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ADPTR_ON
PPDEN
PPD1
PPDEN
PPD
PPD2
PP DEN
PPDH
ADP TR_OFF
PPDEN
PPDH
P PD1
PP D
PPD2
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TPS23754
TPS23754-1
TPS23756
SLVS885D – OCTOBER 2008 – REVISED DECEMBER 2009
www.ti.com
GAT2
GAT2 is the second gate drive output for the dc/dc converter. GAT2’s phase turns the second switch off when it
transitions high, and on when it transitions low. This drives active-clamp PMOS devices per
Figure 1, and driven
flyback synchronous rectifiers per
Figure 28. See the DT Pin Description for GATE to GAT2 timing. Connecting
DT to VB disables GAT2 in a high-impedance condition. GAT2 is low when the converter is disabled.
PPD
PPD is a multifunction pin that has two voltage thresholds, PPD1 and PPD2.
PPD1 permits power to come from an external low voltage adapter, e.g., 24 V, connected from VDD to VSS by
over-riding the normal hotswap UVLO. Voltage on PPD above 1.55 V (VPPDEN) enables the hotswap MOSFET,
inhibits class current, and enables T2P. A resistor divider per
Figure 35 provides ESD protection, leakage
discharge for the adapter ORing diode, reverse adapter protection, and input voltage qualification. Voltage
qualification assures the adapter output voltage is high enough that it can support the PD before it begins to draw
current.
(5)
PPD2 enables normal class regulator operation when VPPD is above 8.3 V to permit type 2 classification when
APD is used in conjunction with diode DVDD (see Figure 34). Tie PPD to VDD when PPD2 operation is desired. The PPD pin has a 5
μA internal pull-down current.
Locate the PPD pull-down resistor adjacent to the pin when used.
PPD may be tied to VSS or left open when not used.
RTN, ARTN, COM
RTN is internally connected to the drain of the PoE hotswap MOSFET, while ARTN is the quiet analog reference
for the dc/dc controller return. COM serves as the return path for the gate drivers and should be tied to ARTN on
the circuit board. The ARTN / COM / RTN net should be treated as a local reference plane (ground plane) for the
dc/dc control and converter primary. RTN and (ARTN/COM) may be separated by several volts for special
applications.
T2P
T2P is an active low output that indicates [ (VAPD > 1.5 V) OR (1.55 V ≤ VPPD ≤ 8.3 V) OR (type 2 hardware
classification observed) ]. T2P is valid after both a delay of tT2P from the start of converter switching, and [VCTL ≤
(VB – 1 V)]. Once T2P is valid, VCTL will not effect it. T2P will become invalid if the converter goes back into
softstart, over-temperature, or is held off by the PD during CIN recharge (inrush). T2P is referenced to ARTN and
is intended to drive the diode side of an optocoupler. T2P should be left open or tied to ARTN if not used.
VB
VB is an internal 5.1V regulated dc/dc controller supply rail that is typically bypassed by a 0.1 μF capacitor to
ARTN. VB should be used to bias the feedback optocoupler.
VC
VC is the bias supply for the dc/dc controller. The MOSFET gate drivers run directly from VC. VB is regulated
down from VC, and is the bias voltage for the rest of the converter control. A startup current source from VDD1 to
VC is controlled by a comparator with hysteresis to implement the converter bootstrap startup. VC must be
connected to a bias source, such as a converter auxiliary output, during normal operation.
10
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