MIC2182 datasheet(18/28 Pages) MICREL

18 / 28 page

MIC2182

Micrel

MIC2182

June 2000

For the low-side switch (N-channel MOSFET), the dc power

dissipation is:

(rms)

switch2(dc)

DS(on)2

SW 2

=×

Since the ac switching losses for the low side MOSFET is

near zero, the total power dissipation is:

low-side MOSFET(max)

switch2(dc)

The total power dissipation for the high side MOSFET is:

highsideMOSFET(max)

SWITCH 1(dc)

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.

80ns

D(avg)

OUT

=× ×

The reverse voltage requirement of the diode is:

(rrm)

diode

The power dissipated by the Schottky diode is:

diode

D(avg)

=×

where:

F = forward voltage at the peak diode current

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. Figure 12 illustrates the

difference in noise on the VSW pin with and without a

Schottky diode.

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:

ESR

OUT

≤

∆

where:

OUT = peak to peak output voltage ripple

PP = peak to peak inductor ripple current

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

output capacitance. The total ripple is calculated below:

∆V

I(1 D)

OUT

ESR

×−









+×

()

where:

D = duty cycle

OUT = output capacitance value

S = switching frequency

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:

(rms)

OUT

The power dissipated in the output capacitor is:

(rms)

DISS(C

ESR(C

)

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.

TIME

Figure 12. Switch Output Noise

With and Without Shottky Diode

MIC2182 Scheda tecnica(PDF) 18 Page - Micrel Semiconductor

MIC2182 Scheda tecnica(HTML) 18 Page - Micrel Semiconductor

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Scheda tecnica Scarica

Link URL