AS384X datasheet(10/20 Pages) ETC

10 / 20 page

ASTEC Semiconductor

Application Information

The AS3842/3/4/5 family of current-mode control

ICs are low cost, high performance controllers

which are pin compatible with the industry stan-

dard UC3842 series of devices. Suitable for

many switch mode power supply applications,

these ICs have been optimized for use in high

frequency off-line and DC-DC converters. The

AS3842 has been enhanced to provide signifi-

cantly improved performance, resulting in excep-

tionally better tolerances in power supply

manufacturing. In addition, all electrical charac-

teristics are guaranteed over the full 0 to 105¡C

temperature range. Among the many enhance-

ments are: a precision trimmed 2.5 volt reference

(+/Ð 1% of nominal at the error amplifier input), a

significantly reduced propagation delay from cur-

rent sense input to the IC output, a trimmed oscil-

lator for precise duty-cycle clamping, a modified

flip-flop scheme that gives a true 50% duty ratio

clamp on 3844/45 types, and an improved 5 V

regulator for better AC noise immunity. Further-

more, the AS3842 provides guaranteed perfor-

mance with current sense input below ground.

The advanced oscillator design greatly simplifies

synchronization. The device is more completely

specified to guarantee all parameters that impact

power supply manufacturing tolerances.

Section 1 Ð Theory of Operation

The functional block diagram of the AS3842 is

shown in Figure 1. The IC is comprised of the six

basic functions necessary to implement current

mode control; the under-voltage lockout; the refer-

ence; the oscillator; the error amplifier; the current

sense comparator/PWM latch; and the output.

The following paragraphs will describe the theory

of operation of each of the functional blocks.

1.1 Under-voltage lockout (UVLO)

The under-voltage lockout function of the

AS3842 holds the IC in a low quiescent current

(² 1 mA) ÒstandbyÓ mode until the supply voltage

CC) exceeds the upper UVLO threshold volt-

age. This guarantees that all of the ICÕs internal

circuitry are properly biased and fully functional

before the output stage is enabled. Once the IC

turns on, the UVLO threshold shifts to a lower

level (hysteresis) to prevent V

CC oscillations.

The low quiescent current standby mode of the

AS3842 allows ÒbootstrappingÓÐÑa technique

used in off-line converters to start the IC from the

rectified AC line voltage initially, after which power

to the IC is provided by an auxiliary winding off the

power supplyÕs main transformer. Figure 14 shows

a typical bootstrap circuit where capacitor (C) is

AS384x

Current Mode Controller

R <

DC MIN

1 mA

>1 mA

16 V/10 V (3842/4)

8.4 V/7.8 V (3843/5)

OUT

IC ENABLE

AC LINE

AS384x

GND

AUX

PRI

SEC

Figure 14. Bootstrap Circuit

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