LTC4269-1 Linear Technology Corporation, LTC4269-1 Datasheet - Page 32

• Manufacturer Part Number: LTC4269-1
• Description: IEEE 802.3 At PD And Synchronous No-Opto Flyback Controller
• Manufacturer: Linear Technology Corporation

www.DataSheet4U.com
APPLICATIONS INFORMATION
LTC4269-1
If we wanted a V
(5%) of hysteresis (on at 36V, off at 34.2V):
Even with good board layout, board noise may cause
problems with UVLO. You can fi lter the divider but keep
large capacitance off the UVLO node because it will slow
the hysteresis produced from the change in bias current.
Figure 13c shows an alternate method of fi ltering by split-
ting the R
more of the resistance on the UVLO side.
Converter Start-Up
The standard topology for the LTC4269-1 utilizes a third
transformer winding on the primary side that provides
both feedback information and local V
LTC4269-1 (see Figure 14). This power bootstrapping
improves converter effi ciency but is not inherently self-
starting. Start-up is affected with an external “trickle charge”
resistor and the LTC4269-1’s internal V
lockout circuit. The V
hysteresis to facilitate start-up.
In operation, the trickle charge resistor, R
to V
of 1mA to charge C
draws only its start-up current. When C
turn-on threshold voltage the LTC4269-1 turns on abruptly
and draws its normal supply current.
Switching action commences and the converter begins to
deliver power to the output. Initially the output voltage is
low and the fl yback voltage is also low, so C
most of the LTC4269-1 current (only a fraction comes
from R
some time (typically tens of milliseconds) the output
voltage approaches its desired value. The fl yback winding
then provides the LTC4269-1 supply current and the V
voltage stabilizes.
32
R
R
IN
B
A
and supplies a small current, typically on the order
=
=
TR
⎛
⎝ ⎝
3 4
A
1 8
.) V
1 23
.
resistor with the capacitor. The split should put
.
36
.
μ
523
V
CC
A
V
V
=
k
voltage continues to drop until, after
IN
–
529
-referred trip point of 36V, with 1.8V
1
TR
⎞
⎠
CC
. Initially the LTC4269-1 is off and
= 18 5
k use
,
undervoltage lockout has wide
. ,
k use
523
k
18 7
TR
.
CC
CC
TR
reaches the V
k
power for the
, is connected
undervoltage
TR
supplies
CC
CC
If C
before stabilization and the LTC4269-1 turns off. The V
node then begins to charge back up via R
threshold, where the part again turns on. Depending upon
the circuit, this may result in either several on-off cycles
before proper operation is reached, or permanent relaxation
oscillation at the V
R
current greater than the maximum rated LTC4269-1 start-up
current, and a worst-case maximum charging current less
than the minimum rated LTC4269-1 supply current.
Make C
behavior described above. This is complicated to deter-
mine theoretically as it depends on the particulars of the
secondary circuit and load behavior. Empirical testing is
recommended. Note that the use of the optional soft-start
function lengthens the power-up timing and requires a
correspondingly larger value for C
TR
R
R R
and
TR
is selected to yield a worst-case minimum charging
TR MAX
TR MIN
is undersized, V
(
(
TR
+
V
IN
V
Figure 14. Typical Power Bootstrapping
large enough to avoid the relaxation oscillatory
CC(ON)
R
)
TR
C
)
TR
>
<
V
THRESHOLD
V
IN MAX
IN MIN
(
(
CC
LTC4269-1
V
I
V
I
VCC
VCC
CC
GND
CC ST MAX
V
PG
node.
0
I
CC
)
)
CC MIN
−
−
(
reaches the V
I
(
VCC
V
V
CC ON MAX
CC ON MIN
_
PG
(
(
)
)
_
_
V
TR
IN
CC
.
)
42691 F14
)
•
•
turn-off threshold
TR
•
to the turn-on
42691f
CC