SP6121 datasheet(11/16 Pages) SIPEX

11 / 16 page

Date: 5/25/04

SP6121 Low Voltage, Synchronous Step Down PWM Controller

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, ven-

dors, such as Fairchild, Siliconix and Interna-

tional 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

SP6121 in 3.3V only applications.

Thermal calculation must be conducted to en-

sure the MOSFET can handle the maximum

load current. The junction temperature of the

MOSFET, determined as follows, must stay

below the maximum rating.

MOSFET

(max)

( max)

where;

TA(max) = maximum ambient temperature

PMOSFET(max) = maximum power dissipation 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

thermal improvement can be achieved in the

maximum power dissipation through the proper

design of copper mounting pads on the circuit

board. For example, in a SO-8 package, plac-

ing two 0.04 square inches copper pad di-

rectly under the package, without occupying

additional board space, can increase the maxi-

mum power from approximately 1 to 1.2W.

For DPAK package, enlarging the tap mount-

ing pad to 1 square inches reduces the RθJA

from 96

°C/W to 40°C/W.

Schottky Diode Selection

When paralleled with the bottom MOSFET, an

optional Schottky diode can improve efficiency

and reduce noise. Without this Schottky diode,

the body diode of the bottom MOSFET con-

ducts the current during the non-overlap time

when both MOSFETs are turned off. Unfortu-

nately, the body diode has high forward voltage

and reverse recovery problem. The reverse re-

covery of the body diode causes additional

switching noises when the diode turns off. The

Schottky diode alleviates this noise and addi-

tionally improves efficiency thanks to its low

forward voltage. The reverse voltage across the

diode is equal to input voltage, and the diode

must be able to handle the peak current equal to

the maximum load current.

The power dissipation of the Schottky diode is

determined by

PDIODE = 2VFIOUTTNOLFS

where;

TNOL = non-overlap time between PDRV and

NDRV.

VF = forward voltage of the Schottky diode.

SP6121

COMP

Figure 4. The RC network connected to the COMP pin

provides a pole and a zero to control loop.

Loop Compensation Design

The goal of loop compensation is to manipulate

loop frequency response such that its gain crosses

over 0db at a slope of -20db/dec. The SP6121

has a trans-conductance error amplifier and re-

quires the compensation network to be con-

nected between the COMP pin and ground, as

shown in Figure 4.

The first step of compensation design is to pick

the loop crossover frequency. High crossover

frequency is desirable for fast transient response,

but often jeopardize the system stability. Cross-

over frequency should be higher than the ESR

zero but less than 1/5 of the switching fre-

quency. The ESR zero is contributed by the ESR

APPLICATIONS INFORMATION: Continued

SP6121 Datasheet(PDF) 11 Page - Sipex Corporation

SP6121 Datasheet(HTML) 11 Page - Sipex Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Datasheet Download

Link URL