LTC4264IDE datasheet(13/24 Pages) LINER

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LTC4264

4264f

During the inrush event as C1 is being charged, a large

amount of power is dissipated in the MOSFET. The LTC4264

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 LTC4264 automatically switches

to ILIMIT_LOW, and load capacitor C1 charging resumes.

The LTC4264 has the option of disabling the normal operat-

ing input current limit, ILIMIT_HIGH, for custom high power

PoE applications. To disable the current limit, connect

ILIM_EN to VIN. To protect the LTC4264 from damage when

the normal current limit is disabled, a safeguard current

limit, ILIMIT_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 LTC4264, engaging the thermal protection circuit. For

normal operations, ﬂoat the ILIM_EN pin. The LTC4264

maintains the ILIMIT_LOW inrush current limit for charging

the load capacitor regardless of the state of ILIM_EN. The

operation of the ILIM_EN pin is summarized in Table 3.

Table 3. Current Limit as a Function of ILIM_EN

STATE OF ILIM_EN

INRUSH CURRENT

LIMT

OPERATING INPUT

CURRENT LIMIT

Floating

ILIMIT_LOW

ILIMT_HIGH

Tied to VIN

ILIMIT_LOW

ILIMIT_DISA

POWER GOOD

The LTC4264 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 activa-

tion 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 LTC4264 to go into

thermal shutdown.

APPLICATIONS INFORMATION

4264 BD

BOLD LINE INDICATES HIGH CURRENT PATH

PWRGD

POWER

NOT

GOOD

INRUSH COMPLETE

AND NOT IN THERMAL SHUTDOWN

VIN < UVLO OFF

OR THERMAL SHUTDOWN

POWER

GOOD

PWRGD

LTC4264

VOUT

VIN 6

VIN

REF

UVLO

THERMAL SD

CONTROL

CIRCUIT

Figure 8. LTC4264 Power Good Functional and State Diagram

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