LX1570 Ver la hoja de datos (PDF) - Microsemi Corporation
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LX1570 Datasheet PDF : 12 Pages
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PRODUCT DATABOOK 1996/1997
PHASE MODULATED AC SYNCHRONOUS SECONDARY-SIDE CONTROLLER
PR E L I M I N A RY D ATA S H E E T
LX1570/1571
IC DESCRIPTION
16V
VSTART UP Cap
VO
tST
VO
VCAP
10V
Output
Voltage - 5V / Div.
Soft-Start
Voltage - 1V / Div.
COMP Pin
tRAMP
OUT DRV
FIGURE 5 — LX157x VCC START-UP VOLTAGE TIMING DIAGRAM
TIMING SECTION
A capacitor connected from the CT pin to ground performs sev-
eral functions. First, it sets the OUT DRV duty cycle to a constant
54% (regardless of the C value) in order to: a) provide the gate
T
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 CT.
CT
=
1
VRPP
∗
fS
∗
1
ICH
G
+
1
IDISCH
Equation 4
where: V ≡ Peak to peak voltage of C (0.6V typ.)
RPP
T
fS ≡ Free-running frequency of the converter.
Selected to be 80% of the minimum freq.
of the seconday side transformer voltage.
ICHG ≡ CT charging current (3mA typ.)
I ≡ C discharge current (3.5mA typ.)
DISCH
T
L1 Current
1ms / Div.
FIGURE 6 — START-UP TIMING DIAGRAM
Example: Assuming the transformer frequency is at 100kHz,
VRPP = 0.6V, ICHG = 3mA, IDISCH = 3.5mA.
CT
=
1
0.6
∗
80
∗103∗
3
∗
1
10−
3
+
1
3.5 ∗10−3
= 0.033µF
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
≈20% from its regulated point, two things will happen. First, the
HICCUP comparator pulls C pin to 6V, which keeps the output
T
drive off and causes C charging current to be disconnected.
T
Second, it sets the HICCUP latch, causing the discharge current
to be turned off until the C capacitor voltage goes below 0.3V.
T
Since both charge and discharge currents are disconnected from
the capacitor, the only discharge path for CT is the internal 2µA
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.
Copyright © 1997
Rev. 0.9.3 1/97
9
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