LX1555MY Microsemi Analog Mixed Signal Group, LX1555MY Datasheet - Page 14
LX1555MY
Manufacturer Part Number
LX1555MY
Description
IC PWM CURRENT MODE 8.4V 8DIP
Manufacturer
Microsemi Analog Mixed Signal Group
Datasheet
1.LX1554CM.pdf
(18 pages)
Specifications of LX1555MY
Pwm Type
Current Mode
Number Of Outputs
1
Frequency - Max
500kHz
Duty Cycle
48%
Voltage - Supply
11 V ~ 25 V
Buck
Yes
Boost
Yes
Flyback
Yes
Inverting
No
Doubler
No
Divider
No
Cuk
No
Isolated
Yes
Operating Temperature
-55°C ~ 125°C
Package / Case
8-DIP
Frequency-max
500kHz
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
LX1552/3/4/5
14
OSCILLATOR
The oscillator is designed such that many values of R
give the same frequency, but only one combination will yield a
specific duty cycle at a given frequency. A set of charts as well
as the timing equations are given to determine approximate
values of timing components for a given frequency and duty
cycle.
1000
FIGURE 27 — OSCILLATOR FREQUENCY vs. TIMING RESISTOR
FIGURE 28 — MAXIMUM DUTY CYCLE vs. TIMING RESISTOR
100
0.1
100
10
90
80
70
60
50
40
30
20
10
1
0
0.1
0.1
C
V
T
C
T
A
CC
T
= 6.8nF
= 25°C
= 22nF
= 15V
(continued)
C
(R
(R
T
= 47nF
T
T
) Timing Resistor - (k )
) Timing Resistor - (k )
1
1
C
T
= 0.1µF
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
U
LTRA
10
10
P
C
-L
T
C
R O D U C T I O N
= 1nF
V
T
T
A
OW
CC
= 3.3nF
= 25°C
T H E O R Y O F O P E R A T I O N
= 15V
S
TART
T
and C
100
100
-U
P
T
C
will
URRENT
D
Given: frequency
Calculate:
Example: A flyback power supply design requires the duty cycle
to be limited to less than 45%. If the output switching frequency
is selected to be 100kHz, what are the values of R
a) LX1552/53, and the b) LX1554/55 ?
, C
A T A
a) LX1552/53
b) LX1554/55
URRENT
1)
Note: R
2)
for duty cycles above 95% use:
3)
Given: f = 100kHz
Dm = 0.45
R
C
f
f
select C
using Figure 27 or Equation 3: R
OUT
OSC
T
T
S
R
operation of oscillator circuit.
C
f
-M
= 267
=
T
T
T
= 200kHz
= ½ f
H E E T
= 277
=
must always be greater than 520
100x10
ODE
R
1.81
1.81 * 0.45
T
1.81 * Dm
T
C
= 1000pf
OSC
f * R
T
PWM
(1.74)
(1.74)
f; maximum duty-cycle
3
(due to internal flip flop)
where R
(1.74)
(1.74)
* 669
T
.45
.55
.45
1
(µf)
1-Dm
Dm
Dm
1
-1
= .012 f
-1
T
-1
-1
5k
= 669
( ), 0.3
T
= 9.1k
Dm
T
Dm
for proper
and C
Copyright © 1994
0.95
Rev. 1.0b
T
for the