ML4821 datasheet(6/12 Pages) MICRO-LINEAR

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ML4821

Micro Linear

FUNCTIONAL DESCRIPTION

OSCILLATOR

The ML4821 oscillator charges the external capacitor

connected to CT with a current equal to 2.5/RT. When the

capacitor voltage reaches the upper threshold, the

comparator changes state and the capacitor discharges to

the lower threshold through Q1.

The oscillator period can be described by the following

relationship:

TOSC = TRAMP + TDISCHARGE

where:

TRAMP = C(Ramp Valley to Peak) ÷ (IRT/2)

and:

TDISCHARGE = C(Ramp Valley to Pk) ÷ (8.4mA – IRT/2)

The ML4821 oscillator includes a SYNC input for

synchronizing to an external frequency source. A positive

pulse on this pin of 2V (typ) resets the oscillators

comparator and initiates a discharge cycle for CT. The RT

and CT component values which set the ML4821

oscillator frequency should be selected to produce a

lower frequency than the external frequency source.

Figure 1. Oscillator Block Diagram.

–

VREF

8.4mA

Ω

SYNC

IRT

1000

100

010

1nF

RT (kΩ)

680pF

470pF

330pF

150pF

Figure 2. Oscillator Timing Resistance vs. Frequency.

CLOCK

RAMP PEAK

RAMP VALLEY

VOLTAGE AND CURRENT ERROR AMPLIFIERS

The ML4821 voltage error amplifier is a high open loop

gain, wide bandwidth amplifier with a class A output. The

soft start circuit controls the input to this amplifier for

closed loop soft start operation.

The current error amplifier (IA) is similar to the voltage

error amplifier but is designed for very low offsets to allow

the selection of a low value resistor for RSENSE.

OUTPUT DRIVER STAGE

The ML4821 Output Driver is a 1A peak output high

speed totem pole circuit designed to quickly drive

capacitive loads, such as power MOSFET gates. The driver

circuit’s output voltage is internally limited to 17V.

GAIN MODULATOR

The ML4821 gain modulator responds linearly to current

injected into the ISINE pin, and in an inverse-square

fashion to voltage on the VRMS pin. At very low voltages

on the VRMS pin, the gain modulator enforces a power

limit, or ”brown-out protection”, upon the overall PFC

circuit (Figures 6 and 7). The rectified line input sine wave

is converted to a current for the ISINE input via a dropping

resistor. In this way, most ground noise produces an

insignificant effect on the reference to the PWM

comparator. This gives the ML4821 a high degree of

immunity to the disturbances common in high-power

switching circuits.

ML4821 Datasheet(PDF) 6 Page - Micro Linear Corporation

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