LTC4269-1 Linear Technology Corporation, LTC4269-1 Datasheet - Page 17

• Manufacturer Part Number: LTC4269-1
• Description: IEEE 802.3 At PD And Synchronous No-Opto Flyback Controller
• Manufacturer: Linear Technology Corporation

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To control the power-on surge currents in the system, the
LTC4269-1 provides a fi xed inrush current, allowing C1 to
ramp up to the line voltage in a controlled manner.
The LTC4269-1 keeps the PD inrush current below the
PSE current limit to provide a well controlled power-up
characteristic that is independent of the PSE behavior.
This ensures a PD using the LTC4269-1 interoperability
with any PSE.
TURN-ON/ TURN-OFF THRESHOLD
The IEEE 802.3af/at specifi cation for the PD dictates a
maximum turn-on voltage of 42V and a minimum turn-off
voltage of 30V. This specifi cation provides an adequate
voltage to begin PD operation, and to discontinue PD op-
eration when the input voltage is too low. In addition, this
specifi cation allows PD designs to incorporate an ON/OFF
hysteresis window to prevent start-up oscillations.
The LTC4269-1 features an ON/OFF hysteresis window (see
Figure 5) that conforms with the IEEE 802.3af/at specifi ca-
tion and accommodates the voltage drop in the cable and
input diode bridge at the onset of the inrush current.
APPLICATIONS INFORMATION
PSE
TO
Figure 5. LTC4269-1 ON/OFF and Overvoltage Lockout
V
LTC4269-1
PORTN
V
VOLTAGE
0V TO ON*
>ON*
<OFF*
>OVLO
*INCLUDES ON/OFF HYSTERESIS
ON THRESHOLD 36.1V
OFF THRESHOLD 30.7V
OVLO THRESHOLD 71.0V
PORTP
OVERVOLTAGE
ON/OFF AND
LOCKOUT
CIRCUIT
– V
PORTN
POWER MOSFET
LTC4269-1
OFF
OFF
OFF
ON
V
PORTP
V
NEG
CURRENT-LIMITED
TURN ON
+
42691 F05
C1
5μF
MIN
LOAD
PD
Once C1 is fully charged, the LTC4269-1 turns on is internal
MOSFET and passes power to the PD load. The LTC4269-
1 continues to power the PD load as long as the input
voltage does not fall below the OFF threshold. When the
LTC4269-1 input voltage falls below the OFF threshold, the
PD load is disconnected, and classifi cation mode resumes.
C1 discharges through the LTC4269-1 circuitry.
COMPLEMENTARY POWER GOOD
When LTC4269-1 fully charges the load capacitor (C1),
power good is declared and the LTC4269-1 load can safely
begin operation. The LTC4269-1 provides complementary
power good signals that remain active during normal op-
eration and are de-asserted when the input voltage falls
below the OFF threshold, when the input voltage exceeds
the overvoltage lockout (OVLO) threshold, or in the event
of a thermal shutdown (see Figure 6).
Figure 6. LTC4269-1 Power Good Functional and State Diagram
V
V
PORTN
PORTN
5
6
LTC4269-1
ON/OFF
OVLO
BOLD LINE INDICATES HIGH CURRENT PATH
TSD
POWER
GOOD
AND NOT IN THERMAL SHUTDOWN
NOT
OR THERMAL SHUTDOWN
CONTROL
CIRCUIT
ON < V
INRUSH COMPLETE
V
V
PORTP
PORTP
PORTP
> OVLO
< OFF
< OVLO
POWER
LTC4269-1
GOOD
42691 F06
30
29
27
26
17
PWRGD
PWRGD
V
V
NEG
NEG
42691f