MIC2182 datasheet(24/40 Pages) MICROCHIP

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MIC2182

DS20006644A-page 24



2022 Microchip Technology Inc. and its subsidiaries

5.4

External Schottky Diode

An external freewheeling diode is used to keep the

inductor current flow continuously while both

MOSFETs are turned off during dead time. This dead

time prevents current from flowing unimpeded through

both MOSFETs and is typically 80 ns 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.

EQUATION 5-23:

The reverse voltage requirement of the diode is:

EQUATION 5-24:

The power dissipated by the Schottky diode is:

EQUATION 5-25:

The external freewheeling Schottky diode, D1, 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 operating conditions, an external

Schottky diode will give a 1/2% to 1% improvement in

efficiency. Figure 5-1 illustrates the difference in noise

on the VSW pin with and without a Schottky diode.

FIGURE 5-1:

Switch Output Noise With

and Without Schottky Diode.

5.5

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:

EQUATION 5-26:

ID AVG





IOUT 2 80ns fSW



VDIODE RRM





VIN

PDIODE ID AVG







Where:

VF = Forward Voltage at the Peak Diode

Current

TIME

ESRCOUT



VOUT

IL PP







------------------



Where:

ΔVOUT = Peak-to-peak output voltage ripple

ΔIL(PP) = Peak-to-peak inductor ripple current

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