SP6120B Datasheet, PDF(12/22 Page) Sipex Corporation – Low Voltage, AnyFETTM, Synchronous ,Buck Controller Ideal for 2A to 10A, High Performance, DC-DC Power Converters

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SP6120B Datasheet, PDF (12/22 Pages) Sipex Corporation – Low Voltage, AnyFETTM, Synchronous ,Buck Controller Ideal for 2A to 10A, High Performance, DC-DC Power Converters

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Program Logic: continued

PROGRAM LOGIC TRUTH TABLE

PROGRAM PIN

NFET OR PFET

MODE

Short to GND

NFET

Continuous

68 kâ¦ to GND

NFET

Discontinuous

Short to VCC

68 kâ¦ to VCC

PFET

Continuous

Discontinous

The NFET/PFET programmability is for the

high side MOSFET. When designing DC/DC

converters, it is not always obvious when to use

an NFET with a charge pump or a simple PFET

for the high side MOSFET. Often, the controller

has to be changed, making performance evalua-

tions difficult. This difficulty is worsened by the

limited availability of true low voltage control-

lers. In addition, by also programming the mode,

continuous or discontinuous, switch mode power

designs that are successful in bus applications

can now find homes in portable applications.

Secondary Loop (3% Window Comparator)

DSP, microcontroller and microprocessor appli-

cations have very strict supply voltage require-

ments. In addition, the current requirements to

these devices can change drastically. Linear

regulators, typically the workhorse for DC/DC

step-down, do a great job managing accuracy

and transient response at the expense of effi-

ciency. On the other hand, PWM switching

regulators typically do a great job managing

efficiency at the expense of output ripple and

line/load step response. The trick in PWM

controller design is to emulate the transient re-

sponse of the linear regulator.

Of course improving transient response should

be transparent to the power supply designer.

Very often this is not the case. Usually the very

circuitry that improves the controllers transient

response adversely interferes with the main PWM

loop or complicates the board level design of the

power converter.

The SP6120B handles line/load transient re-

sponse in a new way. First, a window compara-

tor detects whether the output voltage is above

or below the regulated value by 3%. Then, a

proprietary âRipple & Frequency Independentâ

algorithm synchronizes the output of the win-

dow comparator with the peak and valley of the

inductor current waveform. 3% low detection is

synchronized with inductor current peak; 3%

high detection is synchronized with the inductor

current valley. However, in order to eliminate

any additional loops, the current peak and valley

are determined by the edges associated with the

on-time in the main loop. The set pulse corre-

sponding to the start of an on-time indicates a

MAX

DC Load

Current

MIN

Output

Voltage

VOUT

Reset

Main Loop

Set

V(VCC)

3% High

Latch On

V(VCC)

3% Low

Latch On

TIME

Date: 5/25/04

SP6120B Low Voltage, AnyFETTM, Synchronous, Buck Controller

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