SP6121CN-L datasheet(10/42 Pages) SIPEX

Part #	SP6121CN-L

Description	Low Voltage, Synchronous Step Down PWM Controller

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Manufacturer	SIPEX [Sipex Corporation]

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10 / 42 page

Date: 11/29/04

SP6121 Low Voltage, Synchronous Step Down PWM Controller

10

supplies are connected ‘live’ to low impedance

power sources. Certain tantalum capacitors, such

as AVX TPS series, are surge tested. For ge-

neric tantalum capacitors, use 2:1 voltage derat-

ing to protect the input capacitors from surge

fall-out.

MOSFET Selection

The SP6121 drives a PMOS MOSFET on the

high side and an NMOS MOSFET synchronous

rectifier on the low side. Using a PMOS switch

on the high side negates the need for an external

charge pump and simplifies the application cir-

cuit.

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 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. How-

ever, making the assumption that the turn on and

turn off transition times are equal, the transition

time can be approximated by:

tT = CISSVIN ,

IG

where;

CISS is the PMOS’s input capacitance, or the

sum of the gate-to-source capacitance, CGS, and

the drain-to-gate capacitance, CGD. This param-

eter can be directly obtained from the MOSFET’s

data sheet IG is the gate drive current provided

by the SP6121 (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 = the switching transition time

VF = 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)

2D

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

efficiency even though it gives rise to higher

switching 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 SP6121 gate drive

circuitry. (At 500kHz switching frequency, the

gate charge is the dominant source of power

dissipation in the SP6121.) 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 = gate charge of PMOS

QGL = gate charge of NMOS

Considering that the gate charge current comes

from the input supply voltage VIN, the power

dissipated in the SP6121 due to the gate drive is:

PGATE DRIVE = VINIG(av)

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

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