ltc4268cdkd-1-trpbf Linear Technology Corporation, ltc4268cdkd-1-trpbf Datasheet - Page 20

• Manufacturer Part Number: ltc4268cdkd-1-trpbf
• Description: High Power Pd With Synchronous Noopto Flyback Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
During the inrush event as C1 is being charged, a large
amount of power is dissipated in the MOSFET. The
LTC4268-1 is designed to accept this load and is thermally
protected to avoid damage to the onboard power MOSFET.
If a thermal overload does occur, the power MOSFET turns
off, allowing the die to cool. Once the die has returned to
a safe temperature, the LTC4268-1 automatically switches
to I
The LTC4268-1 has the option of disabling the normal
operating input current limit, I
high power PoE applications. To disable the current limit,
connect I
from damage when the normal current limit is disabled, a
safeguard current limit, I
destructive levels, typically 1.4A. Note that continuous
operation at or near the safeguard current limit will rapidly
overheat the LTC4268-1, engaging the thermal protection
circuit. For normal operations, fl oat the I
LTC4268-1 maintains the I
for charging the load capacitor regardless of the state of
I
in Table 3.
LTC4268-1
20
LIM_EN
LIMIT_LOW
. The operation of the I
LIM_EN
V
V
V
PORTN
PORTN
PORTN
, and load capacitor C1 charging resumes.
5
6
7
LTC4268-1
BOLD LINE INDICATES HIGH CURRENT PATH
to V
THERMAL SD
PORTN
UVLO
LIMIT_DISA
REF
LIMIT_LOW
. To protect the LTC4268-1
CONTROL
CIRCUIT
LIM_EN
Figure 8. LTC4268-1 Power Good Functional and State Diagram
LIMIT_HIGH
keeps the current below
inrush current limit
pin is summarized
LIM_EN
, for custom
pin. The
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Table 3. Current Limit as a Function of I
POWER GOOD
The LTC4268-1 includes complementary power good
outputs (Figure 8) to simplify connection to any DC/DC
converter. Power Good is asserted at the end of the inrush
event when load capacitor C1 is fully charged and the
DC/DC converter can safely begin operation. The power
good signal stays active during normal operation and is
de-asserted at power off when the port drops below the
UVLO threshold or in the case of a thermal overload event.
For PD designs that use a large load capacitor and also
consume a lot of power, it is important to delay activation
of the DC/DC converter with the power good signal. If
the converter is not disabled during the current-limited
turn-on sequence, the DC/DC converter will rob current
intended for charging up the load capacitor and create a
slow rising input, possibly causing the LTC4268-1 to go
into thermal shutdown.
30
29
28
27
26
STATE OF I
PWRGD
PWRGD
V
V
V
Tied to V
NEG
NEG
NEG
Floating
PORTN
LIM_EN
POWER
GOOD
INRUSH CURRENT
AND NOT IN THERMAL SHUTDOWN
NOT
OR THERMAL SHUTDOWN
I
I
LIMIT_LOW
LIMIT_LOW
LIMIT
INRUSH COMPLETE
V
PORT
< UVLO OFF
LIM_EN
OPERATING INPUT
POWER
CURRENT LIMIT
GOOD
I
I
LIMIT_DISA
LIMT_HIGH
42681fa