IC PD/OPTO FLYBACK CTRLR 32-DFN

LTC4269CDKD-1#PBF

Manufacturer Part NumberLTC4269CDKD-1#PBF
DescriptionIC PD/OPTO FLYBACK CTRLR 32-DFN
ManufacturerLinear Technology
TypePower Over Ethernet (PoE)
LTC4269CDKD-1#PBF datasheet
 


Specifications of LTC4269CDKD-1#PBF

ApplicationsPower Interface Switch for Power Over Ethernet (PoE) DevicesVoltage - Supply14 V ~ 16 V
Operating Temperature0°C ~ 70°CMounting TypeSurface Mount
Package / Case32-DFNCurrent - Supply1.35mA
InterfaceIEEE 802.3afController TypePowered Device Interface Controller (PD)
Input Voltage60VSupply Current6.4mA
Digital Ic Case StyleDFNNo. Of Pins32
Duty Cycle (%)88%Frequency100kHz
Operating Temperature Range0°C To +70°CMslMSL 1 - Unlimited
Rohs CompliantYesOperating Temperature (max)70C
Operating Temperature (min)0CPin Count32
MountingSurface MountPackage TypeDFN EP
Case Length7mmScreening LevelCommercial
Lead Free Status / RoHS StatusLead free / RoHS Compliant  
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APPLICATIONS INFORMATION
If we wanted a V
-referred trip point of 36V, with 1.8V
IN
(5%) of hysteresis (on at 36V, off at 34.2V):
1.8V
=
= 529k, use 523k
R
A
3.4µA
523k
=
= 18.5k, use 18.7k
R
B
36V
– 1
⎝ ⎜
⎠ ⎟
1.23V
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
resistor with the capacitor. The split should put
A
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
undervoltage lockout has wide
CC
hysteresis to facilitate start-up.
In operation, the trickle charge resistor, R
to V
and supplies a small current, typically on the order
IN
of 1mA to charge C
. Initially the LTC4269-1 is off and
TR
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
.) V
voltage continues to drop until, after
TR
CC
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.
If C
is undersized, V
TR
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
V
IN
R
TR
+
C
TR
V
CC(ON)
power for the
CC
Figure 14. Typical Power Bootstrapping
undervoltage
CC
R
is selected to yield a worst-case minimum charging
TR
current greater than the maximum rated LTC4269-1 start-up
current, and a worst-case maximum charging current less
, is connected
TR
than the minimum rated LTC4269-1 supply current.
R
reaches the V
TR(MAX)
TR
CC
and
>
R
TR(MIN)
supplies
TR
Make C
large enough to avoid the relaxation oscillatory
TR
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
CC
recommended. Note that the use of the optional soft-start
function lengthens the power-up timing and requires a
correspondingly larger value for C
LTC4269-1
reaches the V
turn-off threshold
CC
CC
to the turn-on
TR
node.
CC
V
IN
I
VCC
V
CC
LTC4269-1
PG
GND
THRESHOLD
V
VCC
I
VCC
0
V
PG
42691 F14
− V
V
IN(MIN)
CC(ON _ MAX)
<
I
CC(ST _ MAX)
− V
V
IN(MAX)
CC(ON _ MIN)
I
CC(MIN)
.
TR
CC
42691fb
31