LX1552 Microsemi Corporation, LX1552 Datasheet - Page 14

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

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. 1.0a 1/01
T
for the