MIC2199

Micrel

MIC2199

12

November 2004

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

is:

PR

I

SWITCH1(dc)

DS(on)1

=×

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

dissipation is:

PR

I

SWITCH2(dc)

DS(on)2

=×

Since the AC switching losses for the low side MOSFET is

near zero, the total power dissipation is:

PP

low-side MOSFET(max)

SWITCH2(dc)

=

The total power dissipation for the high side MOSFET is:

PP

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:

VV

DIODE(rrm)

IN

=

The power dissipated by the Schottky diode is:

PI

V

DIODE

D(avg)

F

=×

where:

V

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 current.

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.

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 less ringing and less

power loss. Depending on the circuit components and oper-

ating conditions, 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.

The output capacitor’s ESR is usually the main cause of

output ripple. The maximum value of ESR is calculated by:

R

V

I

ESR

OUT

PP

≤

∆

where:

V

I

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

output capacitance. The total ripple is calculated below:

∆V

I(1 D)

Cf

IR

OUT

PP

OUT

S

2

PP

ESR

2

=

×

−

×

⎛

⎝

⎜

⎞

⎠

⎟ +×

()

where:

D = duty cycle

C

f

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:

PI

R

DISS(C

C

ESR(C

)

OUT

OUT(rms)2

OUT

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:

∆VI

R

IN

INDUCTOR(peak)

ESR(C )

IN

=×