SP6120EY datasheet(17/22 Pages) SIPEX

17 / 22 page

Date: 5/25/04

SP6120 Low Voltage, AnyFETTM, Synchronous, Buck Controller

thermal improvement can be achieved in the maxi-

mum power dissipation through the proper design

of copper mounting pads on the circuit board. For

example, in a SO-8 package, placing two 0.04

square inches copper pad directly under the pack-

age, without occupying additional board space,

can increase the maximum power from approxi-

mately 1 to 1.2W. For DPAK package, enlarging

the tap mounting 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 noises. Without this Schottky diode,

the body diode of the bottom MOSFET conducts

the current during the non-overlap time when both

MOSFETs are turned off. Unfortunately, the body

diode has high forward voltage and reverse recov-

ery problem. The reverse recovery of the body

diode causes additional switching noises when the

diode turns off. The Schottky diode alleviates

these noises and additionally 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 GH and GL.

VF = forward voltage of the Schottky diode.

SP6120

COMP

Figure 18. 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 SP6120

has a transconductance error amplifier and re-

quires the compensation network to be con-

nected between the COMP pin and ground, as

shown in Figure 18.

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. Since

the SP6120 is equipped with 3% window com-

parator that takes over the control loop on tran-

sient, the crossover frequency can be selected

primarily to the satisfaction of system stability.

Crossover frequency should be higher than the

ESR zero but less than 1/5 of the switching

frequency. The ESR zero is contributed by the

ESR associated with the output capacitors and

can be determined by:

Z(ESR) =

πCOUTRESR

Crossover frequency of 20kHz is a sound first

try if low ESR tantalum capacitors or poscaps

are used at the output. The next step is to calcu-

late the complex conjugate poles contributed by

the LC output filter,

P(LC) =

π√ LCOUT

The open loop gain of the whole system can be

divided into the gain of the error amplifier,

PWM modulator, buck converter, and feedback

resistor divider. In order to crossover at the

selected frequency fco, the gain of the error

amplifier has to compensate for the attenuation

caused by the rest of the loop at this frequency.

In the RC network shown in Figure 18, the

product of R1 and the error amplifier

transconductance determines this gain. There-

fore, R1 can be determined from the following

equation that takes into account the typical error

amplifier transconductance, reference voltage

and PWM ramp built into the SP6120.

1 =

1300V

OUT fCO fZ(ESR)

IN fP(LC)

APPLICATIONS INFORMATION: Continued

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