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
Copyright © 1994
Rev. 1.0b
16
P
RODUCTION DATA SHEET
TYPICAL APPLICATION CIRCUITS
FIGURE 33. — EXTERNAL DUTY CYCLE CLAMP AND MULTI-UNIT
SYNCHRONIZATION
FIGURE 32. — ADJUSTABLE BUFFERED REDUCTION OF CLAMP
LEVEL WITH SOFT-START
Precision duty cycle limiting as well as synchronizing several parts is
possible with the above circuitry.
Soft start and adjustable peak current can be done with the external
circuitry shown above.
f =
(R
A
+ 2R
B
)C
1.44
f =
R
A
+ 2R
B
R
B
V
CS
R
S
V
EAO
- 1.3
5 ( )
R
1
R
2
R
1
+R
2
t
SOFTSTART
= -ln 1 - () C
where; V
EAO
≡ voltage at the Error Amp Output under
minimum line and maximum load conditions.
R
1
R
1
+R
2
R
1
R
1
+R
2
I
PK
= Where: V
CS
= 1.67 () and V
C.S.MAX
= 1V (Typ.)
Unless otherwise specified, pin numbers refer to 8-pin package.
FIGURE 30. — CURRENT SENSE SPIKE SUPPRESSION FIGURE 31. — MOSFET PARASITIC OSCILLATIONS
A resistor (R
1
) in series with the MOSFET gate reduces overshoot &
ringing caused by the MOSFET input capacitance and any inductance
in series with the gate drive. (Note: It is very important to have a low
inductance ground path to insure correct operation of the I.C. This
can be done by making the ground paths as short and as wide as
possible.)
The RC low pass filter will eliminate the leading edge current spike
caused by parasitics of Power MOSFET.
LX155x
3
5
6
7
R
S
C
Q1
V
CC
DC BUS
I
PK
I
PK(MAX)
=
1.0V
R
S
LX155x
6
7
Q1
V
CC
DC BUS
5
R
S
R
1
MPSA63
R
1
R
2
C
1N4148
1
2
4
8
LX155x
5
3
6
7
Q
1
I
PK
V
CS
R
S
V
CC
V
IN
2
6
7
R
B
R
A
5 1
84
3
555
TIMER
4
5
8
LX155x
To other
LX155x devices
0.01
