ML4819CS datasheet(5/15 Pages) MICRO-LINEAR

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ML4819

FUNCTIONAL DESCRIPTION

OSCILLATOR

The ML4819 oscillator charges the external capacitor (CT)

with a current (ISET) equal to 5/RSET. When the capacitor

voltage reaches the upper threshold, the comparator

changes state and the capacitor discharges to the lower

threshold through Q1. While the capacitor is discharging,

the clock provides a high pulse.

The oscillator period can be described by the following

relationship:

tOSC = tRAMP + tDEADTIME

where:

C(Ramp Valley to Peak)

RAMP

SET

and:

C(Ramp Valley to Peak)

8.4mA - I

DEADTIME

SET

The maximum duty cycle of the PWM section can be

limited by setting a threshold on pin 7. when the CT ramp

is above the threshold at pin 7, the PWM output is held

off and the PWM flip-flop is set:

- 0.9)

3.4

LIMIT

OSC

PIN7

≅

where:

DLIMIT = Desired duty cycle limit

DOSC = Oscillator duty cycle

ELECTRICAL CHARACTERISTICS (Continued)

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

UNDER-VOLTAGE LOCKOUT

Start-Up Threshold

Shut-Down Threshold

VREF Good Threshold

4.4

SUPPLY

Supply Current

Start-Up, VCC = 14V

0.6

1.2

Operating, TJ = 25×C25

Note 1: Limits are guaranteed by 100% testing, sampling, or correlation with worst-case test conditions.

Note 2: VCC is raised above the Start-Up Threshold first to activate the IC, then returned to 15V.

Note 3: PWM comparator bias currents are subtracted from this reading.

Figure 1. Oscillator Block Diagram

ISET

CLOCK

DUTY CYCLE

ISET

VREF

+5V

TO PWM

LATCH B

TO PWM

LATCHES

8.4mA

CLOCK

RAMP PEAK

RAMP VALLEY

–

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