AS3843D813 datasheet(12/20 Pages) ETC

12 / 20 page

ASTEC Semiconductor

the following paragraphs. The basic operation of

the oscillator is as follows:

A simple RC network is used to program the fre-

quency and the maximum duty ratio of the

AS3842 output. See Figure 15. Timing capacitor

T) is charged through timing resistor (RT) from

the fixed 5.0 V at V

REG. 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 C

T reaches the upper oscillator

trip point (Å3.0 V), an internal current sink from

pin 4 to ground is turned on and discharges C

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

T reaches the lower trip point (Å1.3 V), the cur-

rent sink is turned off, the output goes high, and

the cycle repeats. Since the output is blanked

during the discharge of C

T, it is the discharge

time which controls the output deadtime and

hence, the maximum duty ratio.

The nature of the AS3842 oscillator circuit is such

that, for a given frequency, many combinations of

T and CT are possible. However, only one value

of R

T 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 sig-

nificantly improved tolerances as explained later

in this section. In addition, the AS3844/5 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 R

and C

The values of R

T and CT can be determined

mathematically by the following expressions:

(1)

AS384x

Current Mode Controller

5 V REG

OSCILLATOR

AS3842

6 OUTPUT

7 V

5 GND

CLOCK

PWM

OUTPUT

Large R

T/Small CT

OUTPUT

Small R

T/ Large CT

Figure 15. Oscillator Set-up and Waveforms









OSC

1.63D

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