TK75003 Ver la hoja de datos (PDF) - Toko America Inc

Número de pieza

componentes Descripción

Fabricante

TK75003

PWM CONTROLLER

Toko America Inc  

TK75003

THEORY OF OPERATION

The TK75003 is intended for use as a primary-side Pulse

Width Modulator (PWM) controller. Using a control

technique referenced in the "Application Information"

section, the TK75003 can be used as a highly cost-

effective controller for power factor correction. The many

features integrated into a simple 5-pin design allow it to be

easily configured for voltage-mode or current-mode control,

fixed-frequency or fixed off-time operation, off-line boot-

strapping, and direct drive of a power MOSFET. The

polarity of the feedback signal allows for simpler interface

with a TL431-derived error signal (see "Applications

Information" section).

fixed off-time control technique can be readily implemented

by using a small transistor to keep the timing capacitor

discharged during the on-time. When the on-pulse is

terminated, the timing capacitor ramps up to a fixed

threshold at a fixed rate to fix the off-time.

The Undervoltage Lockout (UVLO) feature with hysteresis

minimizes the start-up current which allows a low-power

boot-strap technique to be used for the housekeeping

power. The duty ratio of the TK75003 is limited to

approximately 88% by the time required to discharge the

timing ramp.

The most noteworthy integrated feature in the TK75003 is

the way in which the feedback control pin is configured to

receive the error signal and the current signal for current-

mode control. Rather than receiving both inputs into a

comparator, a single input receives both signals summed

together and compares them against a fixed internal

reference. This yields two desirable effects: 1) a current-

limit threshold is automatically established, and 2) the

required error-signal polarity is the inverse of that of a

standard two-input current-mode control system. Generally,

the signal summation requires no additional external

components and the required error-signal polarity is simpler

to achieve.

Two other functions are integrated into the feedback pin.

A current ramp, which can be used to establish either the

slope-compensation ramp for a current-mode control design

or the voltage-comparison ramp for a voltage-mode control

design, flows out of the feedback pin. By adjusting the

terminating resistance at the feedback pin, the desired

ramp magnitude is established. For overcurrent protection,

a second fixed-reference comparator monitors the feedback

pin. If the feedback pin voltage should reach the second

threshold, this indicates that cycle-by-cycle PWM control

is not sufficient for maintaining control of the current (i.e.,

the minimum duty-ratio is too large to achieve volt-second

balance in the magnetics). The overcurrent detection

comparator latches (for one cycle) a reduction in the

source current which feeds the timing capacitor. This has

the effect of reducing the switching frequency and, thus,

effectively, the minimum duty ratio, which is just what is

needed to maintain control of the current.

The switching frequency is determined by an internal

current source charging an external timing capacitor. The

timing capacitor is ramped between internally-fixed

thresholds, valley to peak, and then quickly discharged. A

Page 6

January 1999 TOKO, Inc.

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