TABLE 1. MOSFET Manufacturers

Manufacturer

Model Number

Package Type

www Address

Phone

Fax

Fairchild

Semiconductor

FDC653N

SuperSOT-6

www.fairchildsemi.com

888-522-5372

207-761-6020

General

Semiconductor

GF4420

SO-8

www.gensemi.com

631-847-3000

631-847-3236

International

Rectifier

IRF7807

SO-8

www.irf.com

310-322-3331

310-322-3332

Vishay Siliconix

Si4812DY

SO-8

www.vishay.com

800-554-5565

408-567-8995

Si4874DY

SO-8

Zetex

ZXM64N03X

SO-8

www.zetex.com

(44) 161-622-4422

(44) 161-622-4420

LOW-SIDE MOSFET SELECTION

When operating in synchronous mode, special attention

should be given to the selection of the low-side MOSFET.

Besides choosing a MOSFET with minimal size and on resis-

tance, it is critical that the MOSFET meet certain rise and fall

time specifications. A 30ns deadtime between the low-side

and high-side MOSFET switching transitions is programmed

into the LM2655, as shown in

Figure 1. The prevent shoot-

through current, the low-side MOSFET must turn off before

the high-side MOSFET turns on. Hence, the low-side MOS-

FET has 30ns to turn off from the time the low-side driver

goes low. The fall time of the low-side MOSFET is governed

by the equation:

I

where I

lent capacitance seen at the LDR pin, and V

source voltage of the MOSFET. I

driver of the LM2655, but C

Therefore, it is important that the chosen MOSFET has a

suitable C

within 30ns. An input capacitance of less than 1000pF is rec-

ommended. Several suitable MOSFETs are shown in

Table

1.

EXTERNAL SCHOTTKY DIODE (Syncronous)

A Schottky diode is recommended to prevent the intrinsic

body diode of the low-side MOSFET from conducting during

the deadtime in PWM operation. If the body diode turns on,

there is extra power dissipation in the body diode because of

the reverse-recovery current and higher forward voltage

drop. In addition, the high-side MOSFET has more switching

loss because the diode reverse-recovery current adds to the

high-side MOSFET turn-on current. These losses degrade

the efficiency by 1-2%. The improved efficiency and noise

immunity with the Schottky diode become more obvious with

increasing input voltage and load current.

It is important to place the diode very close to the switch pin

of the LM2655. Extra parasitic impedance due to the trace

between the switch pin and the cathode of the diode will

cause the current limit to decrease. The breakdown voltage

rating of the diode is preferred to be 25% higher than the

maximum input voltage. Since it is on for a short period of

time, the diode’s average current rating need only be 30% of

the maximum output current.

EXTERNAL SCHOTTKY DIODE (Asyncronous)

In asyncronous mode, the output current commutates

throught the schottky diode when the high-side MOSFET is

turned off. Using a schottky diode with low forward voltage

drop will minimize the effeciency loss in the diode. However,

to achieve the greatest efficiency, the LM2655 should be op-

erated in syncronous mode using a low-side MOSFET. Since

the Schottky diode conducts for the entire second half of the

duty cycle in asyncronous mode, it should be rated higher

than the full load current.

BOOST CAPACITOR

The boost capacitor provides the extra votage needed to

turn the high-side, n-channel MOSFET on. A 0.1 µF ceramic

capacitor is recommended for the boost capacitor. The typi-

cal voltage across the boost capacitor is 6.7V.

DS101284-21

FIGURE 1. Low-side/high-side driver timing diagram.

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