TPS23754PWPR datasheet(21/41 Pages) TI

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t-Time-200 s/div

I PI

16VInputstep

V RTN-VSS

V VDD-VSS

CIN completescharge

whileconverteroperates

RecoveryfromPIdropout

VRTN <12V@400 s

Converter Controller Features

TPS23754

TPS23754-1

TPS23756

www.ti.com ....................................................................................................................................................... SLVS885B – OCTOBER 2008 – REVISED MAY 2009

causing a PD reset, provided that recovery lies within the bounds of the hotswap and PSE protection. Figure 26

shows an example of recovery from a 16 V PSE rising voltage step. The hotswap MOSFET goes into current

limit, overshooting to a relatively low current, recovers to ~950 mA full current limit, and charges the input

capacitor while the converter continues to run. The MOSFET did not go into foldback because VRTN-VVSS was

below 12 V after the 400

µs deglitch.

Figure 26. 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 VDD to RTN short cause high power dissipation in the MOSFET. An over-temperature shutdown

(OTSD) turns off the hotswap MOSFET and class regulator, which are restarted after the device cools. The

hotswap MOSFET will be re-enabled with the inrush current limit when exiting from an over-temperature event.

Pulling DEN to VSS during powered operation causes the internal hotswap MOSFET to turn off. This feature

allows a PD with Option three ORing per Figure 27 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:

1. VAPD above VAPDEN (~1.5 V)

2. VDEN < VPD-DIS when VVDD– VVSS is in the operational range

3. PD over-temperature

4. (VVDD– VVSS) < PoE UVLO (~30.5 V).

The TPS23754 dc/dc controller implements a typical current-mode control as shown in the Functional Block

Diagram. Features include oscillator, over-current and PWM comparators, current-sense blanker, dead-time

control, softstart, and gate driver. In addition, an internal slope-compensation ramp generator, frequency

synchronization logic, thermal shutdown, and startup current source with control are provided.

The TPS23754 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.

There is an offset of VZDC (~1.5 V) and 2:1 resistor divider between the CTL pin and the PWM. A VCTL below

VZDC will stop converter switching, while voltages above (VZDC + (2 × VCSMAX)) will not increase the requested

peak current in the switching MOSFET. Optocoupler biasing design is eased by this limited control range.

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TPS23754PWPR Datasheet(PDF) 21 Page - Texas Instruments

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