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IX9908NTR

P1-P3P4-P6P7-P9P10-P10
I
NTEGRATED
C
IRCUITS
D
IVISION
IX9908
R02 www.ixysic.com 7
2.2 Soft-Start
Once V
CC
reaches V
VCC_on
(typically 18V), the device
will initiate a soft-start sequence. This is intended to
minimize the electrical stresses on Q1, D
OUT
, D
VCC
,
and the transformer. The soft-start operates as shown
in Figure 3. The duration of this soft-start is 12mS
nominal and steps V
CS
, the current sense voltage, to
four values, as shown.
Figure 3. Soft-Start
2.3 Normal Operation
Because the IX9908 employs quasi-resonant
operation, its PWM switch-on is set by the zero
crossing of the auxiliary winding voltage, and the
switch-off is set by the current sense voltage.
2.3.1 Zero Crossing & Switch-On Determination
As the application schematic on Page 1 shows, the
voltage from the auxiliary winding is connected to the
zero crossing pin, ZCV, through an RC network. This
network provides a delay so that switch-on can occur
at voltage valley thus enhancing efficiency. The
required time delay, t, should be approximately
one-fourth of the oscillation period (determined by
transformer primary inductor and drain-source
capacitance of Q1) minus the propagation delay from
zero-cross detect to Q1 switch-on, t
delay
.
This time delay, t, should be matched by adjusting
the RC network.
After Q1 is turned off, its V
DS
will show some
oscillation. This will also show on the ZCV input. To
avoid a mis-triggered Q1 turn-on, a ringing
suppression circuit is implemented. The suppression
time has two values that depend on the voltage at
ZCV. If V
ZCV
is greater than 0.7V, then the time is
2.5S nominal. If V
ZCV
is less than 0.7V, then the time
is 42S nominal. Turn-on of Q1 can not occur during
the suppression time, but does occur after a
zero-crossing is detected. In the case of a missed
zero-crossing, a maximum off-time is implemented.
After Q1 has been off for 42S nominal (t
offMax
), it is
turned back on.
2.3.2 Switch-Off Determination
In the application circuit, the primary current is sensed
by R
CS
. The voltage across this resistor, V
CS
, is
applied to the CS input of the device. It is processed
internally, and compared to the voltage at the VR pin,
which is a scaled version of the rectified line voltage.
When the following relation is true, the power switch,
Q1, is turned off.
Leading-edge blanking is used to prevent a false
trigger caused by the voltage spike across R
CS
at the
moment of Q1 turn-on. This blanking time, t
BLKCS
, is
nominally 330nS. To prevent transformer saturation, a
maximum on-time circuit is implemented. Max on-time
for Q1 (GD=H) is 30S nominal.
Time (ms)
012345678 9 101112131415
V
CS_SST
(V)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
Maximum Current (Sense Voltage)
During Soft-Start
t =
t
OSC
- t
delay
4
t
RC
= C
ZCV
(R
ZCV1
// R
ZCV2
)
V
CS
30.7V+ VR
I
NTEGRATED
C
IRCUITS
D
IVISION
IX9908
8 www.ixysic.com R02
2.3.3 Foldback Point Correction
When the AC line voltage increases, the Q1 on-time
decreases, which increases the operating frequency.
As a result, with a constant primary current limit, the
output power increases. To provide output power
regulation with respect to line voltage, the internal
foldback point correction circuit varies the V
CS
limit.
The V
CS
limit is decreased in response to an increase
in AC line voltage. The relationship between V
CSMax
and V
IN
is shown in Figure 4.
Figure 4. V
CSMax
vs. V
IN
The variation in AC line voltage is sensed by way of
the auxiliary winding and an internal clamp and
current sense circuit. When Q1 is on, a negative
voltage proportional to the line voltage is coupled to
the auxiliary winding; the IC will hold the ZCV pin very
close to ground during this time. The line voltage is
thus sensed indirectly through the current in R
ZCV1
.
This current is given by:
The device uses I
ZCV
to vary the V
CS
limit as shown in
Figure 4. The actual implementation is digital and is
shown below:
Figure 5. V
CS
vs. I
ZC
2.3.4 Protection Functions
The IX9908 provides comprehensive protection
features. They are summarized in the table below:
OUTPUT OVER-VOLTAGE
During the Q1 off-time the auxiliary winding voltage
(V
AUX
) will swing positive and in proportion to the
secondary voltage. V
AUX
is connected to ZCV through
a resistor divider. If the voltage at ZCV exceeds a
preset threshold (V
ZCVOVP
) for longer than the blanking
time (t
ZCVOVP
), then the IC is latched off.
SHORTED WINDING
If the voltage at CS exceeds a preset threshold
(V
CSSW
) during Q1 on time the device is latched off.
OVER-TEMPERATURE
If the die temperature exceeds 140°C, then the device
will enter the Auto-Restart Mode.
V
CC
OVER-VOLTAGE / UNDER-VOLTAGE
The IC continuously monitors the V
CC
voltage. In case
of an over-voltage, Q1 is turned off (GD=L) and V
CC
will begin to fall. Once V
CC
goes below V
VCC_off
(10.5V
nominal), the startup circuit is activated, and begins to
charge C
VCC
. When V
CC
exceeds V
VCC_on
(18.0V
nominal), the device initiates a new soft-start. For an
under-voltage the operation is the same except that
the sequence begins with V
CC
< V
VCC_off
so GD=L and
the startup circuit is activated. This operation
describes the Auto-Restart Mode.
During Latch-Off Mode, V
CC
also cycles between
V
VCC_off
and V
VCC_on
, but GD remains low, and no
soft-start is initiated. The line voltage must be turned
off and on again to begin normal operation.
I
ZCV
=
V
IN
• Na
R
ZCV1
• Np
I
ZC
(µA)
200 600 1000 1400 1800 2200
V
CS
(V)
0.60
0.65
0.70
0.75
0.80
0.85
0.90
0.95
1.00
1.05
V
CS
vs. I
ZC
Fault Condition Action Taken
Output Over-Voltage GD Latched Off
Shorted Winding GD Latched Off
Over-Temperature Auto-Restart Mode
VCC Over-Voltage Auto-Restart Mode
VCC Under-Voltage Auto-Restart Mode
I
NTEGRATED
C
IRCUITS
D
IVISION
IX9908
R02 www.ixysic.com 9
3. Manufacturing Information
3.1 Moisture Sensitivity
All plastic encapsulated semiconductor packages are susceptible to moisture ingression. IXYS Integrated
Circuits Division classified all of its plastic encapsulated devices for moisture sensitivity according to the
latest version of the joint industry standard, IPC/JEDEC J-STD-020, in force at the time of product
evaluation. We test all of our products to the maximum conditions set forth in the standard, and guarantee
proper operation of our devices when handled according to the limitations and information in that standard as well as
to any limitations set forth in the information or standards referenced below.
Failure to adhere to the warnings or limitations as established by the listed specifications could result in reduced
product performance, reduction of operable life, and/or reduction of overall reliability.
This product carries a Moisture Sensitivity Level (MSL) rating as shown below, and should be handled according to
the requirements of the latest version of the joint industry standard IPC/JEDEC J-STD-033.
3.2 ESD Sensitivity
This product is ESD Sensitive, and should be handled according to the industry standard
JESD-625.
3.3 Soldering Profile
This product has a maximum body temperature and time rating for lead-free reflow processes as shown below. All
other guidelines of J-STD-020 must be observed.
3.4 Board Wash
IXYS Integrated Circuits Division recommends the use of no-clean flux formulations. However, board washing to
remove flux residue is acceptable, and the use of a short drying bake may be necessary. Chlorine-based or
Fluorine-based solvents or fluxes should not be used. Cleaning methods that employ ultrasonic energy should not be
used.
Device Moisture Sensitivity Level (MSL) Rating
IX9908N MSL 1
Device Maximum Temperature x Time
IX9908N 260°C for 30 seconds
P1-P3P4-P6P7-P9P10-P10

IX9908NTR

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LED Lighting Drivers 8Pin SOIC LED Driver Low Power
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