TPS23753 Datasheet, PDF (19/27 Pages) Texas Instruments – IEEE 802.3-2005 PoE INTERFACE AND ISOLATED CONVERTER CONTROLLER

TPS23753
www.ti.com.............................................................................................................................................................. SLVS853A – JUNE 2008 – REVISED JUNE 2008
R
S
(Ω)
=
_VSLOPE
_
D
(mV
)
−
V_SLOPE

(mV
)
D
MAX




I
SL
_
EX
(
µA)
⋅1000
(6)
GATE
5/09/08
CS
R
S
R
C
CS
S
Figure 22. Additional Slope Compensation
C_S may be required if the presence of R_S causes increased noise, due to adjacent signals like the gate drive, to
appear at the C_S pin. The TPS23753 has an internal pull-down on C_S ( ~500 Ω ) while the MOSFET is OFF to
reduce cycle-to-cycle carry-over voltage on C_S.
FRS and Synchronization
The FRS pin programs the (free-running) oscillator frequency, and may also be used to synchronize the
TPS23753 converter to a higher frequency. The internal oscillator sets the maximum duty cycle at 80% and
controls the current-compensation ramp circuit. R_FRS should be selected per the following equation.
15000
R
FRS
(k
Ω
)
=
f
SW
(kHz
)
The TPS23753 may be synchronized to an external clock to eliminate beat frequencies from a sampled system,
or to place emission spectrum away from an RF input frequency. Synchronization may be accomplished by
applying a short pulse ( > 25 ns) of magnitude V_SYNC to FRS as shown in Figure 23. R_FRS should be chosen so
that the maximum free-running frequency is just below the desired synchronization frequency. The
synchronization pulse terminates the potential on-time period, and the off-time period doesn’t begin until the
pulse terminates. A short pulse is preferred to avoid reducing the potential on-time.
Figure 23 shows examples of non-isolated and transformer-coupled synchronization circuits The pulse at the
FRS pin should reach between 2.5 V and V_B, with a minimum width of 22 ns (above 2.5 V) and rise/fall times
less than 10 ns. The FRS node should be protected from noise because it is high-impedance.
Synchronization
Pulse
47pF
FRS
RTN
V_SYNC
T_SYNC
4/30 /08
Synchronization
Pulse
V_SYNC
1000pF
T_{S YNC}
1:1
47pF
Example:
Pulse PA0184
Figure 23. Synchronization
FRS
RTN
4/3 0/08
Adapter ORing
Many PoE-capable devices are designed to operate from either a wall adapter or PoE power. A local power
solution adds cost and complexity, but allows a product to be used if PoE is not available in a particular
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