NCP1081_13 Datasheet, PDF (14/18 Pages) ON Semiconductor – Integrated High Power PoE-PD Interface & DC-DC Converter Contr

ILIM1 /
INRUSH
NCP1081
Vbg1
VDDA1
VDDA1
Ilim_ref
VPORTNx
NCP1081
Figure 11. Current Limitation Configuration (Inrush & Ilim1 Pins)
Inrush ₀
Ilim1 ₁
I_pass_switch
&
VDS_PGOOD
Vds_pgood
threshold
1 V / 9.2 V
VDDA1
Current_limit_ON
detector
VDDA1
2V
VDDA1
VPORTNx
NCP1081
Pass Switch
RTN
Figure 12. Inrush and Ilim1 Selection Mechanism
When VPORT reaches the UVLO_on level, the Cpd
capacitor is charged with the INRUSH current (in order to
limit the internal power dissipation of the pass−switch).
Once the Cpd capacitor is fully charged, the current limit
switches from the inrush current to the current limit level
(ilim1) as shown in Figure 12. This transition occurs when
both following conditions are satisfied:
1. The VDS of the pass−switch is below the
Vds_pgood low level (1 V typical).
2. The pass−switch is no longer in current limit
mode, meaning the gate of the pass−switch is
“high” (above 2 V typical).
The operational current limit will stay selected as long as
Vds_pgood is true (meaning that RTN−VPORTN_1,2 is
below the high level of Vds_pgood). This mechanism allows
a current level transition without any current spike in the
pass−switch because the operational current limit (ilim1) is
enabled once the pass−switch is not limiting the current
anymore, meaning that the Cpd capacitor is fully charged.
Thermal Shutdown
The NCP1081 includes thermal protection which shuts
down the device in case of high power dissipation. Once the
thermal shutdown (TSD) threshold is exceeded, following
blocks are turned off:
• DC−DC controller
• Pass−switch
• VDDH and VDDL regulators
• CLASS regulator
When the TSD error disappears and if the input line
voltage is still above the UVLO level, the NCP1081
automatically restarts with the current limit set in the inrush
state, the DC−DC controller is disabled and the Css
(soft−start capacitor) discharged. The DC−DC controller
becomes operational as soon as capacitor Cpd is fully
charged.
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