SP6132 datasheet(9/14 Pages) SIPEX

9 / 14 page

Date: 8/4/04

SP6132 Wide Input, 300KHz Synchronous PWM Controller

APPLICATIONS INFORMATION: Continued

The capacitor type suitable for the output capac-

itors can also be used for the input capacitors.

However, exercise extra caution when tantalum

capacitors are considered. Tantalum capacitors are

known for catastrophic failure when exposed to

surge current, and input capacitors are prone to

such surge current when power supplies are con-

nected “live” to low impedance power sources.

MOSFET Selection

The losses associated with MOSFETs can be

divided into conduction and switching losses.

Conduction losses are related to the on resistance

of MOSFETs, and increase with the load current.

Switching losses occur on each on/off transition

when the MOSFETs experience both high current

and voltage. Since the bottom MOSFET switches

current from/to a paralleled diode (either its own

body diode or a Schottky diode), the voltage across

the MOSFET is no more than 1V during switching

transition. As a result, its switching losses are

negligible. The switching losses are difficult to

quantify due to all the variables affecting turn on/

off time. However, the following equation pro-

vides an approximation on the switching losses

associatedwiththetopMOSFETdrivenbySP6132.

OUT

rss

(max)

where

Crss = reverse transfer capacitance of the top

MOSFET

Switching losses need to be taken into account for

high switching frequency, since they are directly

proportional to switching frequency. The conduc-

tion losses associated with top and bottom

MOSFETs are determined by:

OUT

(max)

)

(

(max) =

)

(

(max )

)

(

(max )

OUT

−

where

PCH(max) = conduction losses of the high side

MOSFET

PCL(max) = conduction losses of the low side

MOSFET

RDS(ON) = drain to source on resistance.

The total power losses of the top MOSFET are the

sum of switching and conduction losses. For syn-

chronous buck converters of efficiency over 90%,

allow no more than 4% power losses for high or

low side MOSFETs. For input voltages of 3.3V

and 5V, conduction losses often dominate switch-

ing losses. Therefore, lowering the RDS(ON) of the

MOSFETs always improves efficiency even

though it gives rise to higher switching losses due

to increased Crss.

Top and bottom MOSFETs experience unequal

conduction losses if their on time is unequal. For

applications running at large or small duty cycle, it

makes sense to use different top and bottom

MOSFETs. Alternatively, parallel multiple

MOSFETs to conduct large duty factor.

RDS(ON) varies greatly with the gate driver voltage.

The MOSFET vendors often specify RDS(ON) on

multiple gate to source voltages (VGS), as well as

provide typical curve of RDS(ON) versus VGS. For

5V input, use the RDS(ON) specified at 4.5V VGS. At

the time of this publication, vendors, such as

Fairchild, Siliconix and International Rectifier,

have started to specify RDS(ON) at VGS less than 3V.

This has provided necessary data for designs in

which these MOSFETs are driven with 3.3V and

made it possible to use SP6132 in 3.3V only

applications.

Thermal calculation must be conducted to ensure

the MOSFET can handle the maximum load cur-

rent. The junction temperature of the MOSFET,

determined as follows, must stay below the maxi-

mum rating.

MOSFET

(max)

( max)

where

TA(max) = maximum ambient temperature

PMOSFET(max) = maximum power dissipa-

tion of the MOSFET

RΘJA = junction to ambient thermal resistance.

RΘJA of the device depends greatly on the board

layout, as well as device package. Significant

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