Datasheet

LTC4269-1
16
42691fc
APPLICATIONS INFORMATION
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.
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
Figure 5. LTC4269-1 ON/OFF and Overvoltage Lockout
V
PORTP
C1
5µF
MIN
V
PORTN
V
NEG
LTC4269-1
42691 F05
TO
PSE
ON/OFF AND
OVERVOLTAGE
LOCKOUT
CIRCUIT
PD
LOAD
CURRENT-LIMITED
TURN ON
+
V
PORTP
– V
PORTN
LTC4269-1
VOLTAGE POWER MOSFET
0V TO ON* OFF
>ON* ON
<OFF* OFF
>OVLO OFF
*INCLUDES ON/OFF HYSTERESIS
ON THRESHOLD 36.1V
OFF THRESHOLD 30.7V
OVLO THRESHOLD 71.0V
Figure 6. LTC4269-1 Power Good Functional and State Diagram
42691 F06
BOLD LINE INDICATES HIGH CURRENT PATH
PWRGD
POWER
NOT
GOOD
INRUSH COMPLETE
ON < V
PORTP
< OVLO
AND NOT IN THERMAL SHUTDOWN
V
PORTP
< OFF
V
PORTP
> OVLO
OR THERMAL SHUTDOWN
POWER
GOOD
29
PWRGD
LTC4269-1
30
V
NEG
27
V
NEG
26
V
PORTN
6
V
PORTN
OVLO
ON/OFF
TSD
5
CONTROL
CIRCUIT
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).
The PWRGD pin features an open collector output refer-
enced to V
NEG
which can interface directly with the UVLO
pin. When power good is declared and active, the PWRGD
pin is high impedance with respect to V
NEG
. An internal
14V clamp limits the PWRGD pin voltage. Connecting the
PWRGD pin to the UVLO prevents the DC/DC converter