SI3400-C-GM Datasheet SI3400-E1-GM, SI3400-C-GM, SI3400-BZ-GM | P13-P15

Si3400/Si3401

Rev. 0.9 13

3.2.6. Power Loss Indicator

A situation can occur in which power is lost at the input

of the diode bridge and the hotswap controller does not

detect the fault due to the VPOS to VNEG capacitor

maintaining the voltage. In such a situation, the PD can

remain operational for hundreds of microseconds

despite the PSE having removed the line voltage. If it is

recognized early enough, the time from power loss to

power failure can provide valuable time to gracefully

shut down an application.

Due to integration of the diode bridges, the Si3400 and

Si3401 are able to instantaneously detect the removal

of the line voltage and provide that early warning signal

to the PD application. The PLOSS

pin is an open drain

output that pulls up to VPOS when a line voltage greater

than 27 V is applied. When the line voltage falls below

27 V, the output becomes high-impedance, allowing an

external pull-down resistor to change the logic state of

PLOSS. The benefit of this indicator is that the powered

device may include a microcontroller that can quickly

save its memory or operational state before draining the

supply capacitors and powering itself down. This feature

can help improve overall manageability in applications,

such as wireless access points.

3.3. Switching Regulator

Power over Ethernet (PoE) applications fall into two

broad categories, isolated and non-isolated. Non-

isolated systems can be used when the powered device

is self-contained and does not provide external

conductors to the user or another application. Non-

isolated applications include wireless access points and

security cameras. In these applications, there is no

explicit need for dc isolation between the switching

regulator output and the hotswap interface. An isolated

system must be used when the powered device

interfaces with other self-powered equipment or has

external conductors accessible to the user or other

applications. For proper operation, the regulated output

supply of the switching regulator must not have a dc

electrical path to the hotswap interface or switching

regulator primary side. Isolated applications include

point-of-sale terminals where the user can touch the

grounded metal chassis.

The application determines the converter topology. An

isolated application will require a flyback transformer-

based switching topology while a non-isolated

application can use an inductor-based buck converter

topology. In the isolated case, dc isolation is achieved

through a transformer in the forward path and a voltage

reference plus opto-isolator in the feedback path. The

application circuit shown in Figure 2 is an example of

such a topology. The non-isolated application in

Figure 1 makes use of a single inductor as the energy

conversion element, and the feedback signal is directly

supplied into the internal error amplifier. As can be seen

from the application circuits, the isolated topology has

an increased number of components, thus increasing

the bill of materials (BOM) and system footprint.

To optimize cost and ease implementation, each

application should be evaluated for its isolated or non-

isolated requirements.

Figure 4. Switcher Block Diagram

VSS1

SWITCH

DRIVE

SWITCHER

STARTUP & BIAS

IABS

ITC

VREF

PULSE-

WIDTH

MODULATOR

OSCILLATOR

ERROR

AMPLIFIER

EROUT

FBSSFT

VDD

SWO

VPOSS

HSO

VSS2

VPOSF

VSSA

ISOSSFT

Si3400/Si3401

14 Rev. 0.9

3.3.1. Switcher Startup

The switching regulator is disabled until the hotswap

interface has both identified itself to the PSE and

charged the supply capacitor needed to filter the

switching regulator's high-current transients. Once the

supply capacitor is charged, the hotswap controller

engages the internal bias currents and supplies used by

the switcher. Additionally, the soft-start current begins to

charge the external soft-start capacitor.

The voltage developed across the soft-start capacitor

serves as the error amplifier's reference in the non-

isolated application. Ramping this voltage slowly allows

the switching regulator to bring up the regulated output

voltage in a controlled manner. Controlling the initial

startup of the regulated voltage restrains power

dissipation in the switching FET and prevents overshoot

and ringing in the output supply voltage.

In the isolated mode, a capacitor connected between

pins ISOSSFT and VSSA slowly ramps the duty cycle

clamp in the PWM circuit. Tie this pin to VDD if not

used.

3.3.2. Switching Regulator Operation

The switching regulator of the Si3400 and Si3401 is

constant-frequency, pulse-width-modulated (PWM), and

controller integrated with switching power FETs

optimized for the output power range defined by the

802.3 specification.

Once the hotswap interface has ensured proper turn-on

of the switching regulator controller, the switcher is fully

operational. An internal free-running oscillator and

internal precision voltage reference are fed into the

pulse-width modulator. The output of the error amplifier

(either internal for non-isolated applications or external

for isolated applications) is also routed into the PWM

and determines the slicing of the oscillator.

The PWM controls the switching FET drive circuitry. A

significant advantage of integrating the switching power

FET onto the same monolithic IC as the switching

regulator controller is the ability to precisely adjust the

drive strength and timing to the FET's sizable gate,

resulting in high regulator efficiency. Furthermore,

current-limiting circuitry prevents the switching FET

from sinking too much current, dissipating too much

power, and becoming damaged. Thermal overload

protection provides a secondary level of protection.

The flexibility of the Si3400 and Si3401's switching

regulator allows the system designer to realize either

the isolated or non-isolated application circuitry using a

single device. In operation, the integration of the

switching FET allows tighter control and more efficient

operation than a general-purpose switching regulator

coupled with a general-purpose external FET.

3.3.3. Flyback Snubber

Extremely high voltages can be generated by the

inductive kick associated with the leakage inductance of

the primary side of the flyback transformer used in

isolated applications.

Refer to “AN296: Using the Si3400/01 PoE PD

Controller in Isolated and Non-Isolated Designs” for

more information on the snubber.

Si3400/Si3401

Rev. 0.9 15

4. Pin Descriptions

Table 11. Si3400 and Si3401 Pin Descriptions (Top View)

Pin# Name Description

1 EROUT Error-amplifier output and PWM input; directly connected to opto-coupler in isolated application.

2 SSFT

Soft-start output pin ramps voltage across external soft-start capacitor to allow switcher to ramp

output slowly.

3 VDD 5 V supply rail for switcher; provides drive for opto-coupler.

4ISOSSFT

Isolated mode soft start enable input. Tie to VDD for non-isolated applications. Connect a

0.1 µF capacitor between this pin and VSSA for isolated applications.

PLOSS

Early power loss indicator; open drain output is pulled to VPOS when VPORT is applied.

6RDET

Input pin for external precision detection resistor; also used for establishing absolute current ref-

erence.

7 HSO Hotswap switch output; connects to VNEG through hotswap switch.

8 RCL Input pin for external precision classification resistor; float if optional RCLASS is unused.

9, Pad VNEG

Rectified high-voltage supply, negative rail. Must be connected to thermal PAD node (VNEG)

on package bottom. This thermal pad must be connected to VNEG (pin #9) as well as a 2 in

heat spreader plane using a minimum of nine thermal vias.

10 SP2 High-voltage supply input from spare pair; polarity-insensitive.

11 SP1 High-voltage supply input from spare pair; polarity-insensitive.

12 VPOSF Rectified high-voltage supply, positive rail (force node)

13 CT2 High-voltage supply input from center tap of Ethernet transformer; polarity-insensitive.

14 CT1 High-voltage supply input from center tap of Ethernet transformer; polarity-insensitive.

15 VSSA Analog ground.

16 VPOSS Rectified high-voltage supply, positive rail sense node.

17 VSS1 Negative supply rail for switcher; externally tied to HSO.

18 SWO Switching transistor output; drain of switching N-FET.

19 VSS2 Negative supply rail for switcher; externally tied to HSO.

20 FB Regulated feedback input in non-isolated application.

EROUT

SSFT

RDET

HSO

RCL

SP1

SP2

VPOSF

CT1

PLOSS

ISOSSFT

VDD

5678

151620 19 18 17

VNEG

VSSA

VPOSS

VSS1

SWO

VSS2

VNEG

(PAD)

CT2

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P20

SI3400-C-GM

Mfr. #:

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Manufacturer:

Silicon Labs

Description:

Hot Swap Voltage Controllers

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SI3400-C-GM

SI3400-C-GM

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