LX1552/3/4/5

PRODUCT D A T ABOOK 1996/1997

13

Copyright © 1994

Rev. 1.0a

1/01

THEO R Y OF OPER A TION

VOLTAGE REFERENCE

The voltage reference is a low drift bandgap design which

provides +5.0V to supply charging current to the oscillator timing

capacitor, as well as supporting internal circuitries.

Initial

accuracy for all devices are specified at ±1% max., which is a 2x

improvement for the commercial product when compared to the

SG384x series. The reference is capable of providing in excess

of 20mA for powering any external control circuitries and has

built-in short circuit protection.

FIGURE 25 — SIMPLIFIED SCHEMATIC OF OSCILLATOR SECTION

OSCILLATOR

The oscillator circuit is designed such that discharge current and

valley voltage are trimmed independently. This results in more

accurate initial oscillator frequency and maximum output duty

cycle, especially important in LX1552/53 applications.

The

oscillator is programmed by the values selected for the timing

components (R

is shown in Figure 25. The operation is as follows; Capacitor (C

is charged from the 5V reference thru resistor (R

voltage of 2.7V nominally.

Once the voltage reaches this

threshold, comparator (A1) changes state, causing (S1) to switch

to position (2) and (S2) to (V

This will allow the

capacitor to discharge with a current equal to the difference

between a constant discharge current (I

charging resistor (I

until the voltage drops down to 1V

nominally and the comparator changes state again, repeating the

cycle. Oscillator charge time results in the output to be in a high

state (on time) and discharge time sets it to a low state (off time).

Since the oscillator period is the sum of the charge and discharge

time, any variations in either of them will ultimately affect stability

of the output frequency and the maximum duty cycle. In fact, this

FIGURE 24 — REFERENCE VOLTAGE vs. TEMPERATURE

4.95

4.99

(T

Ambient Temperature - (°C)

4.97

5.00

4.96

5.01

5.02

4.98

-75

-50

-25

0

25

50

75

100

125

5.03

V

I

FIGURE 26 — DUTY CYCLE VARIATION vs. DISCHARGE CURRENT

20

60

(R

Timing Resistor - ( )

100

40

70

600

700

800

900

1000

T

V

V = 1V

V

30

80

90

50

I

I

I

I

SG384x Lower Limit

LX155x Limits

SG384x Upper Limit

C

T

R

T

I

R

REF

5V

R

I

2

1

OPEN

2.8V

1.1V

S2

V

P

V

V

S1

A1

TO OUTPUT

STAGE

variation is more pronounced when maximum duty cycle has to

be limited to 50% or less. This is due to the fact that for longer

output off time, capacitor discharge current (I

decreased by increasing I

Consequently, this increases the

sensitivity of the frequency and duty cycle to any small variations

of the internal current source (I

critical under those conditions. Because this is a desired feature

in many applications, this parameter is trimmed to a nominal

current value of 8.3±0.3mA at room temperature, and guaranteed

to a maximum range of 7.8 to 8.8mA over the specified ambient

temperature range. Figure 26 shows variation of oscillator duty

cycle versus discharge current for LX155x and SG384x series

devices.