Datasheet

TPS23753
www.ti.com SLVS853C –JUNE 2008–REVISED JANUARY 2010
Figure 20. Response to PSE Step Voltage
The PD control has a thermal sensor that protects the internal hotswap MOSFET. Conditions like startup or
operation into a V
DD
to RTN short cause high power dissipation in the MOSFET. An overtemperature shutdown
(OTSD) turns off the hotswap MOSFET and class regulator, which are restarted after the device cools. The PD
state machine will always restart in inrush current limit when exiting from a PD overtemperature event.
Pulling DEN to V
SS
during powered operation causes the internal hotswap MOSFET to turn off. This feature
allows a PD with secondary-side adapter ORing to achieve adapter priority. Care must be taken with
synchronous converter topologies that can deliver power in both directions.
The hotswap switch will be forced off under the following conditions:
• V
APD
above V
APDEN
(~1.5 V)
• V
DE N
≤ V
PD_DIS
when V
VDD-VSS
is in the operational range
• PD over temperature
• V
VDD-VSS
< PoE UVLO (~30.5 V).
Converter Controller Features
The TPS23753 dc/dc controller implements a typical current-mode control as shown in Figure 2. Features
include oscillator, overcurrent and PWM comparators, current-sense blanker, softstart, and gate driver. In
addition, an internal current-compensation ramp generator, frequency synchronization logic, thermal shutdown,
and startup current source with control are provided.
The TPS23753 is optimized for isolated converters, and does not provide an internal error amplifier. Instead, the
optocoupler feedback is directly fed to the CTL pin which serves as a current-demand control for the PWM and
converter. There is an offset of V
ZDC
(~1.5 V) and 2:1 resistor divider between the CTL pin and the PWM. A V
CTL
below V
ZDC
will stop converter switching, while voltages above (V
ZDC
+ 2 × V
CSMAX
) will not increase the
requested peak current in the switching MOSFET. Optocoupler biasing design is eased by this limited control
range.
The internal startup current source and control logic implement a bootstrap-type startup. The startup current
source charges C
VC
from V
DD1
when the converter is disabled (either by the PD control or the V
C
control), while
operational power must come from a converter (bias winding) output. Loading on V
C
and V
B
must be minimal
while C
VC
charges, otherwise the converter may never start. The optocoupler will not load V
B
when the converter
is off. The converter will shut off when V
C
falls below its lower UVLO. This can happen when power is removed
from the PD, or during a fault on a converter output rail. When one output is shorted, all the outputs fall in voltage
including the one that powers V
C
. The control circuit discharges V
C
until it hits the lower UVLO and turns off. A
restart will initiate as described in "Startup and Converter Operation" if the converter turns off and there is
sufficient V
DD1
voltage. This type of operation is sometimes referred to as “hiccup mode,” which provides robust
output short protection by providing time-average heating reduction of the output rectifier.
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