LX1553 MICROSEMI [Microsemi Corporation], LX1553 Datasheet - Page 12

• Manufacturer Part Number: LX1553
• Description: ULTRA-LOW START-UP CURRENT, CURRENT-MODE PWM
• Manufacturer: MICROSEMI [Microsemi Corporation]

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. 1.0a 1/01
ST(max)
).