LTC4268IDKD-1#PBF【PDF 18/46 页】IC PD HIGH POWER W/CNTRL 32-DFN

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LTC4268-1

42681fc

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

LIMIT_LOW

, and load capacitor C1 charging resumes.

The LTC4268-1 has the option of disabling the normal

operating input current limit, I

LIMIT_HIGH

, for custom

high power PoE applications. To disable the current limit,

connect I

LIM_EN

to V

PORTN

. To protect the LTC4268-1

from damage when the normal current limit is disabled, a

safeguard current limit, I

LIMIT_DISA

keeps the current below

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, float the I

LIM_EN

pin. The

LTC4268-1 maintains the I

LIMIT_LOW

inrush current limit

for charging the load capacitor regardless of the state of

LIM_EN

. The operation of the I

LIM_EN

pin is summarized

in Table 3.

Table 3. Summary of IEEE 802.3af Power Classifications and

LTC4268-1 R

CLASS

Resistor Selection

STATE OF I

LIM_EN

INRUSH CURRENT

LIMIT

OPERATING INPUT

CURRENT LIMIT

Floating

LIMIT_LOW

LIMT_HIGH

Tied to V

PORTN

LIMIT_LOW

LIMIT_DISA

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

applicaTions inForMaTion

42681 F08

BOLD LINE INDICATES HIGH CURRENT PATH

PWRGD

POWER

NOT

GOOD

INRUSH COMPLETE

AND NOT IN THERMAL SHUTDOWN

PORT

< UVLO OFF

OR THERMAL SHUTDOWN

POWER

GOOD

PWRGD

LTC4268-1

NEG

PORTN

NEG

PORTN

REF

UVLO

THERMAL SD

CONTROL

CIRCUIT

Figure 8. LTC4268-1 Power Good Functional and State Diagram

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