OBSOLETE PRODUCT
NOT RECOMMENDED FOR NEW DESIGNS
U
LTRA
-L
OW
S
TART
-U
P
C
URRENT
, C
URRENT
-M
ODE
PWM
LX1552/3/4/5
PRODUCT DATABOOK 1996/1997
13
Copyright © 1994
Rev. 1.0b
PRODUCTION DATA SHEET
THEORY OF OPERATION
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
T
) and (C
T
). A simplified schematic of the oscillator
is shown in Figure 25. The operation is as follows; Capacitor (C
T
)
is charged from the 5V reference thru resistor (R
T
) to a peak
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
V
) position. This will allow the
capacitor to discharge with a current equal to the difference
between a constant discharge current (I
D
) and current through
charging resistor (I
R
), 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
A
) Ambient Temperature - (°C)
(V
REF
) Reference Voltage - (V)
4.97
5.00
4.96
5.01
5.02
4.98
-75
-50 -25 0 25 50 75 100 125
5.03
V
CC
= 15V
I
L
= 1mA
FIGURE 26 — DUTY CYCLE VARIATION vs. DISCHARGE CURRENT
20
60
(R
T
) Timing Resistor - ()
100
Oscillator Duty Cycle - (%)
40
70
600 700 800 900
1000
T
A
= 25°C
V
P
= 2.7V
V = 1V
V
REF
= 5V
30
80
90
50
I
d
= 7.5mA
I
d
= 8.0mA
I
d
= 8.6mA
I
d
= 9.3mA
SG384x Lower Limit
LX155x Limits
SG384x Upper Limit
C
T
R
T
I
R
REF
5V
R
T
/C
T
I
D
= 8.3mA
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
D
- I
R
) must be
decreased by increasing I
R
. Consequently, this increases the
sensitivity of the frequency and duty cycle to any small variations
of the internal current source (I
D
), making this parameter more
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.
