SI3402-B Datasheet, PDF (12/21 Pages) Silicon Laboratories – FULLY-INTEGRATED 802.3-COMPLIANT POE PD INTERFACE AND LOW-EMI SWITCHING REGULATOR

Si3402-B
Class
0
1
2
3
4
Table 10. Class Resistor Values
Usage
Default
Peak Power Levels
0.44 to 12.95 W
Nominal Class
Current
< 4 mA
Optional
Optional
Optional
PoE+
0.44 to 3.84 W
3.84 to 6.49 W
6.49 to 12.95 W
12.95 to 17 W
10.5 mA
18.5 mA
28 mA
40 mA
RCL Resistor (1%,
1/16 W)
> 681 
(or open circuit)
140 
75.0 
48.7 
33.2 
3.2.4. Under Voltage Lockout
The 802.3 standard specifies the PD to turn on when
the line voltage rises to 42 V and for the PD to turn off
when the line voltage falls to 30 V. The PD must also
maintain a large on-off hysteresis region to prevent
wiring losses between the PSE and the PD from
causing startup oscillation.
The Si3402 incorporates an undervoltage lockout
(UVLO) circuit to monitor the line voltage and determine
when to apply power to the integrated switching
regulator. Before the power is applied to the switching
regulator, the hotswap switch output (HSO) pin is high-
impedance and typically follows VPOS as the input is
ramped (due to the discharged switcher supply
capacitor). When the input voltage rises above the
UVLO turn-on threshold, the Si3402 begins to turn on
the internal hotswap power MOSFET. The switcher
supply capacitor begins to charge up under the current
limit control of the Si3402, and the HSO pin transitions
from VPOS to VNEG. The Si3402 includes hysteretic
UVLO circuits to maintain power to the load until the
input voltage falls below the UVLO turn-off threshold.
Once the input voltage falls below 30 V, the internal
hotswap MOSFET is turned off.
3.2.5. Dual Current Limit and Switcher Turn-On
The Si3402 implements dual current limits. While the
hotswap MOSFET is charging the switcher supply
capacitor, the Si3402 maintains a low current limit. The
switching regulator is disabled until the voltage across
the hotswap MOSFET becomes sufficiently low,
indicating the switcher supply capacitor is almost
completely charged. When this threshold is reached,
the switcher is activated, and the hotswap current limit
is increased. This threshold also has hysteresis to
prevent systemic oscillation as the switcher begins to
draw current and the current limit is increased, which
allows resistive losses in the cable to effectively
decrease the input supply.
The Si3402 stays in a high-level current limit mode until
the input voltage drops below the UVLO turn-off
threshold or excessive power is dissipated in the
hotswap switch.
An additional feature of the current limit circuitry is
current limiting in the event of a fault condition. When
the current limit is switched to the higher level, 470 mA
of current can be drawn by the PD. Should a fault cause
more than this current to be consumed, the voltage
across the hotswap MOSFET will increase to clamp the
maximum amount of power consumed. The power
dissipated by the MOSFET can be very high under this
condition; therefore, the hot swap switch goes into a
temporary 10 mA current limit mode and turns off the
switcher. After 90 ms have elapsed, and if the switcher
supply capacitor is recharged, the hot swap switch turns
back on in the 470 mA limit mode, and enables the
switcher.
12
Rev. 1.0