AS2845D-8 datasheet(12/20 Pages) ETC

12 / 20 page

AS2842/3/4/5

Current Mode Controller

ASTEC Semiconductor

1.3 Oscillator

The newly designed oscillator of the AS2842 is

enhanced to give significantly improved perfor-

mance. These enhancements are discussed in

the following paragraphs. The basic operation of

the oscillator is as follows:

A simple RC network is used to program the

frequency and the maximum duty ratio of the

AS2842 output. See Figure 15. Timing capacitor

(CT) is charged through timing resistor (RT) from

the fixed 5.0 V at VREG. During the charging time,

the OUT (pin 6) is high. Assuming that the output

is not terminated by the PWM latch, when the

voltage across CT reaches the upper oscillator

trip point (

≈3.0 V), an internal current sink from

pin 4 to ground is turned on and discharges CT

towards the lower trip point. During this dis-

charge time, an internal clock pulse blanks the

output to its low state. When the voltage across

CT reaches the lower trip point (

≈1.3 V), the

current sink is turned off, the output goes high,

and the cycle repeats. Since the output is blanked

during the discharge of CT, it is the discharge time

which controls the output deadtime and hence,

the maximum duty ratio.

Figure 15. Oscillator Set-up and Waveforms

The nature of the AS2842 oscillator circuit is such

that, for a given frequency, many combinations of

RT and CT are possible. However, only one value

of RT will yield the desired maximum duty ratio at

a given frequency. Since a precise maximum

duty ratio clamp is critical for many power supply

designs, the oscillator discharge current is

trimmed in a unique manner which provides

significantly improved tolerances as explained

later in this section. In addition, the AS2844/5

options have an internal flip-flop which effectively

blanks every other output pulse (the oscillator

runs at twice the output frequency), providing an

absolute maximum 50% duty ratio regardless of

discharge time.

1.3.1 Selecting timing components RT and

The values of RT and CT can be determined

mathematically by the following expressions:

5 V REG

OSCILLATOR

AS2842

6 OUTPUT

7 VCC

5 GND

CLOCK

PWM

OUTPUT

Large RT/Small CT

OUTPUT

Small RT/ Large CT









()

OSC

1.63

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