October 2005

11

MIC2198

MIC2198

Micrel, Inc.

For the high-side switch, the maximum DC power dissipa-

tion is:

P

R

I

SWITCH1(dc)

DS(on)1

=

×

For the low-side switch (N-Channel MOSFET), the DC power

dissipation is:

P

R

I

SWITCH2(dc)

DS(on)2

=

×

Since the AC switching losses for the low-side MOSFET is

near zero, the total power dissipation is:

P

P

low-side MOSFET(max)

SWITCH2(dc)

=

The total power dissipation for the high-side MOSFET is:

P

P

P

high sideMOSFET(max)

SWITCH 1(dc)

AC

−

=

+

External Schottky Diode

An external freewheeling diode is used to keep the inductor

current flow continuous while both MOSFETs are turned off.

This dead time prevents current from flowing unimpeded

through both MOSFETs and is typically 80ns The diode

conducts twice during each switching cycle. Although the

average current through this diode is small, the diode must

be able to handle the peak current.

I

I

2 80ns f

D(avg)

OUT

S

=

× ×

×

The reverse voltage requirement of the diode is:

V

V

DIODE(rrm)

IN

=

The power dissipated by the Schottky diode is:

P

I

V

DIODE

D(avg)

F

=

×

where:

The external Schottky diode, D2, is not necessary for circuit

operation since the low-side MOSFET contains a parasitic

body diode. The external diode will improve efficiency and

decrease high frequency noise. If the MOSFET body diode

is used, it must be rated to handle the peak and average cur-

rent. The body diode has 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.

AnexternalSchottkydiodeconductsatalowerforwardvoltage

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 less ringing and less power loss.

Depending on the circuit components and operating condi-

tions, an external Schottky diode will give a 1/2% to 1%

improvement in efficiency.

Output Capacitor Selection

The output capacitor values are usually determined by the

capacitors ESR (equivalent series resistance). Voltage rating

and RMS current capability are two other important factors in

selecting the output capacitor. Recommended capacitors are

tantalum, low-ESR aluminum electrolytics, and OS-CON.

Theoutputcapacitor’sESRisusuallythemaincauseofoutput

ripple. The maximum value of ESR is calculated by:

R

V

I

ESR

OUT

PP

≤

∆

where:

The total output ripple is a combination of the ESR and the

output capacitance. The total ripple is calculated below:

∆V

I

(1 D)

C

f

I

R

OUT

PP

OUT

S

2

PP

ESR

2

=

× −

×











 +

×

(

)

where:

D = duty cycle

The voltage rating of capacitor should be twice the output

voltage for a tantalum and 20% greater for an aluminum

electrolytic or OS-CON.

The output capacitor RMS current is calculated below:

I

I

12

C

PP

OUT(rms)

=

The power dissipated in the output capacitor is:

Input Capacitor Selection

The input capacitor should be selected for ripple current rating

and voltage rating. Tantalum input capacitors may fail when

subjected to high inrush currents, caused by turning the input

supply on. Tantalum input capacitor voltage rating should

be at least 2 times the maximum input voltage to maximize

reliability. Aluminum electrolytic, OS-CON, and multilayer

polymer film capacitors can handle the higher inrush currents

without voltage derating.

The input voltage ripple will primarily depend on the input

capacitors ESR. The peak input current is equal to the peak

inductor current, so:

∆V

I

R

IN

INDUCTOR(peak)

ESR(C )

IN

=

×

The input capacitor must be rated for the input current ripple.

The RMS value of input capacitor current is determined at

the maximum output current. Assuming the peak-to-peak

inductor ripple current is low:

I

I

D (1 D)

C (rms) OUT(max)

IN

≈

×

× −