LTC4269CDKD-2#TRPBF Linear Technology, LTC4269CDKD-2#TRPBF Datasheet - Page 23

IC PD/SYNC FORWARD CTRLR 32-DFN

LTC4269CDKD-2#TRPBF

Manufacturer Part Number
LTC4269CDKD-2#TRPBF
Description
IC PD/SYNC FORWARD CTRLR 32-DFN
Manufacturer
Linear Technology
Type
Power Over Ethernet (PoE)r
Datasheet

Specifications of LTC4269CDKD-2#TRPBF

Applications
Power Interface Switch for Power Over Ethernet (PoE) Devices
Voltage - Supply
14 V ~ 16 V
Operating Temperature
0°C ~ 70°C
Mounting Type
Surface Mount
Package / Case
32-DFN
Current - Supply
1.35mA
Interface
IEEE 802.3af
Controller Type
Powered Device Interface Controller (PD)
Lead Free Status / RoHS Status
Lead free / RoHS Compliant

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applicaTions inForMaTion
Synchronizing
A SYNC pin allows the LTC4269-2 oscillator to be synchro-
nized to an external clock. The SYNC pin can be driven
from a logic-level output, requiring less than 0.8V for a
logic-level low and greater than 2.2V for a logic-level high.
Duty cycle should run between 10% and 90%. To avoid
loss of slope compensation during synchronization, the free
running oscillator frequency, f
to 80% of the external clock frequency (f
resistor chosen for nonsynchronized operation should be
increased by 1.25x (= f
Shutdown and Programming the Power Supply
Undervoltage Lockout
The LTC4269-2 has an accurate 1.32V shutdown threshold
at the SD_V
junction with a resistor divider to define the power supply
undervoltage lockout threshold (UVLO) of the power supply
input voltage (V
before part turn-on, 0µA after part turn-on) allows power
supply UVLO hysteresis to be programmed. Calculation of
the on/off thresholds for the power supply input voltage
can be made as follows:
Connect the PWRGD pin to the resistive divider network
at the SD_V
starting before the PD interface completely charges the
reservoir capacitor, C1 (Figure 9).
The SD_V
be an external source current >11µA to lift the pin past its
1.32V threshold for part turn-on.
Micropower Start-Up: Selection of Start-Up Resistor
and Capacitor for V
The LTC4269-2 uses turn-on voltage hysteresis at the V
pin and low start-up current to allow micropower start-up
(Figure 10). The LTC4269-2 monitors V
allow the part to turn-on at 14.25V and the part to turn-
off at 8.75V. Low start-up current (460µA) allows a large
resistor to be connected between the power supply input
supply and V
V
V
S(OFF)
S(ON)
Threshold = V
SEC
Threshold = 1.32[1 + (R1/R2)]
SEC
SEC
IN
pin must not be left open since there must
. Once the part is turned on, input current
S
pin to prevent the DC/DC converter from
) (Figure 9). A pin current hysteresis (11µA
pin. This threshold can be used in con-
IN
SYNC
S(OFF)
/f
OSC
OSC
+ (11µA • R1)
, should be programmed
).
SYNC
IN
pin voltage to
). The R
SLOPE
IN
increases to drive the IC (5.2mA) and the output drivers
(I
to prevent V
a bias winding in the converter takes over supply to V
This technique allows a simple resistor/capacitor for
start-up which draws low power from the system supply
to the converter.
For system input voltages exceeding the absolute maximum
rating of the LTC4269-2 V
be connected from the V
condition where V
not turn on because SD_V
V
exceeding the rating for the V
should obey V
IN
DRIVE
will continue to charge towards system V
INPUT VOLTAGE (V
). A large enough capacitor is chosen at the V
POWER SUPPLY
C
INPUT VOLTAGE (V
START
R
POWER SUPPLY
R1
R2
START
D1*
Figure 9. Programming Power Supply
Undervoltage Lockout (UVLO)
IN
IN(ONMAX)
Figure 10. Low Power Start-Up
falling below its turn-off threshold before
SD_V
PWRGD
S
IN
*FOR V
(V
)
S
IN ON(MAX)
V
charges past V
)
IN
FROM AUXILIARY WINDING
SEC
S
> 25V, ZENER D1 RECOMMENDED
< V
IN
IN
< V
SEC
11µA
1.32V
pin, an external Zener should
Z
pin to GND. This covers the
Z
1.32V
< 25V)
< 25V.
< 1.32V. In this condition,
IN
pin. The Zener voltage
IN(ON)
+
LTC4269-2
+
but the part does
LTC4269-2
LTC4269-2
42692 F10
IN
42692 F09
, possibly

IN
42692fb
pin
IN
.

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