SP6123 datasheet(11/18 Pages) EXAR

11 / 18 page

Date: 9/13/04

SP6123 Low Voltage, Synchronous Step Down PWM Controller

power sources. Certain tantalum capacitors, such

as AVX TPS series, are surge tested. For generic

tantalum capacitors, use 2:1 voltage derating to

protect the input capacitors from surge fallout.

MOSFET Selection

The losses associated with MOSFETs can be

divided into conduction and switching losses.

Conduction losses are related to the on resis-

tance 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 an external

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, making the assumption that the

turn on and turn off transition times are equal,

the transition time can be approximated by:

tT = CISSVIN ,

where CISS is the MOSFET’s input capacitance,

or the sum of the gate-to-source capacitance,

CGS, and the drain-to-gate capacitance, CGD.

This parameter can be directly obtained from

the MOSFET’s data sheet.

IG is the gate drive current provided by the

SP6123 (approximately 1A at VIN=5V) and VIN

is the input supply voltage.

Therefore an approximate expression for the

switching losses associated with the high side

MOSFET can be given as:

PSH(max) = (VIN(max) + VF)IOUT(max)tTFS ,

where tT is the switching transition time and VF

is the free wheeling diode drop.

Switching losses need to be taken into account

for high switching frequency, since they are

directly proportional to switching frequency.

The conduction losses associated with top and

bottom MOSFETs are determined by

PCH(max) = RDS(ON)IOUT(max)

PCL(max) = RDS(ON)IOUT(max)

2(1 - D),

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

synchronous 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 domi-

nate switching losses. Therefore, lowering the

RDS(ON) of the MOSFETs always improves effi-

ciency even though it gives rise to higher switch-

ing losses due to increased CISS .

Total gate charge is the charge required to turn

the MOSFETs on and off under the specified

operating conditions (VGS and VDS). The gate

charge is provided by the SP6123 gate drive

circuitry. (At 500kHz switching frequency, the

gate charge is the dominant source of power

dissipation in the SP6123). At low output levels,

this power dissipation is noticeable as a reduc-

tion in efficiency. The average current required

to drive the high side and low side MOSFETs is:

IG(av) = QGHFS + QGLFS, where

QGH and QGL are the total charge for the high

side and the low side MOSFETs respectively.

Considering that the gate charge current comes

from the input supply voltage VIN, the power

dissipated in the SP6123 due to the gate drive is:

PGATE DRIVE = VINIG(av)

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 volt-

age. The MOSFET vendors often specify RDS(ON)

on multiple gate to source voltages (VGS), as

well as provide typical curve of RDS(ON) versus

APPLICATIONS INFORMATION

SP6123 Datasheet(PDF) 11 Page - Exar Corporation

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