AS2845D-8 datasheet(10/20 Pages) ETC

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AS2842/3/4/5

Current Mode Controller

ASTEC Semiconductor

Application Information

The AS2842/3/4/5 family of current-mode control

ICs are low cost, high performance controllers

which are pin compatible with the industry

standard UC2842 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 AS2842 has been enhanced to provide

significantly improved performance, resulting in

exceptionally 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 refer-

ence (

±0.5% of nominal at the error amplifier

input), a significantly reduced propagation delay

from current sense input to the IC output, a

trimmed oscillator for precise duty-cycle clamp-

ing, a modified flip-flop scheme that gives a true

50% duty ratio clamp on 2844/45 types, and

an improved 5 V regulator for better AC noise

immunity. Furthermore, the AS2842 provides

guaranteed performance 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— Theory of Operation

The functional block diagram of the AS2842 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 reference; 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 Undervoltage lockout (UVLO)

The undervoltage lockout function of the AS2842

holds the IC in a low quiescent current (

≤ 1 mA)

“standby” mode until the supply voltage (VCC)

exceeds the upper UVLO threshold voltage. This

guarantees that all of the IC’s 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 (hys-

teresis) to prevent VCC oscillations.

The low quiescent current standby mode of the

AS2842 allows “bootstrapping” — a 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 supply’s main transformer. Figure 14

shows a typical bootstrap circuit where capacitor

Figure 14. Bootstrap Circuit

R <

VDC MIN

1 mA

>1 mA

16 V/10 V (2842/4)

8.4 V/7.8 V (2843/5)

OUT

IC ENABLE

AC LINE

AS284x

VCC

GND

AUX

PRI

SEC

VDC

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