Datasheet

R
CS
GATE
CS
RTN
R
S
C
S
5/09/08
15000
( )
( )
FRS
SW
R k
f kHz
Ω =
RTN
FRS
R
FRS
47pF
Synchronization
Pulse
4/30/08
V
SYNC
T
SYNC
RTN
FRS
R
FRS
47pF
Synchronization
Pulse
4/30/08
V
SYNC
T
SYNC
1:1
1000pF
Example:
Pulse PA0184
TPS23753
SLVS853C –JUNE 2008–REVISED JANUARY 2010 www.ti.com
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.
(7)
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.
Figure 23. Synchronization
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
installation. While most applications only require that the PD operate when both sources are present, the
TPS23753 supports forced operation from either of the power sources. Figure 24 illustrates three options for
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