LX1571 Microsemi Corporation, LX1571 Datasheet - Page 9

• Manufacturer Part Number: LX1571
• Description: PHASE MODULATED AC SYNCHRONOUS SECONDARY-SIDE CONTROLLER
• Manufacturer: Microsemi Corporation

Copyright © 1997
Rev. 0.9.3
TIMING SECTION
A capacitor connected from the C
eral functions. First, it sets the OUT DRV duty cycle to a constant
54% (regardless of the C
drive for an N-channel MOSFET, utilizing a simple gate drive
transformer, and b) insure reliable operation with a transformer
duty cycle within a 0 to 50% range. Second, it sets the free-
running frequency of the converter in order to insure the con-
tinuous operation during non-steady state conditions, such as
start up, load transient and current limiting operations. The value
of the timing capacitor is selected so that the free-running fre-
quency is always 20% below the minimum operating frequency
of the secondary transformer voltage, insuring proper operation.
Equation 4 shows how to select the timing capacitor C
where: V
FIGURE 5 — LX157x V
1/97
16V
V
C
START UP
T
OUT DRV
Cap
V
f
I
I
RPP
S
CHG
DISCH
RPP
f
S
I
Peak to peak voltage of C
Free-running frequency of the converter.
Selected to be 80% of the minimum freq.
of the seconday side transformer voltage.
C
C
CHG
1
1
T
T
charging current (3mA typ.)
discharge current (3.5mA typ.)
T
CC
I
value) in order to: a) provide the gate
DISCH
V
START-UP VOLTAGE TIMING DIAGRAM
O
1
P
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
HASE
t
ST
T
pin to ground performs sev-
Equation 4
M
P
ODULATED
R E L I M I N A R Y
T
(0.6V typ.)
V
AC S
V
CAP
O
I C D E S C R I P T I O N
YNCHRONOUS
T
.
10V
D
V
CURRENT LIMITING
Using the main Block Diagram as a reference and the typical
application circuit of Figure 2, note that current limiting is per-
formed by sensing the current in the return line using a current
transformer in series with the switch. The voltage at C.S. pin is
then amplified and compared with an internal threshold. Ex-
ceeding this threshold turns the output drive off and latches it off
until the set input of the PWM latch goes high again. However,
if the current keeps rising such that it exceeds the HICCUP com-
parator threshold, or if the output of the converter drops by
HICCUP comparator pulls C
drive off and causes C
Second, it sets the HICCUP latch, causing the discharge current
to be turned off until the C
Since both charge and discharge currents are disconnected from
the capacitor, the only discharge path for C
current source. When this happens, a very slow discharge oc-
curs, resulting in a long delay time between current limit cycles
which greatly reduces power MOSFET dissipation under short
circuit conditions.
Voltage - 1V / Div.
Voltage - 5V / Div.
S
20% from its regulated point, two things will happen. First, the
RPP
A T A
Example: Assuming the transformer frequency is at 100kHz,
ECONDARY
= 0.6V, I
C
T
L1 Current
Soft-Start
COMP Pin
Output
0 6 80 10
S
.
-S
CHG
FIGURE 6 — START-UP TIMING DIAGRAM
H E E T
IDE
= 3mA, I
C
ONTROLLER
3
T
3 10
DISCH
charging current to be disconnected.
1
1
T
T
capacitor voltage goes below 0.3V.
pin to 6V, which keeps the output
= 3.5mA.
3
3 5 10
.
LX1570/1571
1ms / Div.
1
t
RAMP
3
T
is the internal 2µA
0 033
.
F
9