LX1553MY データシートの表示（PDF） - Microsemi Corporation

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LX1553MY

ULTRA-LOW START-UP CURRENT, CURRENT-MODE PWM

Microsemi Corporation 

PRODUCT DATABOOK 1996/1997

LX1552/3/4/5

ULTRA-LOW START-UP CURRENT, CURRENT-MODE PWM

PRODUCTION DATA SHEET

THEORY OF OPERATION

IC DESCRIPTION

The LX1552/3/4/5 series of current mode PWM controller IC's are

designed to offer substantial improvements in the areas of start-

up current and oscillator accuracy when compared to the first

generation products, the UC184x series. While they can be used

in most DC-DC applications, they are optimized for single-ended

designs such as Flyback and Forward converters. The LX1552/

54 series are best suited for off-line applications, whereas the

1553/55 series are mostly used in power supplies with low input

voltages. The IC can be divided into six main sections as shown

in the Block Diagram (page 4): undervoltage lockout and start-

up circuit; voltage reference; oscillator; current sense comparator

and PWM latch; error amplifier; and the output stage. The

operation of each section is described in the following sections.

The differences between the members of this family are summa-

rized in Table 1.

TABLE 1

PART #

UVLO

Start-up Voltage Hysterises Voltage

(VST)

(VHYS)

MAXIMUM

DUTY CYCLE

LX1552

LX1553

LX1554

LX1555

16V

8.4V

16V

8.4V

6V

0.8V

6V

0.8V

<100%

<100%

<50%

<50%

UNDERVOLTAGE LOCKOUT

The LX155x undervoltage lock-out is designed to maintain an

ultra low quiescent current of less than 250µA, while guarantee-

ing the IC is fully functional before the output stage is activated.

Comparing this to the SG384x series, a 4x reduction in start-up

current is achieved resulting in 75% less power dissipation in the

start-up resistor. This is especially important in off-line power

supplies which are designed to operate for universal input

voltages of 90 to 265V AC.

Figure 23 shows an efficient supply voltage using the ultra low

start-up current of the LX1554 in conjunction with a bootstrap

winding off of the power transformer. Circuit operation is as

follows.

DC BUS

I1 > 250µA

D1

The start-up capacitor (C1) is charged by current through resistor

(R1) minus the start-up current. Resistor (R1) is designed such

that it provides more than 250µA of current (typically 2x I ).

ST(max)

Once this voltage reaches the start-up threshold, the IC turns on,

starting the switching cycle. This causes an increase in IC

operating current, resulting in discharging the start-up capacitor.

During this time, the auxiliary winding flyback voltage gets

rectified & filtered via (D1) and (C1) and provides sufficient

voltage to continue to operate the IC and support its required

supply current. The start-up capacitor must be large enough such

that during the discharge period, the bootsrap voltage exceeds

the shutdown threshold of the IC.

Table 2 below shows a comparison of start-up resistor power

dissipation vs. maximum start-up current for different devices.

TABLE 2

Design Using

SG384x UC384xA LX155x

Max. Start-up Current

Specification (IST)

Typical Start-Up

Resistor Value (RST)

Max. Start-Up Resistor

Power Dissipation (PR)

1000µA 500µA 250µA

62KΩ 124KΩ 248KΩ

2.26W 1.13W 0.56W

(Resistor R1 is designed such that it provides 2X maximum

start-up current under low line conditions. Maximum power

dissipation is calculated under maximum line conditions. Ex-

ample assumes 90 to 265VAC universal input application.)

1ST < 250µA

REF VIN

C1

RT

LX1554

VO

RT/CT

GND

CT

GND

RS

FIGURE 23 — TYPICAL APPLICATION OF START-UP CIRCUITRY

12

Copyright © 1994

Rev. 1.0a 1/01

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