LX1552 Microsemi Corporation, LX1552 Datasheet - Page 13

• Manufacturer Part Number: LX1552
• Description: ULTRA-LOW START-UP CURRENT/ CURRENT-MODE PWM
• Manufacturer: Microsemi Corporation

Copyright © 1994
Rev. 1.0a 1/01
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.
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.
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.
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.
threshold, comparator (A1) changes state, causing (S1) to switch
to position (2) and (S2) to (V
capacitor to discharge with a current equal to the difference
between a constant discharge current (I
charging resistor (I
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
5.03
5.02
5.01
5.00
4.99
4.98
4.97
4.95
4.96
FIGURE 24 — REFERENCE VOLTAGE vs. TEMPERATURE
-75
T
-50
) and (C
(T
A
) Ambient Temperature - (°C)
R
-25
), until the voltage drops down to 1V
T
). A simplified schematic of the oscillator
0
V
Once the voltage reaches this
25
) position. This will allow the
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
U
50
LTRA
D
P
V
I
L
) and current through
CC
75
-L
= 1mA
R O D U C T I O N
= 15V
OW
T H E O R Y O F O P E R A T I O N
100
S
TART
T
) to a peak
125
-U
P
Initial
C
The
URRENT
T
)
D
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
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.
C
, C
I
R
T
A T A
R
T
URRENT
FIGURE 25 — SIMPLIFIED SCHEMATIC OF OSCILLATOR SECTION
FIGURE 26 — DUTY CYCLE VARIATION vs. DISCHARGE CURRENT
R
100
REF
T
/C
90
80
70
60
50
40
30
20
600
T
S
-M
H E E T
2.8V
SG384x Upper Limit
I
ODE
D
T
V
V = 1V
V
= 8.3mA
A
P
REF
= 25°C
= 2.7V
= 5V
PWM
5V
SG384x Lower Limit
700
(R
2
T
) Timing Resistor - ( )
V
V
S1
P
V
1.1V
LX155x Limits
R
. Consequently, this increases the
OPEN
1
D
800
LX1552/3/4/5
), making this parameter more
S2
I
I
d
d
= 8.0mA
= 8.6mA
A1
I
I
d
d
900
= 7.5mA
= 9.3mA
D
- I
R
) must be
1000
TO OUTPUT
STAGE
13