LX1555MY Microsemi Analog Mixed Signal Group, LX1555MY Datasheet - Page 12
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
12
IC DESCRIPTION
The LX1552/3/4/5 series of current mode PWM controller IC's are
designed to offer substantial improvements in the areas of start-
up current and oscillator accuracy when compared to the first
generation products, the UC184x series. While they can be used
in most DC-DC applications, they are optimized for single-ended
designs such as Flyback and Forward converters. The LX1552/
54 series are best suited for off-line applications, whereas the
1553/55 series are mostly used in power supplies with low input
voltages. The IC can be divided into six main sections as shown
in the Block Diagram (page 4): undervoltage lockout and start-
up circuit; voltage reference; oscillator; current sense comparator
and PWM latch; error amplifier; and the output stage.
operation of each section is described in the following sections.
The differences between the members of this family are summa-
rized in Table 1.
UNDERVOLTAGE LOCKOUT
The LX155x undervoltage lock-out is designed to maintain an
ultra low quiescent current of less than 250µA, while guarantee-
ing the IC is fully functional before the output stage is activated.
Comparing this to the SG384x series, a 4x reduction in start-up
current is achieved resulting in 75% less power dissipation in the
start-up resistor. This is especially important in off-line power
supplies which are designed to operate for universal input
voltages of 90 to 265V AC.
start-up current of the LX1554 in conjunction with a bootstrap
winding off of the power transformer. Circuit operation is as
follows.
Figure 23 shows an efficient supply voltage using the ultra low
FIGURE 23 — TYPICAL APPLICATION OF START-UP CIRCUITRY
LX1552
LX1553
LX1554
LX1555
DC BUS
PART #
GND
C
1
Start-up Voltage
8.4V
8.4V
16V
16V
(V
I
1
> 250µA
ST
)
1
ST
TABLE 1
R
C
< 250µA
T
T
UVLO
REF
R
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
Hysterises Voltage
T
/C
LX1554
T
GND
U
V
(V
0.8V
0.8V
IN
LTRA
6V
6V
HYS
P
)
V
-L
O
R O D U C T I O N
OW
D
1
T H E O R Y O F O P E R A T I O N
S
DUTY CYCLE
MAXIMUM
TART
<100%
<100%
<50%
<50%
-U
R
S
P
C
The
URRENT
D
The start-up capacitor (C1) is charged by current through resistor
(R1) minus the start-up current. Resistor (R1) is designed such
that it provides more than 250µA of current (typically 2x I
Once this voltage reaches the start-up threshold, the IC turns on,
starting the switching cycle.
operating current, resulting in discharging the start-up capacitor.
During this time, the auxiliary winding flyback voltage gets
rectified & filtered via (D1) and (C1) and provides sufficient
voltage to continue to operate the IC and support its required
supply current. The start-up capacitor must be large enough such
that during the discharge period, the bootsrap voltage exceeds
the shutdown threshold of the IC.
dissipation vs. maximum start-up current for different devices.
start-up current under low line conditions. Maximum power
dissipation is calculated under maximum line conditions. Ex-
ample assumes 90 to 265VAC universal input application.)
, C
A T A
URRENT
Table 2 below shows a comparison of start-up resistor power
(Resistor R1 is designed such that it provides 2X maximum
Max. Start-up Current
Specification (I
Typical Start-Up
Resistor Value (R
Max. Start-Up Resistor
Power Dissipation (P
Design Using
S
-M
H E E T
ODE
ST
)
PWM
ST
)
R
)
TABLE 2
1000µA
SG384x
2.26W
62K
This causes an increase in IC
UC384xA
124K
500µA
1.13W
LX155x
248K
250µA
0.56W
Copyright © 1994
Rev. 1.0b
ST(max)
).