SP6120B datasheet(17/22 Pages) SIPEX

17 / 22 page

Date: 5/25/04

SP6120B Low Voltage, AnyFETTM, Synchronous, Buck Controller

two 0.04 square inches copper pad directly

under the package, without occupying addi-

tional board space, can increase the maximum

power from approximately 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 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 these noises 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 GH and GL.

VF = forward voltage of the Schottky diode.

SP6120B

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 SP6120B

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 SP6120B is equipped with 3% window

comparator that takes over the control loop on

transient, the crossover frequency can be se-

lected 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 contrib-

uted by the ESR associated with the output

capacitors and can be determined by:

Z(ESR) =

πC

OUTRESR

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 SP6120B.

1 =

1300V

OUT fCO fZ(ESR)

IN fP(LC)

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