AS2845D-8 datasheet(14/20 Pages) ETC

14 / 20 page

AS2842/3/4/5

Current Mode Controller

ASTEC Semiconductor

ing. Standard 2842 devices specify or trim only

for a specific value of discharge current. This

makes precise and repeatable duty ratio clamp-

ing virtually impossible due to other IC toler-

ances. The AS2844/5 provides true 50% duty

ratio clamping by virtue of excluding from its flip-

flop scheme, the normal output blanking associ-

ated with the discharge of CT. Standard AS2844/

5 devices include the output blanking associated

with the discharge of CT, resulting in somewhat

less than a 50% duty ratio.

1.3.3 Synchronization

The advanced design of the AS2842 oscillator

simplifies synchronizing the frequency of two or

more devices to each other or to an external

clock. The RT/CT doubles as a synchronization

input which can easily be driven from any open

collector logic output. Figure 16 shows some

simple circuits for implementing synchroniza-

tion.

1.4 Error amplifier (COMP)

The AS2842 error amplifier is a wide bandwidth,

internally compensated operational amplifier

which provides a high DC open loop gain (90 dB).

The input to the amplifier is a PNP differential

pair. The non-inverting (+) input is internally

connected to the 2.5 V reference, and the invert-

ing (–) input is available at pin 2 (VFB). The output

of the error amplifier consists of an active pull-

down and a 0.8 mA current source pull-up as

shown in Figure 17. This type of output stage

allows easy implementation of soft start, latched

shutdown and reduced current sense clamp func-

tions. It also permits wire “OR-ing” of the error

amplifier outputs of several 2842s, or complete

bypass of the error amplifier when its output is

forced to remain in its “pull-up” condition.

Figure 16. Synchronization

Figure 17. Error Amplifier Compensation

–

COMPENSATION

NETWORK

2.50 V

PWM

E/A

1 COMP

VFB

0.8 mA

VOUT

From

VREG

A 2842

RT/CT

GND

Open

Collector

Output

5 V

RT/CT

Open

Collector

Output

3 K

2 K

RT/CT

3 K

CMOS

SYNC

EXTERNAL CLOCK

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