POWER MOSFETs

Two N-channel logic-level MOSFETs are required for each

output. The voltage rating should be at least 1.2 times the

maximum input voltage.

Maximizing efficiency for a design requires selecting the right

FET. The ON-resistance of the FET determines the ON-state

(conduction) losses, while gate charge defines the losses

during switch transitions. These two parameters require a

trade-off, since reducing ON-resistance typically requires in-

creasing gate capacitance (which increases the charge re-

quired to switch the FET). Improved FETs are currently being

released which are designed specifically for optimized

ON-resistance and gate charge characteristics.

The V

ON time of each switch. In some cases where one FET is on

most of the time, efficiency may be improved slightly by se-

lecting a low ON-resistance FET for one of the FET switches

and a different type with lower gate charge requirement for

the other FET switch. However, for most applications this

would give no measurable improvement.

CURRENT SENSE RESISTOR

A sense resistor is placed between the inductor and the out-

put capacitor to measure the inductor current. The value of

this resistor is set by the current limit voltage of the LM2641

(see Electrical Characteristics) and the maximum (peak) in-

ductor current. The value of the sense resistor can be calcu-

lated from:

Where:

V

Electrical Characteristics).

I

I

TOL is the tolerance (in %) of the sense resistor.

The physical placement of the sense resistors should be as

close as possible to the LM2641 to minimize the lead length

of the connections to the CSH and CSL pins. Keeping short

leads on these connections reduces the amount of switching

noise conducted into the current sense circuitry of the

LM2641.

EXTERNAL DIODES

FET Diodes

Both of the low-side MOSFET switches have an external

Schottky diode connected from drain to source. These di-

odes are electrically in parallel with the intrinsic body diode

present inside the FET. These diodes conduct during the

dead time when both FETs are off and the inductor current

must be supplied by the catch diode (which is either the body

diode or the Schottky diode).

Converter efficiency is improved by using external Schottky

diodes. Since they have much faster turn-off recovery than

the FET body diodes, switching losses are reduced.

The voltage rating of the Schottky must be at least 25%

higher than the maximum input voltage. The average current

rating of the diode needs to be only about 30% of the output

current, because the duty cycle is low.

The physical placement of the Schottky diode must be as

close as possible to the FET, since any parasitic (lead) in-

ductance in series with the Schottky will slow its turn-ON and

cause current to flow through the FET body diode.

Bootstrap Diodes

As shown in the block diagram for the LM2641, the CBOOT

pin has an internal diode which is connected to the 5V inter-

nal rail (which is also connected to the LIN pin). This diode

charges up the bootstrap capacitor to about 5V when the

low-side FET switch turns ON and pulls its drain down to

ground. The internal diode works well until the pulse widths

get extremely narrow, and then the charge applied to the

bootstrap capacitor can become insufficient to fully turn ON

the gate of the FET.

For this reason, an external diode should be used which con-

nects directly between the bootstrap capacitor and the exter-

nal capacitor connected to the LIN pin (C17). A fast-recovery

silicon diode should be used which has an average current

rating

Output Diodes

It is recommended that diodes be placed between the regu-

lated outputs and ground to prevent the outputs from swing-

ing below ground. The diode used may be a Schottky or sili-

con type, and should have a current rating of 1A or more. If

the outputs are allowed to swing below ground more than a

Vbe, the substrate of the LM2641 will become forward bi-

ased which will cause the part to operate incorrectly. Another

potential problem which could be caused by negative output

transients is damage to the output capacitors, since tantalum

capacitors can be damaged if a reverse voltage is forced

across them

The operating conditions where this can occur are not typi-

cal: it can happen if one or both of the outputs are very lightly

loaded, and an undervoltage (or overvoltage) condition is

detected. When this happens, the LM2641 turns off the

switching oscillator and turns on both of the low-side FET’s

which abruptly grounds one end of the inductor. When this

happens, the other end of the inductor (which is connected

to the regulated output) will experience a transient ringing

voltage as the energy stored in the inductor is discharged.

The amplitude and duration of the ringing is a function of the

R-L-C tank circuit made up the output capacitance, inductor,

and resistance of the inductor windings.

Because of this, the choice of inductor influences how large

in amplitude the ringing will be. In tests performed on the

Typical Application Circuit, the Sumida inductor showed less

ringing than the Pulse inductor, but both showed a voltage

transient that would go slightly below ground. For this rea-

son, the output diodes are recommended.

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