Datasheet

LTC4264
13
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 I
LIMIT_LOW
, and load capacitor C1 charging resumes.
The LTC4264 has the option of disabling the normal operat-
ing input current limit, I
LIMIT_HIGH
, for custom high power
PoE applications. To disable the current limit, connect
I
LIM_EN
to V
IN
. To protect the LTC4264 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 LTC4264, engaging the thermal protection circuit. For
normal operations, fl oat the I
LIM_EN
pin. The LTC4264
maintains the I
LIMIT_LOW
inrush current limit for charging
the load capacitor regardless of the state of I
LIM_EN
. The
operation of the I
LIM_EN
pin is summarized in Table 3.
Table 3. Current Limit as a Function of I
LIM_EN
STATE OF I
LIM_EN
INRUSH CURRENT
LIMT
OPERATING INPUT
CURRENT LIMIT
Floating I
LIMIT_LOW
I
LIMT_HIGH
Tied to V
IN
I
LIMIT_LOW
I
LIMIT_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
V
IN
< UVLO OFF
OR THERMAL SHUTDOWN
POWER
GOOD
9
PWRGD
LTC4264
10
V
OUT
8
V
OUT
7
V
IN
6
V
IN
REF
UVLO
THERMAL SD
5
CONTROL
CIRCUIT
Figure 8. LTC4264 Power Good Functional and State Diagram