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
12
P
RODUCTION DATA SHEET
THEORY OF OPERATION
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. The
operation of each section is described in the following sections.
The differences between the members of this family are summa-
rized in Table 1.
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
ST(max)
).
Once this voltage reaches the start-up threshold, the IC turns on,
starting the switching cycle. This causes an increase in IC
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.
Table 2 below shows a comparison of start-up resistor power
dissipation vs. maximum start-up current for different devices.
Max. Start-up Current
Specification (I
ST
)
Typical Start-Up
Resistor Value (R
ST
)
Max. Start-Up Resistor
Power Dissipation (P
R
)
Design Using SG384x UC384xA LX155x
2.26W 1.13W 0.56W
62K
ΩΩ
ΩΩ
Ω 124K
ΩΩ
ΩΩ
Ω 248K
ΩΩ
ΩΩ
Ω
1000µA 500µA 250µA
(Resistor R1 is designed such that it provides 2X maximum
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.)
FIGURE 23 — TYPICAL APPLICATION OF START-UP CIRCUITRY
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.
Figure 23 shows an efficient supply voltage using the ultra low
start-up current of the LX1554 in conjunction with a bootstrap
winding off of the power transformer. Circuit operation is as
follows.
HysterisesVoltage
(V
HYS
)
PART #
Start-up Voltage
(V
ST
)
LX1552
LX1553
LX1554
LX1555
16V
8.4V
16V
8.4V
6V
0.8V
6V
0.8V
<100%
<100%
<50%
<50%
UVLO
MAXIMUM
DUTY CYCLE
TABLE 1
R
S
GND
DC BUS
C
1
D
1
I
1
> 250µA
1
ST
< 250µA
V
IN
REF
R
T
/C
T
V
O
GND
R
T
C
T
LX1554
TABLE 2
