capacitor that is too large causes unnecessary power

dissipation in the resistor, which lowers efficiency.

The snubber components should be placed as close as

possible to the low-side MOSFET and/or external

schottky diode since in contributes to most of the stray

capacitance. Placing the snubber too far from the FET or

using etch that is too long or thin will add inductance to

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